Report: fatigue cracking caused gear failure of Swedish DHC-6 Twin Otter

June 21, 2010

A de Havilland Canada DHC-6 Twin Otter sustained substantial damage in a landing gear failure in Sweden in June 2009. The Swedish accident investigators concluded that the gear fractured as the result of a fatigue crack.

The pilot intended to take off with 21 parachutists on board the aircraft. He did not notice anything abnormal, neither during the preparations for take-off nor during the initial taxiing.
After taxiing for a few minutes at low speed, suddenly the right main landing gear broke, whereupon the aircraft tipped over to the right and the right wing struck the ground. The aircraft then slowed down, turned somewhat to the right, and stopped. No person onboard was injured.
The technical examination of the aircraft has shown that the right main landing gear fractured as the result of a fatigue crack. The crack consisted of several smaller fatigue cracks that had grown and joined. The cracks had initiated in an external welded joint and developed over an extended period of time without being detected.
The aircraft type has been exposed to landing gear fracture before as the result of fatigue cracks in the actual area. Current maintenance system prescribes NDT inspection of the landing gear within intervals of 12,000 flying hours or five years, whichever comes first.

The Swedish Accident Investigation Board recommends that EASA and the Swedish Transport Agency, in conjunction with the manufacturer, consider the need for supplementing the present maintenance system in respect of crack formation in the landing gear.

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