History of the Flight
On 9 June 2011, a Piper PA-34-200T Seneca II, registration EC-KDP, was conducting a local check flight from Tenerife North Airport (GCXO). On board were an examiner and an examinee. During the approach, while configuring for landing by lowering the gear, the examiner (pilot in command) observed that the green light for the left gear leg was off and the red gear unsafe light remained on, accompanied by a continuous horn. The pilot cycled the gear twice without success. A low pass over the control tower allowed tower personnel to visually confirm the legs were down but could not verify if they were locked.
The aircraft proceeded to the downwind leg and the gear was cycled up and down again, still to no avail. The emergency gear extension procedure as per the Pilot Operating Manual was then performed, but the cockpit indications remained unchanged. The pilot decided to land with caution, touching down first on the right gear. After traveling about 200 meters, the left wheel made contact, at which point the left gear leg collapsed, causing the aircraft to yaw sharply left, depart the runway near the E3 exit taxiway, and come to a stop approximately 40 meters from the runway centerline at a 180° angle to the landing direction. Neither occupant was injured.
Aircraft Information
The aircraft, a low-wing twin-engine Piper PA-34-200T Seneca II (serial number 34-7970149), was equipped with two Continental TSIO-360-EB1B engines and retractable landing gear. It had a valid airworthiness certificate and was used as a school and training aircraft. The aircraft had accumulated approximately 3,363 flight hours. Its last 100-hour/annual inspection was performed on 2 December 2010 at a maintenance center in Portugal, at 3,337 hours. The usual maintenance center was on Gran Canaria island.
The aircraft was normally parked outdoors on the tarmac, experiencing long periods of inactivity. Flight logs showed 35 hours 15 minutes in 2009, 57 hours 48 minutes in 2010, and 16 hours in 2011 up to the incident. In May 2011 it flew 2 hours 25 minutes, with the last flight on 18 May 2011 lasting 30 minutes.
The landing gear system is hydraulically deployed and retracted by a reversible electric pump. A lever on the instrument panel selects gear up or down. Three green lights indicate when the gear is down and locked, and a red light indicates an unsafe condition. The leg locking mechanism consists of a tab or hook that lodges in a small pin when fully extended.
Inspection Findings
Initial inspection at the incident site and later at the hangar revealed that the hydraulic pump functioned correctly, and all three legs deployed when the gear was cycled. However, the left leg did not lock. The hydraulic fluid level was adequate, and the gear unsafe warning system operated properly. Examination of the left gear mechanism showed a lack of grease, visible corrosion, and dust accumulation. Some corroded areas had been painted over. The only way to lock the left leg was by manually fully extending it using considerable force with a tool such as a screwdriver to force the locking tabs down.
During the removal of the aircraft, it was hoisted by the engine mounts and the gear was cycled; the hydraulic pump lowered the left leg, but the locking hooks did not lower. One mechanic used an iron bar on the mechanism to gain leverage and managed to lower the locking tabs. Subsequent checks in the hangar confirmed that the left gear failed to lower fully or lock, even when using the emergency extension system.
Conclusions
The investigation concluded that the incident likely resulted from a restriction in the left gear deployment and locking system, preventing the leg and locking tabs from fully extending. The humid and saline environment at Tenerife North Airport, combined with the relative inactivity of the aircraft, led to accumulation of water and dust in the landing gear, causing a restriction that went undetected until the gear was lowered. The corrosion and lack of lubrication hampered the mechanism's operation.