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Academic Journal of Science,
CD-ROM. ISSN: 2165-6282 :: 06(01):191–202 (2016)
USE OF SIX SIGMA METHOD AS A PROCESS IMPROVEMENT
TECHNIQUE: A CASE STUDY IN AEROSPACE INDUSTRY
Atakan Gerger and Ali Rõza Firuzan
Dokuz Eylul University, Turkey
Purpose – The aim of this work is the implementation of Six Sigma (SS) method on a company
operating within aviation industry and the statistical evaluation of the methodological effects on the
performance of company. SS implementation was conducted in order to improve the manufacturing
process which caused a bottleneck within the company. SS project executed in XYZ Company serves as
basis for the implementation. The university academically provided support to XYZ Company during
the realization of the project. The results of the work will be shared with manufacturing industry
professionals and universities via this symposium and related manuscript. Target audience includes SS
implementers, continuous improvement operatives, industrial performance leaders and researchers at
universities, working on Lean Management. The bottleneck was eliminated with the SS project
conducted within the frame of close cooperation of the university and the industry and an increase
regarding the work performance of the company was achieved. Statistical techniques were used much
more actively in the company by the help of the academic support provided by university. This work is
related with production industry and the data were provided by XYZ Company, which operates on an
international scale. The company seeks to ramp up its competitiveness and capabilities in the global
market. SS method is used intensively in the manufacturing industry. The process surveyed in this SS
project is specific to the company. The SS project revealed that not only layout or method change but
also modifications on the machine design were required. The changes were applied in line with this
requirement.
Keywords: Six sigma, Manufacturing industries, Lean six sigma, Aviation industry.
Six Sigma
Companies use several statistical, executive and engineering techniques to improve the quality of their
products/services. By using these techniques; they want to control their processes and improve the quality
of their products/services which they offer to their customers. Therefore, they try to increase their
competitiveness by implementing the methods which have been tried before and obtained successful
results. One of the methods which have been used for this purpose is Six Sigma (SS), which is initiated by
Motorola in 1987 in order to eliminate variations in the process and provide a significant improvement.
SS is increasingly applied in many different countries and sectors (Kuvvetli, Firuzan, Alpaykut and
Gerger, 2016) including accounting, sales, and service. This new branch of Six Sigma was termed
Business Transactions for Six Sigma (Chowdhury, 2002).
For the academic field of quality Six Sigma has become one of the most prominent trends in the past
few years (Gerger and Firuzan, 2012). Many large and well known corporations like Motorola, General
191
192 Use of Six Sigma Method as a Process Improvement ...
Electric, American Express, Ford Motor Co. and AlliedSignal, have appealed to the Six Sigma
methodology (Sandholm and Sorqvist, 2002). The results of this methodology have been quite good and
beneficial for the companies so this lead to a growing interest and tendency to this method (Gerger and
Firuzan, 2012).
Six sigma is a statistical concept that measures a process in terms of defects. The Sigma scale
measures Defects per Million Opportunities (DPMO). Six Sigma equates to 3.4 defects per million
opportunities. The Sigma metric allows dissimilar processes to be compared in terms of the number of
defects generated by the process in one million opportunities (McCarty et al, 2004).
A metric is the measurement of performance: an improvement metric is the measurement of
performance over time. Improvement metrics require that data be maintained on the defect rate reaching
customers or data from internal inspection or evaluation points (Larson, 2003). Process performance is a
measure of how well a process performs. It is measured by comparing the actual process performance
level versus the ideal process performance level (Yang and El-Haik, 2009).
The main focus of Six Sigma is to reduce potential variability from processes and products by using
a continuous improvement methodology, which follows the phases:
x define,
x measure,
x analyze,
x improve and
x control.
This approach is known as DMAIC methodology and is employed in tackling problems associated
with existing processes/products (Banuelas, Antony and Brace, 2005).
