Aircraft Clean Air Requirements Using Bleed Air Systems

Abstract

There are certification and airworthiness requirements relevant to the provision of clean breathing air in the crew and passenger compartments. There have been continuing reports and studies over the years regarding oil fumes in aircraft, including impaired crew performance. Oil fumes are viewed in varying ways ranging from rare seal bearing failures, to low level leakage in normal flight. A Masters of Science (MSc) research degree was undertaken to assess whether there is any gap between the certification requirements for the provision of clean air in crew and passenger compartments, and the theoretical and practical implementation of the requirements using the bleed air system. A comprehensive literature search reviewed applicable certification standards, documented and theoretical understanding of oil leakage. Two types of interviews were conducted to address the research questions. Key aviation regulators were questioned about the process by which they certify and ensure compliance with the clean air requirements. Aerospace engineers and sealing professionals were interviewed about their understanding of how oil may leak past compressor oil bearing seals, and into the air supply under various flight conditions. The outcome of the research showed that there is a gap between the clean air certification requirements, and the theoretical and practical implementation of the requirements using the bleed air system. Low level oil leakage into the aircraft cabin in normal flight operations is a function of the design of the engine lubricating system and bleed air systems, both utilising pressurised air. The use of the bleed air system to supply the regulatory required air quality standards is not being met or being enforced as required.

Full text

Engineering, 2018, 10, 142-172
http://www.scirp.org/journal/eng
ISSN Online: 1947-394X
ISSN Print: 1947-3931
DOI:
10.4236/eng.2018.104011 Apr. 20, 2018 142 Engineering
Aircraft Clean Air Requirements Using Bleed
Air Systems
Susan Michaelis
Faculty of Health Sciences Pathfoot Building R E010, University of Stirling, Stirling, UK
Abstract
There are certification and airworthiness requirements relevant to the prov
i-
sion of clean breathing air in the crew and passenger
compartments. There
have been continuing reports and studies over the years regarding oil fumes in
aircraft, including impaired crew performance. Oil fumes are viewed in var
y-
ing ways ranging from rare seal bearing failures, to low level leakage in no
r-
mal flight. A Masters of Science (MSc) research degree was undertaken to a
s-
sess whether there is any gap between the certification requirements for the
provision of clean air in crew and passenger compartments, and the theoret
i-
cal and practical implementation of the requirements using the bleed air sy
s-
tem. A comprehensive literature search reviewed applicable certification
standards, documented and theoretical understanding of oil leakage. Two
types of interviews were conducted to address the research questions.
Key
aviation regulators were questioned about the process by which they certify
and ensure compliance with the
clean air requirements. Aerospace engineers
and sealing professionals were interviewed about their understanding of how
oil may leak past compre
ssor oil bearing seals, and into the air supply under
various flight conditions. The outcome of the research showed that there is a
gap between the clean air certification requirements, and the theoretical and
practical implementation of the requirements u
sing the bleed air system. Low
level oil leakage into the aircraft cabin in normal flight operations is a fun
c-
tion of the design of the engine lubricating system and bleed air systems, both
utilising pressurised air. The use of the bleed air system to supply the regul
a-
tory required air quality standards is not being met or being enforced as r
e-
quired.
Keywords
Bleed Air, Secondary Air, Gas Turbine Engines, Cabin Air Quality,
Lubricants, Oil Bearing Seals, Labyrinth Seals, Mechanical Seals, Oil Fumes
How to cite this paper:
Michaelis, S.
(201
8)
Aircraft Clean Air Requirements
Using Bleed Air Systems
.
Engineering
,
10,
142
-172.
https:
//doi.org/10.4236/eng.2018.104011
Received:
February 22, 2018
Accepted:
April 17, 2018
Published:
April 20, 2018
Copyright © 201
8 by author and
Scientific
Research Publishing Inc.
This work is licensed under the Creative
Commons Attribution International
License (CC BY
4.0).
http://creativecommons.org/licenses/by/4.0/
Open Access

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DOI:
10.4236/eng.2018.104011 143 Engineering
1. Introduction
The first reports of concerns of exposure to jet engine oils leaking from aircraft
engines into cabin air supplies date back to the early 1950s [1] [2] [3]. This coin-
cided with the introduction of synthetic jet oils that replaced mineral oils and the
introduction of higher performing, higher temperature and pressure turbine en-
gines [4]. Varying types of reports have continued to the present day including
military, airline, manufacturer and crew reports. Furthermore, there have been
airworthiness directives, regulator initiatives, legal and insurance claims, scien-
tific committee studies, published literature and media reports. The “vast major-
ity” of fume events are associated with an abnormal leakage of engine or Aux-
iliary Power Unit (APU) oil [5].
The reported frequency of fume events varies widely including 2.1 events per
10,000 departures [6], oil fumes in 1% of flights [7] and seals leaking as a func-
tion of the design and operation of oil seals reliant upon compressed air [8] [9].
Despite recognised under-reporting, crew impairment has been recorded in
around 30% of fume event reports [8] [10] [11].
Exposure to a range of hazardous substances and pyrolysis by-products, from
engine oils and hydraulic fluids contaminating the aircraft air supply, is increa-
singly recognised as potentially adversely impacting flight safety [12] [13] [14]
[15]. Despite no real time monitoring to detect compressor bleed air contamina-
tion, a growing number of studies have confirmed the presence of low levels of
oil substances in the air supply system in normal operations between 25% and
100% of flights [11] [16] [17]. While the significance of exposure continues to be
questioned, an increasing number of global initiatives continue to be undertaken
[12] [18] [19].
Despite “general acceptance that cabin air can become contaminated by com-
pounds released from pyrolysed oil from engines and APUs” [20] on a some-
what “regular” basis [21], the frequency of such exposures is widely debated.
There are two varying positions held within the aviation industry regarding
the leakage of oil outside the engine oil bearing chamber. In the wider aviation
industry, outside of the seal and engineering specialist areas, a common position
held is that leakage is a function of seal failure or operational deficiencies. Rare
seal or mechanical failures [22] [23] [24], unintentional oil leakage [25], impro-
per work or damage to the main shaft seals [26] or worn seals or overfilled
sumps [27] are commonly cited. A broader approach by the European Aviation
Safety Agency (EASA), recognises fault conditions to include seal or bearing
failure, maintenance irregularities and design deficiency enabling oil contamina-
tion of the air supply [5].
The second view comes from an increasing understanding that low level lea-
kage occurs at various phases of normal flight. Various views are reported, in-
cluding that all engines leak from the seals and bearings [28], which is a feature
of the design using the bleed air system [29], and oils seals may leak at greater
rates during transient operations and while the engine is still achieving optimum

