Engine failure caused by erosion–corrosion of fuel manifold

Abstract

An engine failure caused by erosion–corrosion of a fuel manifold was investigated. The surface of the boost elements (impeller and its cover) of the main fuel pump showed the wear damage by Fe-based particles. This incident reduced the fuel pressure abruptly, and led to engine shut-down. This was confirmed by EDX analysis of the rubbed surface of the boost elements and the metal chips collected form fuel filter. To find the possible source of Fe-based particles the whole fuel system of the aircraft was reviewed. On the basis of this review the fuel manifold attached to the main fuel pump was selected for in-depth analysis. The manifold was cut and its inner surface was examined visually with the aid of a stereoscope revealing internal penetration of the welding metal. This internal welding bead was attacked by erosion–corrosion, partially lifted-off and generated particles. The particles separated from the internal bead and were carried in the fuel. On entering the main fuel pump these particles caused the wear damage. The inner surface of randomly chosen manifolds showed internal welding metal too. Therefore optimization of the welding parameters during the joining process of the manifold was proposed as a remedial action.

Full text

Engine failure caused by erosion–corrosion of fuel manifold
Myounggu Park*
Engine Division, ATRI(Aero-Tech Research Institute), ROKAF, PO Box 304-160, Kumsa dong,
Dong gu, Deagu, 701-799, Republic of Korea
Received 17 December 2001; accepted 30 January 2002
Abstract
An engine failure caused by erosion–corrosion of a fuel manifold was investigated. The surface of the boost elements
(impeller and its cover) of the main fuel pump showed the wear damage by Fe-based particles. This incident reduced
the fuel pressure abruptly, and led to engine shut-down. This was confirmed by EDX analysis of the rubbed surface of
the boost elements and the metal chips collected form fuel filter. To find the possible source of Fe-based particles the
whole fuel system of the aircraft was reviewed. On the basis of this review the fuel manifold attached to the main fuel
pump was selected for in-depth analysis. The manifold was cut and its inner surface was examined visually with the
aid of a stereoscope revealing internal penetration of the welding metal. This internal welding bead was attacked by
erosion–corrosion, partially lifted-off and generated particles. The particles separated from the internal bead and were
carried in the fuel. On entering the main fuel pump these particles caused the wear damage. The inner surface of
randomly chosen manifolds showed internal welding metal too. Therefore optimization of the welding parameters
during the joining process of the manifold was proposed as a remedial action. #2002 Elsevier Science Ltd. All
rights reserved.
Keywords: Aircraft failures; Engine failures; Wear; Erosion; Pump failures
1. Background
During a mission flight, the flame of one of the two engines of an F-5F combat aircraft went out. The
aircraft landed according to its emergency landing procedure. At the initial investigation the failed engine
was disassembled and inspected carefully. From this field investigation, it was discovered that the boost
element of the main fuel pump was rubbed and the filters installed on the fuel system were partially clogged by
metal chips. Because of this the fuel pressure dropped suddenly and could not supply the fuel for continuous
burning in the combustion chamber. No other parts of the engine were seemed to be in bad condition. So this
failure analysis was focused to find the root cause of the boost stage wear damage which made the fuel system
malfunction and the exact failure mechanism to prevent the same type of failure from occurring again.
1350-6307/02/$ - see front matter #2002 Elsevier Science Ltd. All rights reserved.
PII: S1350-6307(02)00005-5
Engineering Failure Analysis 9 (2002) 673–681
www.elsevier.com/locate/engfailanal
* Tel.: +82-53-980-3931/3932; Fax: +82-53-819-1271.
E-mail address: 9planets@hanmail.net

2. Visual examination
Fig. 1(a) shows the MFP (main fuel pump). The MFP is mounted on the forward right hand drive pad of
the accessory gearbox rotating at 8029 RPM at 100% engine speed. MFP is a dual element, self-lubricating
pump consisting of a low pressure or boost stage and a high pressure stage. Both pumping stages are driven
by the same drive shaft. The boost stage is an impeller type centrifugal element [Fig. 1(b)] designed to
increase the inlet pressure 65 psi. The high pressure stage is a positive displacement gear pump increasing
boost pump discharge pressure to 950–1250 psi. Fig. 2 shows the wear pattern of impeller (see arrows). In
comparison with a new one, the rubbed region of the impeller was slightly deformed. Fig. 3 shows the
rubbing pattern of impeller cover indicating typical abrasive wear. From this visual analysis it is thought
that the impeller and its cover at boost stage was scored by particulate debris producing the metal chips.
Fig. 4 indicates the cross sectional view of the boost stage. As seen in Fig. 4 it can be guessed that originally
there was almost no gap between impeller and cover. However because of the enlargement of the gap
between the two elements by wear the fuel pressure might plummet. From the visual analysis it was verified
that particles harder than the impeller and its cover material flow into the boost stage causing the wear
damage.
Fig. 1. (a) Main fuel pump and (b) boost stage impeller.
Fig. 2. The wear pattern of the impeller.
674 M. Park / Engineering Failure Analysis 9 (2002) 673–681