The Six Sigma methodology is not limited to DMAIC. Other problem solving techniques and
methodologies are often used within the DMAIC framework to expand the tool set available to Six Sigma
project teams. These include (McCarty, 2004):
x Theory of Inventive Problem Solving (TRIZ)
x Lean
x Ford 8Ds (Disciplines)
x 5 Whys
x Is/Is Not Cause Analysis
Figure 1. DMAIC methodology (Larson, 2003)
Atakan Gerger and Ali R1za Firuzan 193
Six Sigma gives discipline, structure, and a foundation for solid decision making based on simple
statistics. The real power of Six Sigma is simple because it combines people power with process power
(Kabir, Body and Lutfi, 2013). Although Six Sigma project covers all of the company or any process, it
requires a team to reach target value. Every member of this team have different responsibilities. Every
responsibilities names are named according to Far East, as in martial arts belt colors. These duties and
responsibilities are
x sponsor (top management),
x champion,
x master black belt,
x black belt,
x green belt and
x yellow belt (team members).
The success of a Six Sigma “team” is often the result of just one or two individuals (often the Black
or Green Belt) who end up making major changes to the process in an attempt to achieve short-term
results. These efforts, although often well intentioned, ultimately thwart what Six Sigma is attempting to
do: Change the entire culture of the organization from that of the “firefighter” to a culture based on
everyone being an “arsonist catcher.” Without a cultural change at the facilitative skills level, Six Sigma
results are often short-lived and ultimately frustrating for everyone involved (Eckes, 2003).
Although Six Sigma studies are done project teams, it requires top management and all staffs
participation and supports to reach success.
While executive management plays a critical role in setting the stage for a Six Sigma culture, they
play an indirect role in the tactical piece of Six Sigma. The direct roles and responsibilities of the Six
Sigma project team are discussed in the following pages. The roles are (Eckes, 2003):
x The project sponsor
x The project team leader (called either the Black Belt or Green Belt).
x The project team members.
To guide managers in their task of improving short and long term results, Six Sigma uses a five-step
process known as the DMAIC model, named for the five steps in the process (Levine, 2006):
x Define: The problem is defined along with the costs, benefits, and impact on the customer.
x Measure: Operational definitions for each critical-to-quality (CTQ) characteristic are developed.
In addition, the measurement procedure is verified so that it is consistent over repeated
measurements.
x Analyze: The root causes of why defects occur are determined, and variables in the process
causing the defects are identified. Data are collected to determine benchmark values for each
process variable.
x Improve: The importance of each process variable on the CTQ characteristic are studied using
designed experiments. The objective is to determine the best level for each variable.
x Control: The objective is to maintain the benefits for the long term by avoiding potential
problems that can occur when a process is changed.
There are two perspective of Six Sigma. These are statistical view point and business view point
(Shah and Deshpande, 2015):
x Statistical view point: Sigma is statistical major which denotes variation and symbolize as ı”.
When defects produce are less than 3.4 per million opportunities then it can be said process is
operating at six sigma level under the assumption that the process average may shift over the long
term by as much as 1.5 standard deviation.
x Business view point: In context of business world, it can be consider as business strategy to
satisfy costumer in fullest way and parallel improve business profitability
194 Use of Six Sigma Method as a Process Improvement ...
A six sigma is good initiative by top management which is designed to change the culture via break
through improvements by focusing on thinking out of the box in order to achieve strategic goals. When
successfully applied, it results in to gain of knowledge and capital which finally transform into shared
bottom line result (Shah and Deshpande, 2015).
A Case Study in Aerospace Industry
XYZ Company founded 100 years ago in Germany. Area of activity is production of structural
components and tubing system of Boeing and Airbus airplanes. XYZ portfolio are:
x Fuel tanks
x Pressure (compression) walls and surface
x RAT transporters
x Inner Flap’s
x Cargo fixing canals
x Ground panels
x Tubing systems for different Airbus and Boeing planes
Aerospace industry is very dynamic. So, products must be produced on time. Products must
delivered without any missing time or condition. Variance is not accepted for this industry.
This study covers delivery problem of XYZ Company. Because, XYZ’s customer have started to
complaint due to delay time. To solve this problem XYZ Company had decided to make Six Sigma
project based DMAIC (define, measure, analysis, improve and control) methodology.