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temperature and pressures [30]. Oil seal leakage is also reported to occur during
certain engine events, such as engine switching or at top of descent; and in older
aircraft with chronic exposure to vapours that “continuously leak through the
seals in “tiny” amounts” [31]. Oil leaking from bearings can be either “slowly
varying and somewhat continuous, or sporadic and quite intermittent” [32]. The
specialist oil sealing and engineering sector tends to support the latter position,
however their views are not commonly available.
There are also varying ways in which oil leakage effects are seen. Lower level
leakage related to system design is often viewed as normal, safe and acceptable,
associated with minor discomfort only, with increased levels due to wear or fail-
ure possibly affecting occupant health and flight safety [30]. Fume events are
said to range from rare and serious smoke events to “simple dirty socks smells”
[33], whilst improved seals lead to concentrations of oil in the bleed air being
“negligible” [30]. Some suggest that air is safe and meets all the applicable regu-
latory standards [34], however hazards associated with the oils are recognised
[15] [35]-[41]. Additionally ground based safe exposure limits do not apply to
the aircraft environment [30] [42] [43]. With regard to oil/air sealing flows, in
1995 the NASA Seal Development Workshop stated that “oil vapours and coking
smells are obnoxious at best and health hazards at worst to the customer” [44].
Maintenance diagnostics have been referred to as being of a “trial and error”
nature [27], while failure to eliminate the source of the contamination will lead
to repeated occurrences [45]. The financial implications related to oil fume
events are suggested to range from approximately $40,000 per incident to
$2,000,000 per day [46] [47].
There are clear regulatory standards and guidelines available that outline the
requirements for clean air to be supplied to the crew and passenger compart-
ments. Therefore, given the regulatory frame work applicable to air quality re-
quirements and the discrepancy in the two varying positions on oil leakage fre-
quency, a research question was identified. A question was raised regarding
whether oil leakage out of the bearing chamber occurs only in the occasional
failure or maintenance deficiency situation, or as a normal part of engine opera-
tion when using pressurised oil seals and compressor bleed air to supply cabin
air?
The aim of this research was to therefore assess whether there is any gap be-
tween the certification requirements for the provision of clean air in the crew
and passenger compartments, and the theoretical and practical implementation
of the requirements using the bleed air system [11].
The objectives were to: 1) evaluate the aircraft certification requirements for
the provision of clean air in crew and passenger compartments, and the
processes in ensuring their compliance; 2) assess the theoretical documented
understanding of bleed air contamination of the air supply; and 3) assess the fea-
sibility of the implementation of the aircraft certification requirements for the
provision of clean air in crew and passenger compartments, in the context of the

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potential contamination caused by various conditions in the aircraft bleed air
system.
The research was structured reviewing the applicable certification standards
and the existing documented theoretical understanding of turbine engine oil
leakage past oil-bearing seals into the cabin. Further research was then underta-
ken to understand how the certification process is undertaken in practice and
how oil may contaminate the aircraft bleed air supply.
This research should be beneficial for a wide range of interests within and
outside the aviation industry including engineering and design, maintenance,
airlines, manufacturers, regulators, occupational health and safety, environmen-
tal health occupational and public health and policy makers.
2. Materials and Methods
This research consisted of 3 elements:
1) A review of the certification standards and guidelines applicable to the ven-
tilation air provided to the aircraft cabin;
2) A review of the theoretical and documented understanding of how oil may
contaminate the aircraft air supply when using the bleed air system;
3) Research addressing the real world implementation of the certification re-
quirements requiring clean bleed air.
In order to understand how the certification process is undertaken in practice
and how oil may contaminate the aircraft bleed air supply, two separate inter-
view processes were utilised.
An extensive review of the literature and databases was undertaken addressing
current documented understanding of oil leakage and certification standards.
This was required to meet two of the research objectives, in order to then ad-
dress the final aim of identifying whether there is a gap between the require-
ments for clean air and the theoretical documented understanding and practical
implementation.
A semi-structured qualitative interview approach was undertaken. Written
questionnaires were sent to the two sets of interview candidates, with follow up
phone interviews undertaken where required.
EASA and the Federal Aviation Administration (FAA) were selected as the
regulatory authorities to interview as many countries utilise the EASA and FAA
certification and type certificate process or use essentially the same standards
when undertaking their own certification. Seven questions were directed to the
engine/APU and airframe airworthiness departments to understand the process
by which they certify and ensure clean aircraft air requirements are met with the
use of bleed air.
Ten aviation engineering professionals and two seal supplier experts were se-
lected to conduct the interviews involving their professional judgement on how
oil may leak past oil bearing seals into the air supply under various flight opera-
tional conditions. The respondents were identified based upon professional

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contact with the researcher due to the researcher’s previous expertise in this area
[15]. All were required to have extensive relevant aviation expertise and hold or
have held senior positions within the industry. Ten of the 12 experts had an av-
erage of 43 years in their respective fields. The experts selected were based in
four countries in three continents. Eight questions were utilised to gain an un-
derstanding of the professional view of the interviewees about oil leakage past
bearing seals into the compressor core air, including into the aircraft cabin air
supply.
3. Results
3.1. Certification
The key relevant European Certification Specifications (CS) and Federal Avia-
tion Regulations (FAR) and suggested non-mandatory acceptable means of
compliance (AMC) or guidance material related to the clean air requirements
are outlined below. Specific wording related to some of the requirements and
AMC are outlined in Table 1.
The Guidance material for the engines and APU above note that when dealing
with such low probabilities, absolute proof is not possible, with reliance placed
on good engineering judgement, previous experience, sound design & test phi-
losophies.
CS 25.1309 and the FAR equivalent airframe airworthiness standards require
equipment, systems and installations to be designed ensuring they perform their
intended functions under any foreseeable operating condition, including fluid or
vapour contamination, according to the AMC. The FAR requires failures caus-
ing the prevention of safe flight and landing to be extremely improbable and re-
duced ability of the crew to cope with adverse operating conditions, improbable.
The CS specifications utilise three categories including “hazardous” failures, as
extremely remote and “major” failures as remote. As shown in Figure 1 and
Figure 2, the AMC shows each failure condition should have a probability in-
versely related to its severity.
The EASA AMC lists major failure conditions as those that could impair crew
efficiency, or cause physical discomfort to the pilots, or physical distress or in-
jury to the passengers or cabin crew. Such conditions must be remote, unlikely
to occur to each aeroplane during its total life, but may occur a few times during
the total life of all aircraft of type, with average probability per flight hour of 1 ×
10−5 or less but greater than 1 × 10−7. See Table 2 for further detail outlining the
CS 25.1309 AMC Relationship between Probability and Severity of Airframe
Failure Conditions.
Warning systems must be provided to alert the crew to unsafe system operat-
ing conditions and to enable them to take corrective action FAR and CS
25.1309C.
A safety analysis of the engine, including the compressor bleed system is re-
quired under Certification Standard-Engines (CS-E) 510 and FAR 33.75, with

