Page 431 - Failure Analysis Case Studies II

Failure Analysis Case Studies Ii
D.R.H. Jones (Editor)
0 2001 Elsevier Science Ltd. All rights reserved 415

AN AIR CRASH DUE TO FATIGUE FAILURE OF
A BALL BEARING

I. SALAM, A. TAUQIR*, A. UL HAQ and A. Q. KHAN
Metallurgy Division, Dr. A. Q. Khan Research Laboratorics, GPO Box 502, Rawdlpindi, Pakistan
(Receitied I5 June 1998)

Abstract-The failure analysis of an air crash conclusively shows that the cage of the central main bearing of
the compressor region failed due to fatigue. The broken piece of the cage got struck between the bearing balls
and the races and impaired the function of the bearing resulting in the crash. Q 1998 Elsevier Science Ltd. A11
rights reserved.
Keywords: Accident investigation, aircraft failures, bearing failure, engine failurcs, fatigue.

1. INTRODUCTION
A fighter plane crashed in 1996; Fig. la. The plane had two engines. The on site investigations
pointed out that the fault was probably in the compressor region of one of the engines and the
problem was suspected to be in a bearing of the compressor region. In this region there are three
main bearings. Those at the front and back are roller bearings while the central one (CMB) is the
ball bearing type. The bearings are lubricated by way of a closed loop lubrication system and
metallic filters are inserted to clean the debris. The life of the CMB, which is suspected to have
caused the accident, is 600 h. The bearing of the engine which failed had completed a total of 467 h
and 5 h had elapsed since it was last inspected. The inspection report shows that the parameters
measured were within the specified limits. The CMB of the other engine had completed about 130
h and was found undamaged. There are some previous cases in which an aircraft landed safely after
an indication of a problem in the engine; in all these cases failures in the cages of the CMB were
detected. The cage of the CMB from the plane under consideration in this study was retrieved in a
broken condition (Fig. Ib).
To isolate the cause of failure, the bcaring components were subjected to detailed analysis. The
main components of the bearing included the steel balls, inner and outer races and the cage. The
material of these components was investigated and the details are summarized in Table I. The cage
material was subjected to detailed metallurgical investigations.
The retrieved balls exhibited significant wearlerosion. They were studied for smearing of foreign
material. Similarly the surfaces of the races were extensively deformed and these were analyzed to
investigate the deformation details and the material smeared on them.

2. CMB ASSEMBLY

The bearing is mounted on the compressor shaft and is located in a housing [I]. The main
components of the CMB are inner and outer races on which the bearing ball roll and a soft cage to
keep the balls at a distance from each other. The arrangement of these components is shown in
Fig. 2.

*Author to whom correspondence should be addressed.
Reprinted from Engineering Failure Analysis 5 (4), 26 1-269 (1 998)