1 Define
Problem is defined as Backlog occurrences and often visits to the infirmary and sickness reports of the
employees due to musculoskeletal disorders in the alkaline department. At this step “project statement” is
created. This statement includes are project definition, goals and savings of the project, constraints of the
project, team members of the project and timeline of the project
Scope of the project is «Cleaning and rinsing of inner surface of all tubes by using alkaline
solution». Aimed gaining’s of the project:
x Increasing production efficiency about 20% by reduction the cycle time
x Increasing the energy efficiency by 20% (Thereby, environmental pollution and operational costs
will be decreased)
x Reducing of nonconformance quantities by increasing the process quality
x Decreasing risks of the work accidents and occupational illnesses by 50%.
Project constraint is determined as «Tube Production Alkaline Cleaning and Rinsing» production
step.
Project leader was selected from Lean Management, Statistics and Reporting department. Because he
has black belt certification. The project team consists the employees of tube production, technical
services, Lean Management statistics and reporting, workplace doctor, work safety and health expert, and
finance.
At this phase, six sigma team decided to define core of problem reason. These studies will be
handled in measure phase.
2 Measure
Six Sigma team firstly investigated core of problem. When six sigma team investigate core of the
problem, they saw bottle neck in alkaline cleaning process. Tube production flow chart and bottle neck
shown Figure 1.
Atakan Gerger and Ali R1za Firuzan 195
Figure 1. Tube production flow chart and bottle neck
To understand of reason of problem and improve of cycle time in alkaline cleaning process, six
sigma team was investigated process as details. Alkaline cleaning process consists three steps. These steps
are cleaning, rinsing and drying.
At the measure phase, the factors which are effecting the Alkaline Process is determined by using
cause and effect diagram. Cause and effect diagram study shown Figure 2.
ImprovingAlkaline
CleaningEfficiency
Connectingatubetotheadapter
istoodifficult
Brreakingtheseal
offastener
TheAlkelinefluid
ischangingfrequantlyNonͲworkingdaydueto
medicalexaminationarehigh
Medicalexamination
quantitiesarehigh.
Insufficientnumberofstaff
Machinecapacityis
insufficient
Missingbasketsafety
Machineconnection
quantiesinsufficient
Productionareatohot
Chemicalvaporin
Productionarea
Equippingistooheavy
Overtimeworking
Electric,waterand
Chemicalconsumption
Figure 2. Cause and effect diagram
196 Use of Six Sigma Method as a Process Improvement ...
Main factors are shown below:
x Connecting a tube to the adapter takes 26 seconds.
x The adapters that are used to connect the tubes to the Alkaline cleaning machine are very heavy.
The weight of each adapter is shown in the Table 1.
Table 1. The weight of each adapter
Adapter’s Diameter Adapter’s Weight(Gr)
1/4” 1500
3/8” 1495
1/2” 2714
5/8” 2702
3/4” 2690
1” 1156
x Tubes are being carried to the Alkaline Cleaning Machine manually by the operator. In order to
reach the machine, the operator walks nearly 10 meters and use the stairs (in total, 6 stairs) in two
different section.
x In order to connect the tubes to the alkaline machine, operator steps up the stair (33 cm) in front
of the machine and bends over to the transportation basket. Since the operator has to bend over to
the machine, the operators who work in this process have musculoskeletal disorders. Between the
years of 2014-2015 there were 14 requests for medical examinations and it resulted with 33 non-
working days.
x In the Alkaline pool there are 6 adapter exits supported by one pump. This situation makes
capacity insufficient for production and it causes bottleneck on this process.
x The grease oil used in the previous process (bending process) causes the alkaline pool get dirty in
a short time. Cleaning pool is changed (on average) 2 times in a month.
x Due to insufficient capacity of the drying shelves of the tubes, the tubes are placed to the area
disorderly and tightly. Because of these problems surface damage occurs on 3 tubes in a month.
Additionally; because of the lack of capacity at the drying area, finding a tube that belongs to the
work order takes (averagely) 91 seconds (1,5 mins)
x The gaskets which are used for connecting the tubes to adapters and providing impermeability,
become deformed due to being used in the process and its pieces get ruptured and these pieces
remains inside the tubes. This situation causes Alkaline cleaning pool to get dirty in a shorter
notice and also it may cause customer complaints due to gasket pieces that remain inside the
tubes. Therefore; there must be an inspection of the gaskets during the process of connecting the
gaskets to the adapters. And it takes (averagely) five seconds to control a gasket.
x Due to lack of cover in the Alkaline Cleaning pool and wrong placing of the air conditioning
device, the air stream goes directly to the operator. The responsible operator is effected by both
the hot steam coming from the pool and cold air coming from the air conditioning and this
situation causes various medical and physical conditions.