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Table 1. Airframe (25.1309) failure conditions and engine/APU safety analysis effects for EASA and FAA (§33.75; CS-E 510; CS-APU
210) regulation and standards as well as guidance material relevant to clean air requirements (see ref 11 for complete table).
Airframe Level
FAA
EASA
Regulation/standard
CFR 14 25.1309 - Airworthiness standards equipment
Failure condition:
1. Reducing ability of crew to cope with
adverse operating conditions.
• Improbable
CS 25.1309 - Equipment, systems and installation design requirements Failure condition:
1. Major
•
Remote
2. Hazardous
• Extremely remote
Guidance Material (Advisory Circular - CS AMC)
AC 25.1309
-1A - Failure conditions
1. Minor:
Crew actions well within
capabilities
- slight increase in workload - some
inconvenience to occupants.
•
Probable
•
>1 × 10−5/fh
2. Major
:
-
Conditions impairing crew efficiency or some
discomfort to occupants;
-
Higher workload or physical distress such that
crew can’t be relied upon to perform tasks
accurately or completely
.
•
Improbable
•
≤1 × 10−5 - > 1 × 10−9/fh
AMC 25.1309 - Failure conditions
1. Minor:
Crew actions well within capabilities-slight increase in workload - some physical
discomfort to cabin crew or passengers.
•
Probable
•
>1 × 10−5/fh (
Table 2
)
2. Major:
-
Conditions impairing crew efficiency or discomfort to flight crew
-
Physical distress to cabin crew or passengers, possibly including injuries
•
Remote
•
≤ 1 × 10−5 - > 1 × 10−7/fh.
3. Hazardous:
excessive workload or physical distress such that flight crew can’t be relied
upon to perform tasks accurately or completely
- serious or fatal injury to a small number
of occupants other than flight crew
.
•
Extremely remote
•
≤1 × 10
−7
- > 1 × 10
−9
/fh
Anticipation of failure conditions
•
Probable: One or more times during entire operational life of each aeroplane;
•
Improbable (FAA): Will not occur during entire operational life of a single random aeroplane - may occur occasionally during life of
all aeroplanes of type;
•
Remote (EASA): Unlikely to occur to each aeroplane during its total life, but may occur several times during life of a number of aircraft of type;
•
Extremely remote (EASA): Will not occur to each aeroplane during its life but may occur a few times during total life of all aeroplanes of type.
Compliance shown by analysis and where necessary, appropriate ground, flight or simulator tests.
Engine-APO Level
FAA
EASA
Regulation/standard
CFR 14 33.75 - Safety analysis - Engines
1. Hazardous engine effects -
•
Extremely remote
•
10−7 to 10−9/efh
•
Concentration of toxic products in engine bleed air intended for the
cabin sufficient to incapacitate crew or passengers
.
2. Major engine effects
•
Remote
• 10
−5
to 10
−7
/efh
CS-E 510 & CS-APU 210 - Safety analysis - Engines & APU
1. Hazardous engine/APU effects
•
Extremely remote
•
<10−7/efh or APU operating hour (APU o/h)
•
Concentration of toxic products in engine/APU bleed air
intended for the cabin sufficient to incapacitate crew or passengers
2.Major engine effects
•
Remote
• <10
−5
/efh or APU o/h
Safety analysis: must include compressor bleed systems
Guidance Material (FAA Advisory Circular-EASA CS AMC)
FAA - AC 33.75-1A (engines)/CS AMC E 510 & CS - APU 210 (engines & APU)
1. Hazardous Engine effects: Toxic products:
•
Generation and delivery of toxic products caused by abnormal engine operation sufficient to incapacitate crew or passengers during flight.
•
Degradation of oil leaking into compressor airflow.
Intent is to address relative concentration of toxic products in bleed air delivery.
No assumptions including cabin air mixing/dilution.
2. Major engine effects:
• Concentration of toxic products in engine/APU bleed air for the cabin sufficient to degrade crew performance.

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Figure 1. EASA AMC 25.1309 acceptable means of compliance: the rela-
tionship between probability and severity of failure condition effects.
Figure 2. FAA Advisory Circular (AC) 25.1309-1A: probability V Conse-
quence of failure condition effects.
acceptable means of compliance provided as shown in Table 1. Concentrations
of toxic products in the engine bleed air for the cabin deemed sufficient to inca-
pacitate crew or passengers are regarded as a “hazardous” engine effect under
the FAR or CS standard and must be predicted to occur as extremely remote, at
less than 10−7 per engine flight hour/efh. “Major” engine effects must be remote
at less than 10−5/efh. “Hazardous” effects include no effective means to prevent
the flow of toxic products to crew or passenger compartments, or toxic products
impossible to detect or stop prior to incapacitation. Degradation of oil leaking
into the compressor airflow is listed as a toxic product. Concentrations of toxic
products slow enough acting and/or readily detectable so as to be stopped prior

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Table 2. EASA CS 25.1309 AMC: Relationship Between Probability and Severity of Failure Condition.
Effect on Aeroplane No effect on operational
capabilities or safety
Slight reduction in
functional capabilities
or safety margins
Significant reduction
in functional
capabilities or safety
margin
Large reduction in
functional capabilities or
safety margins
Normall
y with hull
loss
Effect on occupants
excluding flight crew Inconvenience Physical discomfort
Physical distress,
possibly including
injuries
Serious or fatal injury to a
small number of
passengers or cabin crew
Multiple fatalities
Effect on flight crew No effect on flight crew Slight increase in
workload
Physical discomfort or
a significant increase in
workload
Physical distress or
excessive workload impairs
ability to perform tasks
Fatalities or
incapacitation
Allowable qualitative
probability
No Probability
Requirement <…Probable….> <…Remote….>
Extremely <
--------
> Remote
Extremely
Improbable
Allowable qualitative
probability: Average
probability per flight
hour on the order of:
No Probability Require-
ment
<………>
<10−3
Note 1
<………>
<10−5
<………>
<10−7 <10−9
Classification of
Failure Condition No safety effect <…Minor..> <…Major….> <…Hazardous….> Catastrophic
Note 1: A numerical probability range is provided here as a reference. The applicant is not required to perform a quantitative analysis, nor substantiate by
such an analysis, that this numerical criteria has been met for Minor Failure Conditions. Current transport category aeroplane products are regarded as
meeting this standard simply by using current commonly-accepted industry practice.
to incapacitation are considered “major” engine effects. These include sub-
stances sufficient to degrade crew performance.
CS-APU 210 safety analysis and its AMC are similar to CS-E 510, while a US
APU Technical standing order (TSO)-C77b requires that failures do not gener-
ate an unacceptable concentration of toxic products in the bleed air.
Prior to the 2007 FAR 33.75 amendment, there was no requirement to review
toxic bleed air components, while the 2001 Joint Aviation Requirements (JAR)
acceptable compliance referred to unacceptable concentrations of toxic products
in the bleed air supplied to the cabin.
CS-E 690 requires contamination or purity tests of the bleed air when it is di-
rectly used in the cabin. An analysis of the defects which could affect the purity
of the bleed air must be prepared and where necessary defects must be simulated
and tests undertaken to establish the degree of contamination that is likely to
occur.
The airworthiness ventilation and heating requirements are set out under CS
and FAR 25.831. CS 25.831a requires that each crew compartment has enough
fresh air enabling crew members to perform duties without undue discomfort or
fatigue. FAR 25.831a is very similar but covers normal and probable failure con-
ditions, uses the term “sufficient amount of uncontaminated air” and references
reasonable passenger comfort. CS and FAR 25.831b require that the crew and
passenger compartment air must be free of harmful or hazardous concentrations
of gases or vapours. Only carbon monoxide (CO), carbon dioxide (CO2), ozone
(O3) levels and fresh airflow rates are listed.
An unsafe condition includes events that occur more frequently than the

