Summary
The fuel pump in an aircraft failed, resulting in engine flame-out. One of the seven pistons had broken into several pieces. This failure resulted in the fracture of the quill shaft of the pump. The fracture surface of the quill shaft had suffered severe rubbing. The available fracture surface of the piston indicated beach-mark-like features. Diametrically opposite the apparent origin of the beach marks was a nick on the barrel surface. These facts suggest that there was a preexisting crack that propagated by fatigue.
Background
In an aircraft that met with an accident, the fuel pump had failed.
It was reported that the failure of the fuel pump resulted in engine flame-out just after takeoff.
Visual Examination of General Physical Features
The quill shaft of the pump had fractured and the fracture sur-face had rubbed due to the running of its mating part (Fig.
CH13.1). The rotor is shown in Fig. CH13.2. Of the seven piston holes in the rotor, one of them had suffered severe damage. The metal around the hole had plastically deformed in the direction of rotation. Significant metal flow around this hole can be seen in Fig. CH13.3.
The corresponding piston also had suffered severe damage and had broken into several pieces. It was reported that some brass inside this piston hole had to be scooped out before recovering the broken pieces of this piston. The slipper pad of this piston was not available for investigation. Some pieces of the broken piston were
Fig. CH13.1 Fracture surface of the quill shaft Fig. CH13.2 Rotor of the fuel pump
DOI:10.1361/faes2005p092 www.asminternational.org
missing. Part of the fracture surface had been cut for earlier in-vestigations elsewhere. Figure CH13.4 shows a sketch of the bro-ken piston.
The other six pistons and their respective slipper pads did not show any abnormalities.
All of the seven springs were in good condition with no sign of excessive wear. The seven spring guides showed some wear at the spring seating. Two of these are shown in Fig. CH13.5. The spring seats did not show any abnormality.
Testing Procedure and Results
Microscopy
The available fracture surface on the broken piston, “A” in Fig.
CH13.4, was examined under a stereobinocular microscope. About 50% of the cross-sectional area was flat, and the rest showed a jagged surface. The features on the flat region had the appearance of beach marks, although the surface had been rubbed (Fig.
CH13.6). Scanning electron microscopy did not clearly reveal fa-tigue striations.
The mating fracture surface, “B” in Fig. CH13.4, showed a small crack. A small region near this crack corresponded to the flat region of the mating fracture surface A. When the mating surface B was examined in the SEM, a beach-mark-like appear-ance was seen only on a heavily rubbed fracture area, with brass from the rotor smeared on it. There was no brass deposit on the mating surface A.
Diametrically opposite the area where the beach marks appeared to have originated, a small nick, which could have been the result of final failure of the piston against the top edge of the rotor hole, was found on the barrel surface. Fretting damage to the barrel surface could be seen (Fig. CH13.7).
The microstructure of the piston material was satisfactory. The hardness on the barrel surface was 830 HV, and that on the core was 620 HV.
Fig. CH13.3 Deformation and metal flow around the piston hole
Fig. CH13.4 Sketch showing the fracture pattern of the failed piston Fig. CH13.5 Wear on the spring guides
1 mm
Fig. CH13.6 Features resembling beach marks on the fracture surface of the failed piston. SEM fractograph
20 m
Fig. CH13.7 Fretting damage on the barrel surface of the failed piston.
SEM photograph
Discussion
The primary cause of the failure of the fuel pump was due to the fracture of one of the seven pistons in the rotor. The failure of the quill shaft was the result of the failure of this piston.
The piston made of case-hardened steel failed by fretting fa-tigue, the crack initiating at the surface. The beach-mark-like
ap-pearance on the fracture surface could not be due to rubbing be-cause the two mating surfaces showed similar marking, and one of the surfaces had brass smeared on it. If the beach-mark ap-pearance were due to rubbing after fracture, then the brass smear should be on both the fracture surfaces. Because the patterns were similar and only one had the brass smear, it could be concluded that the beach marks were there before final fracture. The other corroborative evidence is the secondary cracking observed on one of the fracture pieces.
All these facts and the nick opposite the fracture origin proved that there was a preexisting crack that propagated by fatigue, leav-ing beach marks. Fatigue striations could not be observed on this surface possibly because the piston was made of a hard material and, secondly, the fracture surfaces had suffered damage.
The cause and type of loading pattern for fatigue crack initiation and propagation could not be ascertained.
The probable cause could be fretting fatigue. The exact se-quence could not be established because of the absence of all the fractured pieces.
The pump had failed due to the fracture of one of the pistons.
This piston failed due to fretting fatigue arising from fretting fa-tigue crack on the barrel, and the exact cause of the fretting fafa-tigue could not be identified.
Conclusion
Most Probable Cause
The most probable primary causes were:
● The spring guides not being properly positioned
● Spring seat not in proper position
● Spring stiffness not being uniform with respect to other springs
● Slipper pad jamming
Any one of these conditions singly or collectively can cause fret-ting fatigue or nonuniform loading on the piston, leading to failure.
Recommendations
● Change all the springs, spring seats, and spring guides every time the pump is overhauled.
● Check and use springs of correct length/stiffness.
● Check the clogging of slipper pad lubricating holes.
● The pistons should be of correct dimensions. In the pump itself, it was observed that the taper at the end of the piston varied from piston to piston. The taper is critical for avoiding spring wear.
● While dismantling the pump, it is always a good practice to identify the parts and their relative locations correctly.