3 Analyze
At this step the data gathered from the measure step is analyzed.
– Cycle time for one tube is 0.6 minute and standard deviation is 0.12V.
Atakan Gerger and Ali R1za Firuzan 197
– The weight of the adapters are determined for each diameter and the average weight of the
adapters is calculated as 2043 Gr. The average weight of the tubes is 360 gr.
– Per month one personnel averagely carries
x 10,8 tons of tubes
x 26,5 tons of adapters
In total a personnel carries 37.3 Tons of weight.
Figure 3. Effect of weight on human bone
The non-value adding movements of the personnel for the process are shown below:
x Manually handling the tubes (which will be cleaned) for 10 meters.
x Unnecessary going ups and downs in the stairs while handling the tubes.
The non-value adding movements for the process causes:
x production losses,
x slipping and
x falling
risks which may cause accidents.
During the process which the tubes are being connected to the adapters and put into the alkaline
cleaning basket, personnel climbs up to the stair in front of the pool and bends down to connect the tubes
to the pool. Six tools and (averagely) 14 tubes are being connected to the machine at one time. The
weights that personnel is being exposed to during this step of the process are shown below.
x Adapter’s weight : 12,3 kg
x Tube’s weight : 5 kg
x Total weight : 17.3 kg
While connecting the tool to the Alkaline cleaning pool, personnel is being exposed to nearly 195
kgs of weight. During connecting the tubes to the alkaline cleaning pool process, the operator does
bending down movements for 84 times.
XYZ firms risk score is calculated as 32 (within the range of 25-50) by using “Key Indicator Method
Lifting/Holding/Carriying - KIM LHC” (Klussmann et al., 2010) as a risk assessment and evaluation
method. Their purpose is the recognition and removal of job design deficits. The risk assessment is
carried out in two stages. The first stage is the ordinal scaled description of workload items. The second
stage is the evaluation of the degree of probability of physical overload (Steinberg, 2012).
198 Use of Six Sigma Method as a Process Improvement ...
x Highly increased weight situation.
x There is a probability of physical problems occurrence for normal working groups.
x Re-design of the working place is highly recommended
Fourteen tubes can be processed at once (averagely). Total number 6 personnel works in this cell as
2 personnel/shift. In order to prevent the backlogging, the personnel in the Alkaline cell works monthly
(on average) 42 hours overtime.
Due to incapacity of the drying area (shelves) for the tubes that are processed in the Alkaline
washing cell, in a month on average 3 tubes are being reproduced due to the damages as a result of
incapacity problem.
As a result of dirty Alkaline pool and rupture of the gaskets in a month on average 15 tubes are not
processed properly and this situation causes re-processing cost.
The pumps that are used before the improvement works with start-stop mechanism. That situation
causes tubes to disconnect from the currently used adapters due to sudden increasing of pressure.
It takes at least 3 minutes of process in order to notice that the tube is disconnected and these tubes
are re-cleaned in the alkaline process. According to production instructions when a tube is cleaned in the
alkaline process more than two times, the tube becomes scrap and cannot be used in the process again.
Due to the problems related to the impermeability parts in the adapters, surface inside of the tube is
not cleaned properly and undesirable grease oil leftovers from the previous process is determined during
endoscopy control.
According to the production instructions in the Alkaline rinsing pool, the water must be A quality
(pure water, particle size100 µm, water hardness0.5ƕdH, pH=5.0-7.0, amount of leftovers: 15mg/l,
electrical conductivity: 20 µs*cm-1) and the conductivity of the water must be maximum 400 µs. Due to
grease oil and other chemicals that are mixed to the water during the process, the conductivity increases
as time passes and pool is changed averagely 5 times in an month.
During the changing of the pool, production stop occurs and it takes 8-12 hours for the rinsing pool
and 12-16 hours for changing the cleaning pool including the time it takes according to the level of
dirtiness of the pool and for heating. The total time spent during this changing step may take up to
28 hours.