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safety objectives allow, or that may reduce the ability of the crew to cope with
adverse operating conditions, impair crew efficiency or cause discom-
fort/injuries to occupants—EASA AMC 21. A.3Bb.
Historically, Military Standard (Mil Spec) MIL-E-5007 specification was uti-
lised as one form of certification guidance compliance. Oil leakage within en-
gines was not to cause oil discharge upon starting after previous shutdown or
cause contamination of the bleed air or deposits. A compressor bleed air analysis
was to be undertaken to ensure contaminant levels were within specified limits,
including oil breakdown products.
There are various other voluntary standards or recommended practices such
as the Society of Automotive Engineers Aviation Recommended Practice SAE
ARP4418, SAE Aerospace Standard AS 5780A and the previous Military Specifi-
cation, MIL-PRF-23699F and the American Society of Heating, Refrigerating
and Air-Conditioning Engineers ASHRAE standard and associated guideline
161-2013.
3.2. Oil Sealing System
Around 25% of the engine core airflow is extracted and utilised to supply engine
internal air and various aircraft systems. This secondary air, also known as bleed
air is primarily tapped off the compressor and used for cooling the engine, and
accessory components, bearing chamber oil cooling and sealing, control of tur-
bine tip clearances, cavity ventilation bearing load controls, cabin pressurisation,
ventilation, anti-icing and other services. Around 3% - 4% of the air bled off the
compressor core airflow is used for cabin ventilation purposes. The extracted
secondary/bleed air is controlled and minimised as it reduces power and effi-
ciency of the engine. To do this a number of oil and air seals are required.
Figure 3 shows a typical engine oil system. The minimum quantity of oil is
used to perform lubricating, cooling, corrosion protection and sealing functions
and then returned to the lubrication system, taking into account the permissible
consumption of oil, usually around 0.1 to 0.5 US Quarts/ hour per engine [48].
Engine bearings grouped in bearing chambers require a continuous supply
and removal of oil. In addition to lubricating and cooling the bearings, the oil
washes away metal parts released from the bearings in normal operations and
supports the sealing of a particular type of seal, the carbon face seal.
The philosophy behind engine bearing compartment sealing involves using
compressor pressurised air (see Figure 4) to maintain the bearing compartment
at a lower pressure than its surroundings, therefore inducing an inward flow to
prevent an outward oil leak [50]. Too much airflow around the chamber to pre-
vent oil leakage is a performance penalty and increases the heat load to the oil in
the chamber [51]. The pressurised air from the compressor in addition to pre-
venting oil leaking out over the bearing seals is also used to cool and ventilate
the bearing sumps to prevent a build up of combustible gas mixtures. Oil leakage
outside the bearing sumps may result in performance loss due to the contamina-
tion of aerodynamic parts, engine fires, vibration due to oil accumulating in

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Figure 3. Typical jet engine oil system [49].
Figure 4. Oil bearing sump [49].

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rotating parts, or pollution in the bleeds resulting in cabin air contamination
[49] [50].
The ability of contaminants to migrate from the core air into the cabin bleed
air supply will be influenced by a number of factors including, the design and
location of the bleed off-takes and the specific operating conditions.
Pressurised oil bearing seals are generally clearance labyrinth seals or me-
chanical contact face seals, both relying upon compressor air as a part of the
sealing function [48]. The seals operating at high speeds require either a clear-
ance or well lubricated seal [52]. The compressor sealing air flowing across the
seal into the bearing compartment is responsive to variations in engine operat-
ing conditions [53]. Sealing bearing compartments at near ambient pressures is
difficult [54] [55]. The pressure difference between inside and outside the
chamber is very small, so as to not blow oil out through the oil system breather.
However, the small differential in transient modes provides a much greater
chance of pressure reversal.
3.2.1. Labyrinth Seals
Non-contacting clearance labyrinth seals (see Figure 5 and Figure 6) rely upon
a small tight clearance between the stationary and rotating members to reduce
leakage air flows. An inward flow or controlled leakage of air, through a series of
restrictions, followed by a clear volume creates expansion of the air, therefore
reducing pressure over the seal. The clearance is set by aero-thermal mechanical
conditions allowing for rotor and axial excursions and minimising rotor contact
with the shroud [54]. Labyrinth seals provide simplicity, reliability, reduced wear
and low cost, however they are subject to high air leakage and loss of engine
performance. However, they do not in isolation provide a complete barrier to
leakage [56]. They do not respond well to dynamics, with permanent increases
Figure 5. Labyrinth seal [51].

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Figure 6. Labyrinth seal function [58].
in seal clearances from shaft excursions on stop/start operations, other transient
conditions [54] and other factors such as engine age and a variety of operating
conditions [57].
The clearance permits fluids to flow in either direction, dependent on pres-
sures and the momentum of the fluid [59]. Leakage of fluid will always occur
across the seal from high to low pressure [52] [59]. The seal is essentially a con-
trolled leakage device [60], relying on pressurisation to minimise oil leakinga-
long the compressor shaft [61].
3.2.2. Mechanical Contact Seals
Mechanical positive contact seals, such as carbon face seals form a seal between a
stationary and rotating flat precision-finished surface, thereby preventing lea-
kage [62]. These seals are often used to seal bearing sumps, thereby restricting
air leakage into the bearing sumps and preventing oil vapours passing into the
cabin air stream. However, they are more complex, maintenance intensive, ex-
pensive, subject to higher wear rates, and have a shorter life than labyrinth seals.
Figure 7 shows the faces are held in sealing contact by a combination of force
by a spring and positive system pressure to ensure adequate loading of the car-
bon elements to minimise leakage and wear [55]. Carbon seal performance is af-
fected by excessive seal wear during transients, finite rates of wear and coked oil
deposits.
A small amount of oil is forced across the flat faces. The minimal film of oil is
a compromise ensuring the oil is sufficiently thick, providing adequate lubrica-
tion of the seal and a long life and being as thin as possible to minimise leakage
[52]. A normal contact seal will leak a very small amount of oil vapour from a
few parts per million (ppm) to 10 cc/min [62]. The flat faces, providing the seal,
will distort with thermal and pressure effects, encouraging increased oil between
the faces then pumped out to the air high-pressure side of the seal [52]. Various
features may be utilised to help prevent oil leakage into the compressor at vari-
ous phases of engine operation.

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Figure 7. Carbon contact seal [51].
3.2.3. Common Aspects of Oil Bearing Seal Operation
There is a fundamental assumption regarding oil bearing seals that the com-
pressor gas path air will be at a higher pressure than the oil in the bearing
chamber, ensuring that leakage will always be into the bearing housing and not
into the gas path. It is commonly reported that oil seals only leak when there is a
failure or under reverse pressure conditions, but the literature suggests this is
not always the case due to a variety of factors as outlined below.
• Oil leakage may flow against the positive pressure gradient with both types of
seals, that is from low to high pressure. A positive gradient is difficult to ob-
tain under all operating conditions and not a guarantee of zero oil leakage
and sealing bearing compartments at near ambient pressures is difficult [53]
[55].
• Pressure generated in the oil film between mechanical face seals can cause
liquid in the film to overcome the pressure gradient to leak both with and
against the pressure gradient [52] [63].
• Dalton’s law of partial pressures with gas trying to create a constant partial
pressure, indicating high pressure will not prevent oil vapour from permeat-
ing through the labyrinth against the pressure gradient [52].
• Reverse pressures over the seals during engine operation causing higher
pressure on the oil side of the bearing chamber, allowing both types of seals
to allow leakage in the opposite direction [52].
Just about all known seals will leak, with seals designed to limit leakage and no
such thing as a seal that does not leak, even if a very small amount, perhaps an
emission, rather than a leakage [52]. Chupp
et al.
2006 state that “a zero leakage
seal is an oxymoron” [54]. Only very small amounts of oil need to leak to gener-
ate a noticeable odour in the cabin [45], with odours noticeable before high oil-

