Corrosion Causing 727 Landing Gear Failures Safety Board Urges Mandatory Inspections, Repairs, Sealant

Air Safety Week, Jan 25, 1999

Operators of that venerable workhorse, the Boeing 727, need to devote attention and care to the main landing gear. Operators who have failed to do so have suffered landing gear collapses. The most recent case, involving a right main landing gear collapse last August of a B-727-200 operated by DHL Worldwide Express, prompted an investigation by the National Transportation Safety Board (NTSB).

In a Jan. 19th letter to the FAA, the NTSB said the DHL jet's original-equipment landing gear failed at 44,554 cycles, and that DHL had not inspected the gear assembly per a 1990 Boeing service bulletin. This bulletin and its two revisions recommended periodic inspections of the forward trunnion bearing support fitting until it is replaced or a sealant is applied to provide additional protection against corrosion. The fitting is a key part of the landing gear assembly, and it was the component that failed in the DHL incident. There are three support points in the 727 main gear design: the forward and aft trunnions and the side strut. A spherical bearing features splines that lock it into the forward trunnion lug. Gaps in those splines, though, have proven to be an entryway for moisture. The moisture has caused corrosion, fatigue cracking and a phenomenon known as intragranular stress corrosion cracking (SCC). The components on the accident aircraft were found to be severely corroded. Indeed, the corrosion found in 1998 was the reason behind the Boeing service bulletin of 1990. However, NTSB investigators found that compliance was spotty. Some operators developed their own inspection protocols, doing them every 30 months instead of the 6- month intervals recommended by Boeing. Others did not perform the inspections at all. "The Safety Board is concerned that additional B-727 forward trunnion bearing support fittings may fail because of corrosion, fatigue and SCC, which could jeopardize safety," the NTSB's letter intoned. Ultrasonic inspection intervals and sealant application should be made mandatory, the Board concluded.