What happened
On 6 July 2010, a Piper PA-23-250 Aztec, registration G-BATN, was performing a series of training landings at Cambridge Airport. The flight was intended to maintain the pilot's currency for flying with passengers. During the third of three planned landings, which was a practice low-level circuit in poor weather, the pilot confirmed that all landing gear indicators showed the gear was down and locked.
As the aircraft touched down, the nose landing gear collapsed. The aircraft's nose lowered beyond the normal pitch attitude, making contact with the runway. The impact caused the propellers to strike the ground and the engines to stop, after which the aircraft slid to a halt. The pilot escaped the single-engine aircraft without injury, though the Airport Fire Service noted a small fuel leak from the left wing.
The investigation
An investigation into the collapse of the nose gear revealed that while the main gears remained locked, the nose gear downlock had failed to engage fully. Although the cockpit indicators initially showed a green light for the nose gear, subsequent testing demonstrated that the gear could not be manually locked in the down position.
An inspection of the nose landing gear assembly identified damage to the link plate that connects the actuator to the downlock link. The bolts securing this plate were found to be dry of lubricant and exhibited visible surface rust on the bolt shanks. While the operator had followed the standard lubrication schedule, the investigation noted that the maintenance procedure did not require the removal of these specific bolts, meaning it was impossible to verify if lubricant had reached the shank.
Findings
- The nose landing gear retracted upon landing because the downlock had not fully engaged.
- A lack of lubrication on the link plate attachment bolts caused a mechanical restriction that prevented the complete engagement of the downlock.
- The pilot was unaware of the incomplete lock because the cockpit green indicator light was illuminated at the time of touchdown.
- The existing maintenance procedure did not include the removal of the link plate bolts, preventing the verification of lubrication penetration to the bolt shanks.