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consumption is noticed [64]. There will be a variety of other factors that influ-
ence the volume of leakage.
3.2.4. Oil Bearing Seal Leakage
Military jet aircraft commenced using pressurised air bled off the compressor in
the late 1940s. It was soon recognised that engine bleed air used for the ventila-
tion was increasingly subject to unacceptable contamination, with the compres-
sor bearing seals being the main source of oil leakage [65]. The bleed air conta-
minated by oil was said to be non toxic, ranging from objectionable odours to
severely irritating and therefore initially unsuitable for commercial airliners [65].
With higher performing civil aircraft necessitating larger turbocompressors to
compress the outside air and as there was “no discernable difference in quality
between ram air and bleed air”, it was decided to use direct bleed air to ventilate
the cabin [25].
Upon a closer review, industry awareness of seal leakage is well established.
There is a general view that mechanical and labyrinth seals will leak as a part of
their normal function, along with the need for more advanced seals. Limited
examples include: recognition that carbon seals will always leak a small amount;
sealing technical challenges including low leakage, long life at high temperatures
and speeds; seals needing to act as seals not flow restrictors; seals varying in ef-
fectiveness at different stages of aircraft operation, especially during transients.
The major part of consumption in Rolls-Royce gas turbines was said to represent
“loss of liquid oil arising from permissible leakage past certain seals, escape of
mist or aerosol through breathers and losses incurred during inspections, made
good by ‘topping up’ the system with fresh oil” [66].
The actual bearing seal arrangements are complex, differing widely, with spe-
cific engine design details not publicly available. The selection of one type of seal
over another involves acceptance of advantages and penalties, varying with dif-
fering engine designs. There seem to be contradictory reports on which seals are
optimal for sealing the bearing compartment. A few examples are listed below.
• Air/oil face seals requiring improved reliability and future research on the
transient behavior of the seals [67].
• Carbon seals suggested to be more effective for bearing compartment sealing
and preventing oil leakage into the cabin [55].
• Carbon face seals are industry workhorse but have problems with face blisters
[68].
• Use of secondary sealing practices in aero engines is unknown, although sin-
gle seals appear to be utilized [52].
• Original Equipment manufacturer’s OEMs satisfied with labyrinths for main
shaft sealing despite mechanical seals suggested to be “seals of the future” for
aircraft engines [69]. Labyrinths will be around for a long time [70].
The problems associated with conventional shaft oil sealing were clearly hig-
hlighted with seal technology not keeping pace with other major engine compo-
nent advances [71]. It was recommended that in order to address some of the

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concerns, seal design should be thoroughly integrated into the engine design
process [70].
While labyrinth seals are noted to have high air leakage leading to high oil
consumption, both labyrinth and mechanical contact seals have high oil loss and
oil pollution in the cabin during reverse pressure conditions [71] [72]. Axial lift
mechanical seals are suggested to prevent oil pollution in the cabin during re-
verse pressure scenarios [72]. Advanced seals are being developed to reduce lea-
kage, improve life and offering wider operating conditions than available for
conventional seals [54].
Fluid control measures appear to depend on how leakage is regarded. The
aviation industry is suggested to be unique in that environmental factors drive
sealing requirements rather than emission limits as occurs in other critical in-
dustries and the general environment [69]. It is suggested that customer re-
quirements for the cabin free of smells and performance parameters drive aero-
space sealing technology [54] [69]. Where emission limits apply, single, double
or tandem seals may be utilised, however few limits apply to the aerospace in-
dustry where leakage may be defined as 10,000 ppm or as a visible oil mist [69].
The air bled from the compressors is parasitic to the main engine cycle and
costs up to 6% of the specific fuel consumption (SFC) in a modern air transport
turbofan [73]. Higher performing gas turbine engines and the drive for im-
proved SFC, have necessitated greater sealing efficiencies to prevent increased
performance losses [71]. A 1% reduction in secondary air extracted gives a 0.4%
reduction in SFC [54]. However, the literature strongly reports on leakage paths,
generally referring to minimisation of airflow leakage into various components,
including into the bearing chambers, so as to reduce performance penalties.
There are only minor references to air/oil leakage out of the bearing chambers.
Lower level oil leakage is expected under a range of circumstances and is
widely reported amongst the specialist engine/oil sealing community, while ref-
erence to oil bearing seal failure is far more limited. Seals for the aerospace in-
dustry are suggested to be far more demanding than those used in industrial ap-
plications, given frequent speed changes and seal operation at high altitude,
start-up and shut-down [74]. Further sources of increased oil leakage into the air
are referenced, such as misalignment of shafts and bearings before engine stabi-
lisation, rapid throttle advancement and autothrottle adjustment [75].
3.3. Research Results
The interview questions and responses provided by the engineers and regulators
are set out below: [11]
3.3.1. Engineers
Q1.What areas can oil leaking out of the engine or APU bearing chamber
go? Oil leakage can occur within and outside the engine along with normal oil
consumption as part of the oil system via the aircraft breather. There was clear
recognition that internal oil leakage from the compressor bearing chamber can

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allow oil to enter the core flow with potential to enter the cabin bleed air system.
Q2. What are the factors that may allow oil to leak past compressor
bearing seals? This complex and specialist area identified a variety of factors in-
cluding design factors such as the use of seal types that are not an absolute de-
sign and will leak; reliance of the seals on pressure differentials; thermal and axi-
al/ radial changes in engine structures; leakage affected by engine speed, power;
and design parameters that do not account for all flight conditions. Other key
engine operation factors include seal wear and degradation, on condition main-
tenance, installation, maintenance and in-use factors.
Q3. Does the phase of flight effect oil leakage rates? The three areas identi-
fied included: changing pressure differentials and balances over the seals with
differing transient engine power, application and ambient conditions affecting
seal efficiency and leakage rates; mechanical variations thermal, axial and radial
in structures over the engine operating range changing gaps requiring to be
sealed to prevent oil crossing the seals; and oil leakage at low power settings with
low internal pressures such as start, spool up, top of descent and descent.
Q4. Do some types of oil bearing seals leak more than others and why?
Both carbon and labyrinth seals leak for varying reasons with some leakage in-
evitable, as it is inherent in the design. Labyrinth seals rely more on pressure
differentials with the clearance allowing leakage both with and against the pres-
sure drop including reverse pressures over the seal. Carbon seals are designed to
have low leakage rates as lubrication is required between the faces and rely more
on physical contact and more subject to wear and high temperatures. Leakage
also occurs with and against the pressure drop.
Q5. How is lower level leakage of oil from the compressor-bearing
chamber at various phases of flight perceived with regard to regulatory
compliance? Responses provided indicate there are no regulations, limits or
measurement methods for air contamination by oil. Differing views indicate ac-
tion is only required if leakage is above useable limits and alternatively that low
level leakage is expected as part of the system design and fails to meet published
design requirements. Regulatory enforcement is regarded as a low priority with
standards available ignored.
Q6. What can be done to address oil leakage from the compressor bearing
chamber? Preventative maintenance, real time measurement, electric air supply
rather than cabin bleed air and mitigating oil leaks into the cabin to be given a
higher priority, are a few of the suggested ways forward to address compressor
oil leakage.
Q7. What is considered oil leakage? Oil leakage is seen in two key differing
ways. Any oil that leaves the intended area, resides in areas in a greater amounts
than intended or loss over seals is leakage. Alternatively, only loss above permit-
ted oil consumption levels or inadequate system pressure differentials are re-
garded as leakage, but not lower level oil emissions.
Q8. Are all oil leakage events documented? The majority believe that not all
leakage events are reported for a variety of reasons including under reporting,

