Accident Overview
On 30 May 2009 at 1340 hours, a Piper PA-30 Twin Comanche (registration G-ATEW, built 1965) experienced landing gear retraction during the ground roll after landing at Sturgate Airfield, Lincolnshire. The flight was a private trip from Newcastle Airport. The sole pilot (aged 66, holding a Private Pilot's Licence with 711 total hours, 475 on type) and one passenger were uninjured. The aircraft sustained damage to both propellers, the fuselage, and the wing underside, and was determined to be beyond economic repair.
History of the Flight
The pilot reported that the weather at Sturgate was CAVOK with a south-easterly wind of 8 to 10 kt. The aircraft joined overhead at 2,000 feet and descended to a downwind leg for Runway 09. The pilot decided to carry out a flaps-up approach to the paved runway (unlicensed length of 820 metres). During final approach, he made a radio call stating "Finals gear down" and checked that the single green gear-down-and-locked light was illuminated. He did not visually check the gear position using the mirror on the left engine cowling. The passenger, also a pilot, noted seeing the green light.
The pilot stated that the aircraft arrived over the runway threshold at 100 KIAS, then he reduced power and flared slightly. The landing was firmer than normal but not "over-hard." After a ground roll of about 300 feet, the pilot looked to his right and moved his hand to turn off the fuel pumps. At about the same time, he became aware of a sinking feeling that continued until the aircraft was sliding on its belly. When the aircraft came to rest, the passenger opened the main door and exited; the pilot turned off the fuel selectors and electrical switches before exiting. The airfield's fire service arrived shortly after, but there was no fire.
Landing Gear System Description
The Piper PA-30's landing gear retraction and extension system consists of an electric motor and transmission assembly, torque tube assembly, push-pull cables to each main gear leg, and a push-pull tube to the nose gear leg. Limit switches shut off the motor when the gear is fully extended or retracted and operate the gear indicator lights. The gear is held down by an over-centre geometric locking mechanism. An anti-retraction safety switch on the left main gear leg prevents gear retraction until sufficient weight is lifted to allow the strut to extend to within 0.75 inches of full extension. A gear warning horn sounds if engine manifold pressure drops below 10 to 12 inches while the gear is not selected down.
Examination of the Aircraft
The chief engineer from the maintenance organisation at the airfield examined the aircraft. He found the gear selector in the down position and the master switch off. After turning the master switch on, he heard the landing gear motor whirring but no gear movement, indicating the motor's drive had sheared. When the aircraft was lifted, the nose gear and right main gear dropped down, but the left main gear came down only to a 45° angle. The engineer pulled the nose gear leg forward, which appeared to cause the right main gear leg to lock down, but the left main gear leg did not move. He noticed a bend in the push-pull cable to the left main gear leg. After cutting this cable, the left gear dropped to the extended position. The engineer assessed that there was no internal wing damage that would have been expected from a heavy landing. No further examination was carried out as the aircraft was beyond economic repair, and the anti-retraction safety switch was not checked.
Pilot's Assessment
The pilot considered that the landing gear either collapsed or retracted. He thought a retraction might have been caused by the gear switch not latching down or his arm catching it when he moved his hand to turn off the fuel pumps. He did not believe he had mistaken the gear lever for the flap lever because it was a flaps-up landing.
Analysis
The analysis (as presented in the AAIB report) noted that the 300-foot ground roll after landing indicated the gear was down and locked initially, consistent with the green light observation. The simultaneous retraction or collapse of all three gear legs suggested a gear retraction rather than a collapse of a single leg. The bent push-pull cable could have resulted from the motor trying to re-extend the gear leg while the aircraft's weight was retracting it. However, the anti-retraction switch should have prevented a commanded retraction if weight was on the left main gear leg. Since the switch was not tested, a fault could not be ruled out. Ultimately, there was insufficient information to determine why the gear retracted.