Alkaline cleaning pool fluid is changed (on average) once in 15 days, and the rinsing pool is
changed once at 5 days (on average) in a month. 6 tons of water is used for changing process of each
pool. During the changing of the pool, production stop occurs and it takes 28 hours to start production
again. Additionally the amount of 69.6 kW/h is consumed in order to heat the water and reach desired
water temperature after the changing process.
4 Improvement Phase
Instead of using current screwed connections, in the new design cam system is used and as a result the
problem of expansion of the threads due to heat is removed (Working temperature: 70±10°C). By the new
design the time it takes to connect a tube to the adapter is reduced from 26 secs to 3 secs.
By decreasing the adapter (on average) weight from 2,053 to 0,606 kg the adapter weight (on
average) is reduced by %70.4. In the meantime; cost of adapter is reduced by %90 with the new design.
Table 2. Comprising of ex and new apparatus weight
Adapter
Diameter
Old Adapter
Weight (Gr)
New Adapter
Weight (Gr)
Improvement
Rate
1/4” 1.500 535 64,3%
3/8” 1.495 512 65,8%
1/2” 2.714 762 71,9%
Atakan Gerger and Ali R1za Firuzan 199
5/8” 2.702 756 72,0%
3/4” 2.690 744 72,3%
1” 1.156 325 71,9%
Total 12.257 3.634 70,4%
Since there is no need of using gasket, by using new designed tool average number of errors due to
rupture of gasket (monthly) is reduced from 0 to 15.
By changing the layout in the Alkaline Cleaning Cell, the stairs are removed. Therefore; the
possibility of work accidents and unnecessary movements is decreased.
By re-designing the working area as a flat and single area, the work step that operator manually
handles (carries) the weight is removed. And by this improvement:
x The weight personnel had to carry is reduced by %80
x Tube external surface damage is reduced by %67
x By reducing the number of required personnel from 6 to 3 yearly cost of the personnel is reduced
by %50.
The curtains that are used in the current system are deactivated. Instead of using curtains, 3
automatic, fast and heat resistant gates are implemented. By new gate mechanism the working conditions
are improved. The problems about bad smell and steam around the pool and splashing of chemical fluid
outside of the cleaning pool during the process where pool basket goes down to the cleaning pool are
prevented. Also energy saving is provided by increasing the work efficiency.
When operator presses the button to make the basket go down, at first the gate is closed immediately.
Therefore the work accidents that may happen as personnel getting hit when the basket is going down are
prevented. When operator presses the button to make the basket go up at first the basket goes up and then
the gate is opened.
The basket safety belts which were not part of the previous system are added to the current system.
In case the belts of engine that carries the basket breaks off these belts will be activated and it will prevent
the possible work accidents by preventing falling down of the basket.
As a result of the improvement; lifting ramps are designed for each diameter (¼”-3/8”-1/2-5/8-
3/4”-1”), therefore by doing a soft lifting (by giving the pressured alkaline fluid into the tube step by step)
the disconnection tubes from the adapters is prevented and by doing so re-cleaning of the tubes is also
prevented.
When determining the soft lifting ramp, cross sectional area of the tube is considered and the
maximum time for reaching the cycle «t» is determined according to this cross sectional area. Therefore
the grease oil remaining’s and air bubbles inside the tubes are removed completely from the tube during
the lifting and maximum cycle is reached by a complete flow. This situation, increase the quality of
cleaning the tubes and re-cleanings are prevented by videoskop control results.
By adding 6 new adapter exits to the pool, total number of adapters are increased to 12 and the
capacity of the pool for cleaning the pool is doubled. To satisfy the needs of the new adapters a new pump
is added to the system. System can work either with one pump or both two pumps in case it is needed.
With a system that designed by XYZ, by putting a filtration system between the return line and the
cleaning pool, changing of pool water is decreased from 2 times in a month to 2 times in 3 months. And
by this new system a 67% of improvement is obtained.
The water need of cleaning pool is provided from rinsing pool and therefore the water that does not
have any conductivity problems is added to the Alkaline cleaning pool. In this situation; a quality water
with 0-20 µs of conductivity is prepared and provided for the rinsing pool.
Since by these improvements it is possible to estimate the time for changing the pool, the pool
changes are started to be made as a planned schedule.