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varying record keeping and maintenance dependent on crews identifying odours
and then reported in the technical log. A small minority focused on the re-
quirement to report, mandatory maintenance procedures being recorded and
higher level events only.
Answers provided clearly identified a comprehensive picture and opinions of-
ten based on specific area of expertise, rather than a complete overview, given
the specialised topic.
3.3.2. Regulators
Q1. What is the certification process that an engine and APU manufac-
turer must follow to demonstrate compliance with the requirements of the
quality of bleed air utilised for the aircraft cabin? While the applicant must
show compliance with the regulations or standards and the means by which
compliance is met, there is no specific process to follow to demonstrate this,
however guidance is provided. The regulator interactively will review the data
submitted to enable agreement that compliance is met.
Q2. What are the relevant certification standards and acceptable means of
compliance AMC used to demonstrate bleed air compliance? The safety
analysis process and published acceptable methods of compliance for both regu-
lators is essentially identical. “Hazardous” engine effects, including concentra-
tions of toxic products resulting from degradation of oil leaking into the com-
pressor air flow, sufficient to incapacitate crew or passengers must be predicted
to be extremely remote 10−7 to 10−8/ engine flight hour (efh). EASA reported that
“major” engine effects must not occur at a rate greater than remote 10−5/efh.
Toxic products are considered a “hazardous” engine effect for several reasons
including there being no effective means to stop the flow of incapacitating toxic
products to the crew or passenger compartments. While both require analysis of
concentrations of toxic products in the bleed air, the EASA specifications are
more specific with engine bleed air purity tests and an analysis of possible de-
fects effecting purity also required.
Q3. Which substances are reviewed and what limits are applied demon-
strating compliance? While there is a requirement to prevent incapacitation
from toxic bleed air substances, there are no specified regulatory limits. EASA
however referred to SAE standard limits ARP 4418A as a means to demonstrate
compliance.
Q4. Was there any difference in previous years with what was deemed
acceptable to demonstrate compliance? The reference to toxic products did
not exist under the FAA safety analysis regulations for aircraft certified before
2007. The initial EASA CS E section in 2003 is effectively the same as the present
version. The last version of the JARs published in 2001 referred to “unacceptable
concentrations” of toxic products generated in air supplied in the guidance ma-
terial.
Q5. What defects that could affect the purity of the bleed air might be
considered and what tests may be undertaken? The FAA referred to the Ad-

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visory Circular guidance material listing oil leakage and degradation of abrada-
ble materials into the compressor airflow, without supplying specific defects
enabling this to occur. While tests for toxic substances are not defined, EASA
standards require analysis and possible simulation of defects as part of the con-
tamination cabin bleed air tests.
Q6. What is the cabin air quality certification process and acceptable
means of compliance at the airframe level and which substances and limits
are included? The airframe requirements are very similar requiring enough
fresh air to avoid discomfort and fatigue and provide reasonable comfort, a
minimum airflow and are interpreted to consider carbon monoxide (CO), car-
bon dioxide (CO2) and ozone (O3) only. However, the FAA has required recent
certification programs to address the National Research Council (NRC) [25] ca-
bin air quality recommendations including oils and the degradation products
into the cabin air. A range of additional standards and guidelines are listed as
optionally utilized by manufacturers demonstrating compliance. Examples in-
clude ASHRAE standard 161, the European AECMA-STAN standard for ac-
ceptable air quality and company specific design specifications.
Q7. What are the general sources of data used to indicate that the power
units and aircraft meet the required standards? The FAA utilized source of
data to show compliance is up to the manufacturers. The data provided is inter-
preted as evidence that incapacitation will not occur above the given rate and a
range of sources at the airframe level are utilised. Examples include those listed
in Q6 above and ASHRAE research project 957-RP 1999 and NIOSH hazard
evaluation report, Alaska Airlines 1993. An interactive process between EASA
and the manufacturers is undertaken.
4. Discussion
The aim of this research was to assess whether there is any gap between the air-
craft certification requirements for the provision of clean air in crew and pas-
senger compartments using the bleed air system and the theoretical and practical
implementation of the requirements. The research results obtained and existing
literature has clearly identified differing understanding of bleed air supply con-
tamination, between seals and aero engine experts compared to the wider avia-
tion industry.
The qualitative nature of this research, enabled the respondents to provide a
wide range of detailed responses, which were then narrowly categorised, so as to
avoid loss of detail in this highly specialist area [11]. Overall the response rates
were high and provide an overall picture. While broader categories could have
been used to capture higher response rates in each category, this approach would
have lost detail and not helped provide a comprehensive understanding.
4.1. Standards and Guidance Material
There are various certification requirements and associated AMC published for

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the provision of clean air in the crew and passenger compartments, which ought
to be acceptable in demonstrating compliance. The requirements and guidelines
have been outlined previously, some of which can be seen in Table 1. However,
there are a number of deficiencies in the descriptive terminology and the pres-
entation of the requirements between standards and guidance material. This
could enable the compliance requirements and AMC to be interpreted in a
number of ways or with lesser priority. The engine safety analysis lists the toxic
products in the bleed air sufficient to cause incapacitation in the standard. Oil
leakage into the airflow and degradation of crew performance are included in
the non-mandatory guidance material. This may allow a lesser priority to be
placed on leakage causing impairment.
Prior to 2007 the FAR engine safety analysis §33.75 did not reference toxic
products in the bleed air. The past and presently used phraseology, “concentra-
tion of toxic products sufficient to cause incapacitation or degrade crew perfor-
mance or unacceptable concentrations of toxic products”, do not provide specif-
ic guidance to acceptable levels. Warning systems required for “unsafe system
operating conditions” may allow room for interpretation on whether detection
systems are required for oil leakage. There may also be room for interpretation
regarding the ventilation standard CS/FAR 25.831. The terms “enough clean air”
or “sufficient amount of uncontaminated air” may allow the focus to be on the
airflow rates listed rather than fresh air preventing undue discomfort. The re-
quirement for air to be free of harmful and hazardous gases and vapours could
be interpreted to refer to all substances or CO, CO2 and O3 only.
EASA CS bleed air purity tests require analysis and possible tests of defects af-
fecting the purity of the air, however, no further guidance is provided. The safety
analysis for both the FAR and CS must include toxic products in the compressor
bleed air, yet no guidance is provided. It is left to the manufacturer to demon-
strate compliance.
There are however, some broader requirements. Systems must be designed to
perform their intended functions under foreseeable operating conditions. Unsafe
conditions refer to events occurring more frequently than intended causing im-
paired crew efficiency, discomfort or injuries.
4.2. Theoretical Understanding
There is a clear discrepancy in the understanding of oil contamination of the
bleed air supplied to the cabin. The general understanding within and outside
the aviation industry varies markedly to seal and aero engine experts, specifically
those involved in the bearing chamber/engine design and maintenance areas
[11].
The general understanding primarily supports rare oil leakage due to failed
bearing seals. Further damaged or worn seals, seals not working properly under
abnormal conditions or overfilled sumps are commonly referenced. There is a
less well publicised recognition that oil leakage may occur as a design factor. Oil