3. EDX analysis
3.1. The surface of the boost elements
To identify the particles flowing into the MFP, EDX (energy-dispersive X-ray) analysis of the rubbed
surface of the boost element was done. Firstly the normal surface was EDX analyzed and secondly the
rubbed surface. From the EDX spectrum of the normal surface it is shown that the base material of the
impeller and its cover is Aluminum (Fig. 5). Also by its surface condition it is thought that the boost element
was manufactured by a casting process. However the EDX results of the rubbed region clearly showed the
presence of Fe (Fig. 6). From theses analysis result it was revealed that the particle base material was iron.
3.2. The surface of the metal chips
Fig. 7 shows the chips from the filters installed on the fuel system. These chips were collected and EDX-
analyzed one by one. The analysis results revealed that the chip base material was Al, indicating that the
chips were form the impeller and its cover. Fig. 8 shows the EDX spectrum of a chip surface showing
Fig. 3. (a) The plan view of rubbed impeller cover (see arrow)—the scale is in centimeters; (b) close up view of the scoring mark.
Fig. 4. The cross sectional view of the boost syage of the MFP.
M. Park / Engineering Failure Analysis 9 (2002) 673–681 675

Fig. 5. EDX spectrum of the normal surface of the boost elements.
Fig. 6. EDX spectrum of the rubbed surface of the boost elements.
676 M. Park / Engineering Failure Analysis 9 (2002) 673–681

major elements of stainless steel—Fe, Cr, Ni. Therefore from the EDX analysis results, it is obvious that
Fe-based particles (belonging to the category of stainless steel) induced the wear damage on the surface of
the boost element.
4. Fuel flow analysis
4.1. Fuel flow path
For the investigation of the potential source of stainless steel particles, the fuel flow was examined from
fuel cell to MFP. The fuel flow of the F-5F aircraft is illustrated in Fig. 9. Main filtering was done by a fuel
Fig. 7. (a) The metal chips from the fuel filters; (b) the enlarged view of some metal chips.
Fig. 8. EDX spectrum of the surface of a metal chip—seen in Fig. 7(b).
M. Park / Engineering Failure Analysis 9 (2002) 673–681 677

strainer and the fuel flow transmitter has its own screen for filtering of fuel. Therefore the most probable
source of stainless steel particles is the fuel manifold attached directly to the MFP. The shaded part in
Fig. 9 is the manifold attached to the MFP. Fig. 10 shows the lower front side of the engine indicating the
fuel manifold.
4.2. Internal examination of the fuel manifold
The manifold was cut for internal examination. At the bend of the fuel manifold is revealed the erosion–
corrosion attack (Fig. 11). Especially at the site where molten metal penetrates into the inner wall of the
manifold during the welding process the erosion–corrosion was very severe (Fig. 12). Also the surface
metal was removed and became particles (Fig. 13). These particles might have entered into the main fuel
pump inlet and resulted in fuel system malfunction. The most significant effect of erosion–corrosion is the
constant removal of protective films (which may range from thick visible films of corrosion products to
thin invisible passivating films) from the metal surface, thus resulting in localized attack at the areas at
Fig. 9. The fuel flow schematic of F-5F aircraft.
Fig. 10. The lower front side of the engine.
678 M. Park / Engineering Failure Analysis 9 (2002) 673–681

Fig. 11. The erosion–corrosion surface morphology at the bend.
Fig. 12. (a) The inner surface of the welded area of the manifold; (b) the close up view of the internal welding bead—the localized
attack is noticed.
Fig. 13. The surface metal was partially removed becoming metal particles.
M. Park / Engineering Failure Analysis 9 (2002) 673–681 679

which the film is removed. This can be caused by movement at high velocities, and will be particularly
prone to occur if the solution contains solid particles (e.g. insoluble salts, sand and silt) that have an
abrasive action [1]. Thus the cleanliness of the fuel is one of the important factors to be controlled.
4.3. EDX analysis of the internal welding bead
Fig. 14 shows the EDX spectrum of a internal welding bead of the fuel manifold indicating the major
elements of stainless steel. This result is in good agreement with the EDX results of the rubbing surface of
the boost element and the chip surface. Thus it is a reasonable guess that the particles removed from the
surface of the internal welding bead entered into the MFP and resulted in the engine failure.
5. Discussion
Fig. 15 shows the internal surface of a fuel manifold which was randomly chosen from the field. From
the internal inspection of several fuel manifolds using a borescope it is thought that most of the manifold have
an internal welding bead. And this internal welding bead can be a potential source of small particles if this sites
are attacked by erosion–corrosion. Therefore for the elimination of this kind of failure, the optimization of
welding process not to make internal welding beads are necessary. The manifold joining process might
adopt one of the arc welding processes—the most frequently used welding methods in the aerospace
industry. The joining process requires the selection or specification of the welding voltage, welding current,
arc polarity (straight polarity, reversed polarity, or alternating), arc length, welding speed (how fast the
electrode is moved across the workpiece), arc atmosphere, electrode or filler material, and flux. For many
of the processes, the quality of the weld also depends on the skill of the operator. Automation and robotics
Fig. 14. EDX spectrum of the internal welding bead of the fuel manifold.
680 M. Park / Engineering Failure Analysis 9 (2002) 673–681

are reducing this dependence, but the selection and training of welding personnel are still of great importance
[2].
6. Conclusion
The internally penetrated welding metal of fuel manifold was attacked by erosion–corrosion and from
this reaction the surface metal were lifted-off. This metal particles entered into the main fuel pump and
caused wear damage. During this incident, the fuel pressure dropped abruptly and the engine stopped. The
remedial action to prevent this kind of failure from occurring again was suggested—the optimization of the
welding practice and the proper training of welding personnel.
References
[1] Corrosion, vol. 1. 3rd ed., Butterworth Heinemann, 1994. p. 1:190–5.
[2] Degarmo EP, Black JT, Kosher RA. Materials and processes in manufacturing, 8th ed. Prentice Hall International, 1997. p. 978–80.
Fig. 15. The internal examination of the fuel manifold from the field using borescope.
M. Park / Engineering Failure Analysis 9 (2002) 673–681 681