200 Use of Six Sigma Method as a Process Improvement ...
Due to the connection between Alkaline cleaning-rinsing pool and pumping system simultaneously
pool changes (which was not possible before), is provided and production stops are minimized.
The time it takes to change the Alkaline cleaning and rinsing water is reduced from 28 hours to 11
hours. Therefore; the time it takes to change the pool is reduced by %61.
In order to prevent surface damages and piling ups of the tubes in the drying area, the shelf is
renovated as using both sides of the shelf instead of using just one side. Therefore; the surface damages
that happens in the drying area in the Alkaline process is reduced from (on average) 3 tubes to 1 tube.
By the improvements of the project the number of personnel that works in this process is reduced
from 6 to 3, and on average overtime is 42 reduced from 42 hours to 0 hours.
The time it takes to find a tube in the Alkaline drying area is 91 seconds to 13 seconds.
In the Alkaline process; for the rinsing pool, new automatic doors are designed instead of the
curtains. The ventilation exits which are placed at the side of the pool, are transported over the pool in
order to assemble the doors to the pool. Therefore the direct explosion of the hot steam on the responsible
personnel in the area is prevented.
5 Control Phase
At this step:
x The consistency of the improvements are followed.
x Permanent changes are made.
x Gaining’s of the project are calculated.
At this part, the savings of the improvements of the Alkaline Cleaning and Rinsing Process are listed
and compared before the project situation. All improvement shown at Table 3.
Table 3. Comparison of project results
Before Project:
Cycle Time: 0.6 minute
Connect a tube to adapter: 26 minute
Average of adapter weight: 2,053 kg
Pool adapter capacity: 6
Number of stay in tube gasket: 15 piece/year
KIM LHC risk score: Between25-50
Staff quantity: 6
Freight transport distance: 100 unit
Pipe call time: 91 second
Overtime working: 42 hour/month
Water consumption: 504 ton/year
Electricity consumption: 35280 kWh/year
Chemical consumption: 3600 kg/year
The number of monthly inspection request: 4.5
request/month
Number of lost working day: 2.1 day/month
Cost:
17215 $
After Project:
Cycle Time: 0.4 minute (%50 improvement)
Connect a tube to adapter: 3 minute (%11,5 improvement)
Average of adapter weight: 0,606 kg (%71 improvement)
Pool adapter capacity: 12 (%100 improvement)
Number of stay in tube gasket: 0 piece/year (%100
improvement)
KIM LHC risk score: Between 10-25
Staff quantity: 3 (%100 improvement)
Freight transport distance: 20 unit (%80 improvement)
Pipe call time: 13 second (%85,7 improvement)
Overtime working: 0 (%100 improvement)
Water consumption: 360 ton/year (%71,4 improvement)
Electricity consumption: 10080 kWh/year
(%71.4 improvement)
Chemical consumption: 1200 kg/year (%66,7
improvement)
The number of monthly inspection request: 1.25
request/month (%72,2 improvement)
Number of lost working day: 0 day/month
(%100 improvement)
Profit:
For first year: 596937 $
For each additional year: 167895$ (staffs salary, operating
expenses etc.)
Atakan Gerger and Ali R1za Firuzan 201
Continuity of the gains obtained after improvement phase is an important part of the whole process.
In this phase it is essential to achieve permanence by following up the results. Subsequent to the
improvement phase, the project was followed up for seven months. Eventually, it is assessed that the
project reached its main goals.
Result
Today Six Sigma projects are being actively applied in various business sectors and branches. Putting
aside the support of the upper management, the success of these projects depends on the effectiveness of
the Six Sigma team. The reason that Six Sigma is actively employed in the industrial establishments is to
provide customers with the products/services of the quality level they requested, by minimizing the
variability of the processes or products. Therefore, not just the assigned team or upper management
involvement but company-wide participation is required in Six Sigma projects.
In this article it is shown that aerospace industries company XYZ has been employing the Six Sigma
successfully. With this project, significant progress has been achieved with the alkaline cleaning process
which caused a bottleneck in company’s production rates. The company arranged not only the punctual
delivery of the goods to the customers, but also was able to make various profits with the process.
Thanks to the statistical support provided by the university it was achieved that the Six Sigma tools
were employed much more efficiently in this project. These circumstances have shown the importance of
the cooperation between the university and the industry.
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