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seals are required to seal across the entire engine operating range, but are less ef-
ficient during transient engine manoeuvres. Oil substances are repeatedly being
identified at background levels in monitoring studies. Some even report conti-
nuous tiny amounts of oil crossing the seal. Studies undertaken generally report
exposures to oil fumes as safe, with low level exposures regarded as normal and
safe, associated with discomfort only.
The literature supporting the engine and sealing experts understanding of oil
leakage is not readily accessible or referenced when the topic of oil leakage is
raised [11]. However seal leakage at lower levels is widely recognised. Pressu-
rised compressor air is used to seal the bearing compartment, but is responsive
to variations in engine operating conditions. The commonly used bearing com-
partment seals, both allow lower level oil leakage across the seal. Labyrinth seals
rely on a clearance and do not in isolation prevent leakage. Mechanical carbon
face seals require oil to lubricate the faces with minimisation of leakage across
the faces. Various operational factors allow increased oil leakage over both seals,
including wear, changes in clearances, seals not at operational temperature or
pressures and during transients. Positive pressure gradients over the seal do not
fully prevent leakage. Reverse pressures, which do occur, will allow leakage in
the opposite direction. While selected aviation standards related to clean air do
exist, several factors stand out as outlined below as why some experts may regard
lower level oil leakage as acceptable.
• Leakage over seals is a normal part of permissible oil consumption limits.
• Belief that permissible leakage is driven by consumer perceptions rather than
regulatory emission limits.
• Sealing the bearing chamber at near ambient pressures is difficult.
• Oil leakage is viewed differently - high level mist or low level emission.
• High awareness of seal technological limitations and concerns about oil lea-
kage out of the bearing chamber, yet no real moves towards advanced sealing,
particularly for current aircraft.
The literature [11] identifies that the different groups are not suitably com-
municating with each other to fully understand the risks.
4.3. Feasibility of Implementation of Standards
Despite, the small sample size, the engineering and seal experts were highly ex-
perienced. Eleven out of the twelve experts recognised low level oil leakage or
emissions over the oil seals are a part of the system function of utilising pressu-
rised oil bearing seals. A wide variety of factors, including those set out below,
were identified that allow oil to enter the compressor air and the bleed air sys-
tem:
• Changes in pressures and balances during different engine operating and
ambient conditions/transient performance changes reducing seal efficiency;
• Thermal, axial and radial changes in engine structures cause changes in gaps
needing to be sealed over whole engine operating range;

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• Low internal pressures at various phases of engine operation;
• Standards and designs modeled on steady state conditions, not transients;
• Seals are not an absolute design, enabling leakage;
• Seal wear/component degradation.
Based upon the responses provided by the engineering and seal experts and
regulators, there appears to be a discrepancy between the design standards and
their implementation with the use of the bleed air system. Table 1 shows some
of the key EASA and FAA requirements and non-mandatory compliance ma-
terial, for the airframe and engine and APU where applicable.
The standards require “major” engine/APU effects to not be greater than
“remote” 10−5 - 10−7/efh. “Major” effects compliance guidance includes oil lea-
kage into the compressor airflow sufficient to degrade crew performance. The
regulators however place the emphasis on the regulation/standard involving
“hazardous” effects including toxic products sufficient to cause incapacitation,
with no mention of “major” effects (FAA) and effectively no reference by EASA.
Reliance on the regulation/standard was clear with the compliance guidance ef-
fectively ignored.
The FAA airframe standards do not allow failure conditions, reducing the
crew’s ability to cope with adverse operating conditions to be more than “im-
probable”. EASA airframe standards require “major” failure conditions to be no
more than “remote”. Major failure conditions under the EASA AMC include
impaired crew efficiency, flight crew physical discomfort or physical distress of
other occupants occurring no more than remotely 1 × 10−5/flight hour (fh). Re-
mote EASA failure conditions may occur several times during the total life of a
number of aeroplanes of type, but are unlikely to occur to each aeroplane. The
FAA terminology varies (Table 1), but the intent is similar.
The regulator responses regarding compliance at the airframe level took part
of the requirements into account only. CS and FAR 25.831 requiring a “suffi-
cient amount of uncontaminated” or “fresh air” were highlighted, while general
airworthiness requirements including “major” effects and impairment (25.1309
and AMC) were ignored. This identifies that in terms of CAQ and oil contami-
nation, the airframe certification requirements are not being adequately applied.
As shown in this research, exposure to lubricants is associated with adverse
effects and is expected to occur more than remotely or improbably, based on the
design, hazard recognition and frequency reported.
Based on engineering judgment provided in this thesis, “major” engine effects
involving oil leakage are occurring more than 1 × 10−5/engine/APU flight hour.
As the oils are accepted in a variety of ways as being associated with adverse ef-
fects [15] [35]-[41], impaired crew efficiency or degraded crew performance can
occur with exposures.
The frequency meets the definition of “probable”. As shown in Table 1,
probable failure conditions may not be greater than minor and may not have
adverse effects on occupants (FAA) or flight crew (EASA). Table 2 outlines that

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EASA airframe probable failures should not be more than 1 × 10−3/fh, with those
more frequent again stated to have no effect on flight crew or inconvenience on-
ly to others and no safety effect. Exposure to oils via the bleed air system does
not meet this. Major effects are expected which must be improbable or remote.
Those responsible for certification and continuing airworthiness are primarily
relying on engineering judgment and analysis to determine the probability of
failure conditions and engine effects, without adequately reviewing other factors.
In-operation occurrences, under-reporting, hazardous substances, design enabl-
ing low-level exposures, and adverse effects on occupants are examples of other
factors to be considered.
The ventilation standard CS/FAR 25.831 was interpreted to include only a
sufficient amount of uncontaminated air, enough fresh air to prevent discomfort
and fatigue, specified ventilation rates, CO, CO2 and O3. More recent certifica-
tion programs have included reference to a number of industry air quality stu-
dies and guidelines to determine what is deemed acceptable. However, not all are
relevant and some such as AECMA-STAN, no longer exist.
The regulatory emphasis is focusing on the “hazardous” engine effects of toxic
products sufficient to incapacitate, with little or no recognition of “major” effects
causing impairment. Impairment and discomfort related to the airframe are ei-
ther being ignored or limited to selected flow rates, limited substances and in-
dustry studies.
Alternatively, as lower level leakage occurs as an expected function of various
phases of engine operation, it could be suggested that the oil system is working
within its intended function. Oil leakage over the seals may be a normal as dis-
tinct from a failure condition.
Despite accepted oil leakage in normal operations, there were various con-
trasting views on acceptability of the leaking oil including: no action required if
the leakage is below the permissible leakage levels and within engine pressure
limits; transients not measured; no oil published limits or standards exist; con-
taminants must be within established limits; and normal low level leakage fails to
meet the standards. Other key issues include that low level emissions are ig-
nored, under-reporting is occurring, and low priority is given to preventative
maintenance and regulatory enforcement.
The non-specified or limited substances referenced under the engine/APU
safety analysis, and ventilation requirements help explain the difficulty in deter-
mining the acceptability of oil contamination of the air supply. The FAA inter-
pretation of the engine analysis requirements was limited to toxic substances
sufficient to incapacitate. EASA referenced an industry standard SAE ARP
4418A that list limits for a few substances and relates to steady-state engine op-
eration only. The previously used compliance specification MIL-E-5007 did not
allow any oil leakage into the bleed air.
The lack of detection systems and warning indicators to identify oil fumes in-
flight fails to meet the regulatory requirement 25.1309c, and causes compliance
problems. This also poses difficulties in post flight maintenance rectification.

