The following NDT relevant parts have been extracted from the Aircraft Incident Report No: 2/98 (EW/C95/9/4) :
 Appendix B - Photograph of the damaged tail rotor shaft with gearbox s/n 187 after removal from G-PUMH  Appendix G - Photograph of fracture face |
The fatigue crack had been initiated by fretting and corrosion of the flapping hinge retainer bore induced by abnormal cyclic loading of the retainer, which was attributed to the effects of a defective flap needle-roller bearing during some previous period of the tail rotor drive shaft's life.
The inspection provisions within the aircraft Maintenance Manual and associated Maintenance Requirements did not specify periodic visual inspections of such retainers, since they had been designed and certificated on a 'safe-life' basis.
A crack had penetrated the surface of a tail rotor shaft for a significant distance, it was not detectable during daily visual inspections. Significant degree of corrosion and/or fretting damage on the bore of the flapping hinge retainer.
1.16.2 Non-destructive test (NDT) technique trials
Shortly after the incident, the operator and manufacturer devised a non-destructive test (NDT) eddy current technique. This technique was used on the operator's fleet. No crack indications were found. The nature of the technique gave rise to concerns that eddy current methods might produce erroneous results due to the use of steel bushes against the bore surfaces where a crack might originate. Some further experimentation with ultrasonic pulse echo techniques was carried out, but this was not particularly successful.
Another operator developed both eddy current and ultrasonic pulse echo techniques. These were demonstrated to the AAIB using a test specimen with an artificially generated crack. The ultrasonic technique was particularly effective in the trial, but in use it gave false crack indications when the flap bearings were in place. That operator carried out a 'one off' eddy current inspection across its Super Puma fleet and found no cracks.
The tail rotor shaft is protected with an epoxy primer and paint treatment as manufactured. In service, this protective treatment may be repaired from time to time. It is possible, therefore, that subsequent layers of paint may not adhere fully to the original finish and that this could result in subsequent cracks at the surface of the metal remaining invisible under such paint layers. Oil and grease in the area could also obscure crack indications. Magnetic particle NDT techniques would be effective in such circumstances.
The continuing airworthiness of the fleet is currently controlled by daily visual inspections.
From the manufacturer's view that daily visual inspections were adequate. In view, however, of the nature of daily inspections and the size and location of this type of crack, it might be considered advisable to supplement them with periodic dye penetrant checks, for example.
The manufacturer should revise the Maintenance Manual and Master Servicing Recommendations to require periodic detailed inspection.
Full Report by E J TRIMBLE
Inspector of Air Accidents Air Accidents
Investigation Branch Department of the Environment, Transport and the Regions
November 1997
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