ANC07LA060

The private pilot was landing a tundra tire-equipped airplane on an off-airport grass surface area. As he applied the brakes during the landing roll, the left main landing gear collapsed. The airplane pivoted to the left, and the left wing struck the ground. His inspection of the landing gear revealed that the outboard end of the left main landing gear strut extension was broken through the outer radius of the attaching lug, where it normally would bolt to the axle. In addition, the landing gear safety cable was broken. The airplane was equipped with hydrasorb landing gear shock units, which consist of automotive type oleo struts, combined with light shock cords. Examination of the outboard end of the hydrasorb extension tube revealed a flat fracture surface that was perpendicular to the long axis of the tube. It had extensive evidence of corrosion. The landing gear had been installed since 1977, and the most recent annual inspection of the airplane was about 1 year before the accident.

On July 1, 2007, about 1745 Alaska daylight time, a tundra tire-equipped Piper PA-18 airplane, N3195Z, sustained substantial damage during the landing roll at a remote airstrip along the Kahiltna River, about 25 miles west-southwest of Talkeetna, Alaska. The airplane was being operated as a visual flight rules (VFR) cross-country personal flight under Title 14, CFR Part 91, when the accident occurred. The airplane was operated by the pilot. The private certificated pilot, the sole occupant, was not injured. Visual meteorological conditions prevailed. The flight originated at the Wolf Lake Airport, Wasilla, Alaska, about 1600, and no flight plan was filed.

During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), on July 2, the pilot reported that he was landing on a grass surface area that was oriented north/south. The area was about 800 feet long, and about 50 feet wide, and he was landing toward the south. He said that during the landing roll, as he applied the brakes, the left main landing gear collapsed. The airplane pivoted to the left, and the left wing struck the ground. His inspection of the landing gear revealed the shock strut and the safety cable were broken. The pilot said he did not think the landing was in anyway hard, and the wind conditions were about 180 degrees at 8 knots.

On July 6, the owner of a maintenance facility reported that the airplane received structural damage to the left wing and both wing spars.

The airplane was equipped with hydrasorb landing gear shock units, which consist of automotive type oleo struts, combined with light shock cords. The outboard end of the hydrasorb has an extension tube that is normally installed between the wheel axle and the hydrasorb unit. In addition, the landing gear had upper and lower safety cables. The safety cables limit the upward travel of the wheel if a strut unit should fail. The upper safety cable is attached between the fuselage tubing and the center cabane. The lower safety cable is attached between the main wheel axle and the center cabane.

On July 10, the pilot brought the broken left hydrasorb strut extension tube to the NTSB Alaska Regional Office. The outboard end of the tube was fractured through the outer radius of the attaching lug, where it normally would bolt to the axle. Visual examination of the broken tube revealed a flat fracture surface that was perpendicular to the long axis of the tube. It had extensive evidence of rust and pitting.

In the Pilot/Operator Aircraft Accident Report (NTSB Form 6120.1) submitted by the pilot, the pilot indicated that the most recent annual inspection was July, 2006, but he did not indicate any airframe total time, or time accrued since the inspection. He did indicate that the main landing gear struts had been installed since 1977.

According to FAA aircraft certification staff, the accident airplane does not have a manufacturer's maintenance manual. The military version of the accident airplane, the L-21, did have a maintenance manual. Paragraph 2-85, Inspection of Main Landing Gear, of the L-21 manual states, in part: "a. Examine all attaching nuts and bolts for wear, distortion, and damaged threads. Replace damaged part. b. Inspect all metal parts for cracks, distortion, corrosion, and other damage. Replace parts that are damaged beyond repair. Corroded spots must be sanded down to good metal and coated with primer..."

Information about aviation maintenance standards are contained in the FAA's advisory circular, AC 43.13-1B, Acceptable Methods, Techniques, and Practices - Aircraft Inspection and Repair. Chapter 9 of the AC, Aircraft Systems and Components, Section 1, Inspection and Maintenance of Landing Gear, states, in part: "9-2. General Inspection; A thorough inspection of the landing gear involves the entire structure of the gear, including attachment, struts, wheels, brakes... . The manufacturer's inspection procedures should be followed where applicable. 9-4. Fixed gear inspection. Fixed landing gear should be examined regularly for wear, deterioration, corrosion, alignment, and other factors that may cause failure or unsatisfactory operation. During a 100-hour or annual inspection of the fixed gear, the aircraft should be jacked up to relieve the aircraft weight. The gear struts and wheels should be checked for play and corrected... (g) The entire structure of the landing gear should be closely examined for cracks, nicks, cuts, corrosion damage, or any other condition that can cause stress concentrations and eventual failure."

Chapter 6, Section 5, Visual Corrosion Inspection Guide for Aircraft, of AC 43.13-1B, states, in part: "6-63. General; This guide provides a general inspection checklist for those parts or surfaces that can be visually inspected without disassembly of the aircraft. 6-68. Wheel wells and landing gear; Inspect wheel well area and landing gear components for damage to exterior finish coating and corrosion. Particular attention should be given to exposed surfaces of struts, oleos, arms, links, and attaching hardware; axle interiors, exposed position indicator switches and other electrical equipment; crevices between stiffeners, ribs, and lower skin surfaces; magnesium wheels, particularly around bolt heads, lugs, and wheel web areas; and exposed rigid tubing at "B" nuts and ferrules under clamps, and tubing identification tapes."