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Leakage of oil into the bleed air meets the definition of an “unsafe condition”,
and an unsafe air supply system operating condition.
Type certificate applicants submit a report to the regulator showing how
compliance has been met. However, there is no requirement to follow a specified
procedure. In a similar manner, bleed air analysis for certification is shown to be
non-specific. The FAA does not refer to specific tests that are to be part of the
safety analysis, yet engines are required to provide bleed air without adverse ef-
fects on the engine. EASA refers to bleed air purity tests, but does not outline
what the tests are or under what conditions they need to be undertaken.
In addition to the non-specific requirements related to bleed air contaminated
by oil and oil sealing of the bearing chamber, this is a highly specialist area. Dif-
ferent experts have their part of the picture only and interpret acceptability in
light of their experience. This becomes problematic in such a safety critical area.
A number of ways to improve the situation were presented including: im-
proved preventative maintenance; better seal, oil sealing system and bleed air
designs; increased seal replacement frequency; elimination of bleed air and use
of an electric air supply; inflight real time monitoring; bleed air filtration; define
emission through seals; avoidance of oil fume exposure in the cabin; better regu-
latory and air quality standards; and improved compliance and reporting.
Despite lower level oil leakage recognition within the seals and aero engines
design community, the aviation industry has failed to address the situation. A
number of factors were identified in the research allowing the problem to re-
main unaddressed. These include data not collected and reviewed adequately.
No manufacturer will make significant changes without regulatory requirement
given assumed high cost, apparent disincentive to change and regulations, stan-
dards and intent of AMC are inappropriately being deemed to be acceptable and
met. Furthermore, there is inadequate understanding of low level exposure to
hazards. It is likely that the industry expects the regulator to take the leading role
to enforce change.
5. Conclusions
In current transport aircraft, exposures to lower level oil fumes containing ha-
zardous and harmful substances, was found to be occurring in normal flight via
the aircraft bleed air supply. Resulting adverse effects are creating a risk to flight
safety.
The research undertaken has found that there is a gap between the aircraft
certification requirements for the provision of clean air in crew and passenger
compartments using the bleed air system and the theoretical and practical im-
plementation of the requirements. Oil bearing seals are not an absolute design
and will allow low level oil leakage over the seals into the compressor and bleed
air supply as a normal function of the engine cycle. Lower level oil leakage is not
exclusive to failure or mechanical abnormalities. Key conclusions are outlined
below.

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1) Regulations & standards
Based on a review of the applicable regulations, standards and guidance ma-
terial and interviews with highly experienced aero and seal experts and regula-
tors, the required bleed air quality is not being met. The standards and com-
pliance material are not specific enough to ensure suitable bleed air quality. The
focus is placed on the standard and prevention of incapacitation, with com-
pliance guidance material and impairment almost ignored. The clean air re-
quirements are open to interpretation and are not taking into account the
in-operation environment, including hazardous substances and adverse effects,
low level normal leakage, the frequency, under-reporting and lack of detection
systems.
2) Design
Low level oil leakage over the bearing seals into the bleed air, at various phases
of engine operation is an expected normal condition, according to the seals and
aero engine experts. While many suggest that enough is being undertaken to
meet the certification requirements, careful review of the literature and research
undertaken with engineering and seal experts, shows the regulations are not be-
ing met. As demonstrated in the literature and supported by the engineer and
seal experts interviews, the airframe failure conditions and engine/APU safety
analysis requirements are not being met. Oil leakage past the seals, associated
with impaired or degraded performance, occurs more frequently than the “ma-
jor” EASA, and FAA regulatory and compliance criteria allow in Table 1. Oil
leakage, capable to cause degraded performance and efficiency is occurring on a
greater than “remote” or “improbable” basis. Oil leakage in normal operations is
probable or above (Figure 1 and Figure 2, Table 1 and Table 2) and meets the
definition of an unsafe condition.
3) Compliance
Although inadequate, compliance is undertaken at certification. However, no
detection systems are available in-flight to monitor the quality of the air, includ-
ing low level leakage in normal operations. The ventilation requirements are not
specific enough to ensure occupants will remain free of adverse effects.
4) Preventative Control Measures
Low level and transient oil emissions are not adequately taken into account
when considering acceptable leakage levels. Designs are based on steady state
conditions, although oil leakage will be minimally occurring during certain en-
gine power conditions and transients. There are no contaminated bleed air de-
tection or filtration systems to identify and protect occupants from oil fumes.
Rigorous controls are lacking including improved designs, better maintenance
and procedures, and suitable air quality emission definitions.
5) Retrospectively
Previous engine certification requirements either did not include toxic effects
or were not specific enough to prevent oil leakage into the air supply.
6) Expertise and Communication
Oil contamination of the air supply is a highly specialist area, with inadequate

S. Michaelis
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10.4236/eng.2018.104011 166 Engineering
communication between all relevant parties to ensure compliance and airwor-
thiness.
6. Recommendations and Future Research
Based upon the literature and the research, it has been demonstrated in terms of
clean cabin air supply that the standards and compliance guidance are inade-
quate and not being met. This is a highly specialist area with various actions
suggested to be undertaken to meet the requirements for the supply of clean ca-
bin air. These include the establishment of a specialist task group, including the
regulators, to review the following issues outlined below.
• The adequacy of the air quality related standards and compliance guidelines,
in light of the real-world understanding of oil leakage into the bleed air
supply;
• Solutions and preventative measures that could be introduced to prevent ex-
posure to engine lubricants in normal operations;
• The reasons why the industry is inadequately addressing the prevention of
inflight exposure to lubricants;
• Oil contamination of the bleed air supply should not be linked exclusively to
rare failure conditions or maintenance irregularities;
• The frequency should be seen in terms of design factors rather than the rate
of reporting;
• Actions should be undertaken to prevent oil leakage into the aircraft air
supply in normal operations;
• Aircraft certified prior to the current standards should be retrospectively
re-certified for bleed air quality;
• Future aircraft air supply systems should use bleed free designs;
• Far greater priority should be placed on clean air regulatory compliance in-
cluding low level oil emissions in normal flight;
• Inflight oil fume detection systems and flight-deck warning should be im-
plemented on all future aircraft.
Acknowledgements
The MSc degree was funded by the Global Cabin Air Quality Executive, for
which the author is Head of Research and by the author.
Conflicts of Interest
The author declares no conflict of interest. The funding sponsors had no role in
the design of the study; in the collection, analyses, or interpretation of data; in
the writing of the manuscript, and in the decision to publish the results.
Ethics Approval and Consent to Participate
Permission to conduct this study was obtained from Cranfield University Air
Safety and Accident Investigation MSc supervisors as part of an MSc research

S. Michaelis
DOI:
10.4236/eng.2018.104011 167 Engineering
project in 2012. All participants were asked to sign consent forms.
Availability of Data and Material
The dataset supporting the conclusions of this article is available in full on the
authors website as part of the authors Cranfield University MSc research project.
Available publicly at:
http://www.susanmichaelis.com/pdf/2016_Susan%20Michaelis_MSc%20Cranfiel
d-Clean%20air%20requirements%20using%20bleed%20air%20system.pdf
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Abbreviations
AECMA-STAN: Association Européenne des Constructeurs de Matériel Aéro-
spatial Normalisation; AMC: Acceptable Means of Compliance; APU: Auxiliary
Power Unit; ARP: Aviation Recommended Practice; AS: Aerospace Standard;
ASHRAE: American Society of Heating, Refrigerating and Air-Conditioning
Engineers; CO: Carbon monoxide; CO2: Carbon dioxide; CS: Certification
Specifications; CS-E: Certification Specification-Engines; EASA: European
Aviation Safety Agency; FAA: Federal Aviation Administration; FAR: Federal
Aviation Regulations; JAR: Joint Aviation Requirements; MIL-SPEC: Military
Standard; MIL: Military Specification; MIL-PRF: Military Specification;
NIOSH: National Institute for Occupational Safety and Health; NRC: National
Research Council; O3: Ozone; SAE: Society of Automotive Engineers; SFC: Spe-
cific fuel consumption; TSO: Technical Standard Order;
Abbreviations-Units
CC/min: Cubic centimetre/minute; EFH: Engine flight hour; FH: Flight hour;
PPM: Parts per million.