ANC93IA085

The right main gear upper strut assembly separated on landing. Metallurgical examination showed that the strut assembly failed due to stress corrosion cracks occurring across the forged parting plane.

On May 26, 1993, at 1530 Alaska daylight time, a retractable gear, wheel equipped Piper PA-31-350, Navajo airplane, N3588Z, registered to and operated by Peninsula Airways, Inc. of Anchorage, Alaska, experienced a right main gear collapse during landing at King Salmon, Alaska. The positioning flight, operating under 14 CFR Part 91, last departed Chignik, Alaska, and the destination was King Salmon. The Airline Transport Certificated Pilot-in-Command, the sole occupant, was not injured, and the airplane received only minor damage. Visual meteorological conditions prevailed and a company flight plan was in effect.

Examination of the airplane showed that the right main landing gear strut housing had separated into 4 major pieces. The pieces were submitted for metallurgical examination and the results revealed the following information.

The strut housing was made from 2014-T6 aluminum alloy which can be susceptible to stress corrosion cracking if it is exposed to halogens, such as fluoride, chloride, etc., in aqueous solutions and is assisted by either external or internal applied stress. The metallurgical examination determined that isolated areas of corrosion that emanated from the outer diameter of the strut housing assembly along the forge parting plane was caused by a chloride contaminant. The metallurgical report further stated that: "Branch cracking and intergranular cracks are indicative of stress-corrosion cracking in aluminum alloys. Although branch cracking was not noted in the metallurgical sections, intergranular cracks were observed on the fractures in section "1-1" (of the attached metallurgical report) to suffice that intergranular cracking was the preferred corrosion mechanism that caused separation of the strut housing."

Furthermore, the metallurgical report stated that another factor which assisted in the separation of the strut housing assembly was the perpendicular stress applied to the forge parting plane. "It is believed that the strut housing assembly was subjected to such a stress when the main landing gear touched the ground causing the pre-existing stress-corrosion cracks to propagate rapidly through he forging parting plane and the assembly to separate."

According to the State of Alaska, none of the state owned airports use a chloride-based solution for sanding runways, taxiways, or roadways in close proximity to runways and/or taxiways. The military installations use either UREA or glycol in the sand.

Examination of the environment in which the airplane operations, the Aleutian Chain, shows that it is surrounded by salt water and is subjected to high winds. An example given was Dutch Harbor, Alaska, where the ocean spray and waves obliterate the ends of the runway on a regular basis. The airplane is operated consistently on the Aleutian Chain, including Dutch Harbor.