Aircraft

Airbus A330/A340 mystery cracking forces emergency airworthiness directive extension

By Ian Goold

Airbus's inability to explain cracking in an undercarriage support beam of an A330 that could lead to a gear collapse has resulted an emergency airworthiness directive (EAD) requiring repetitive inspections on all A330/A340s indefinitely.

The crack was discovered last year in the main landing gear rib 6 forged support beam during routine lubrication of an A330 operated by an unidentified carrier for long-range flights. Rib 6 is a main structural member that carries all landing-gear loads and such a crack could lead to undercarriage collapse, says John Grother, vice-president customer services A330/A340.

Based on experience with a rib-cracking problem on the A320 family Airbus had initially tacked the A330 discovery with a fleet-inspection programme, while the European Aviation Safety Agency issued an EAD in December to mandate compliance with relevant service bulletins on all A330/A340s at least five years old (except those previously fitted, or originally manufactured, with high-interference bushes). This called for inspections of certain aircraft at prescribed flight-cycle and age thresholds.

However, subsequent metallurgical examination and analysis of the damaged rib revealed a different crack propagation to that on the A320. After several months' consideration, the manufacturer remains mystified because it has found no solid evidence of why the cracking occurred.

A new EAD supersedes the December alert that mandates compliance with new Airbus service bulletins and also introduces flight-hour thresholds. It has extended inspections to cover all aircraft indefinitely and makes clear that even complete replacement of rib 6 does not remove the necessity for continuing repetitive inspections.

If the event remains a one-off, Grother thinks checks will continue for at least two years, during which period most aircraft are expected to require inspection up to 12 times at typical flight-cycle rates.

The new flight-hour threshold has been introduced not because normal operational loads would contribute to crack propagation, but to recognise different aircraft operations. It also reduces as permitted maximum landing weight increases.