Incident
On 6 July 2010 at 1259 UTC, a Piper PA-23-250 Aztec, registration G-BATN, was conducting training at Cambridge Airport. The pilot, the sole occupant, planned three landings to maintain currency. The first two landings were normal, with all gear down and locked, indicated by three green lights. The third circuit was a practice low-level bad weather approach. At approximately 200 ft on final, the pilot confirmed the gear was down and locked. The approach and touchdown appeared normal until the nose was lowered, at which point it continued beyond the normal landing pitch attitude and contacted the runway. The propellers also struck the runway, the engines stopped, and the aircraft slid to a halt on its nose. The pilot, wearing a lap and diagonal harness, was uninjured. The Airport Fire Service attended and applied foam due to a small fuel leak from the left wing.
Aircraft Details
The Piper PA-23-250 Aztec is a twin-engine aircraft with hydraulically-actuated retractable tricycle landing gear. The nose landing gear extends forward. When the gear lever is selected down, hydraulic pressure extends a drag link until it reaches an over-centre position. The final actuator movement pivots the downlock link about a link plate to engage the downlock pawl and activate a microswitch. Green indicator lights in the cockpit illuminate when each gear is down and locked. An amber light indicates all gears up. A warning horn sounds when both engine power is reduced below 10-12 inches of manifold pressure and any gear is not locked down.
Examination
Post-incident examination was conducted by an external assessor contracted by the aircraft operator. Initial tests showed that manually lowering the nose landing gear would not achieve full downlock, and the nose gear green light did not illuminate. With throttles at idle, the gear unsafe horn sounded. Functional tests were performed: when the gear lever was selected down, both main landing gears engaged and their green lights illuminated, but the nose landing gear downlock did not engage fully and its green light remained off. Repeated cycling yielded the same result.
Inspection of the nose landing gear revealed damage to the link plate that attaches the actuator to the downlock link. The bolts securing the link plate were removed and found to be 'dry of lubricant and had visible surface rust on the bolt shank.' After straightening the link plate and reinstalling the bolts with engine oil lubrication, further extension and retraction tests were successful; the nose landing gear downlock pawl engaged each time and the cockpit green light illuminated.
Maintenance History
Between 2006 and the incident, G-BATN had undergone several inspections for nose landing gear down indication issues. The service manual's troubleshooting pages suggested that no green light with gear down was likely electrical. Because the gear had remained locked in previous cases, maintenance personnel considered it an intermittent electrical problem. During the April 2010 annual inspection, the microswitch was replaced. No restriction at the link plate had been identified during these maintenance actions; all previous cycling of the gear during maintenance resulted in successful downlock and indication.
The aircraft maintenance manual specifies lubrication of the nose landing gear downlock link plate attachments every 100 hours using a general-purpose low-temperature lubricating oil conforming to MIL-PRF-7870C. However, the lubrication procedure does not require removal of the downlock link plate bolts, making it impossible to verify that lubrication penetrated to the bolt shank. The operator confirmed that the correct lubrication schedule had been followed.
Discussion
The nose landing gear retracted on landing because it was not fully locked down. A lack of lubrication on the shanks of the downlock link plate bolts was considered to have caused sufficient restriction to prevent complete engagement of the downlock. For reasons that could not be established, the pilot was unaware that the gear was not locked down. He stated that it would be "very unlikely" that the green light was not illuminated at touchdown and that the gear unsafe horn was sounding but that he had failed to notice. The aircraft manufacturer noted that no similar events had been reported since 1995. Following this incident, the operator was considering introducing an additional maintenance activity to periodically remove and inspect the bolts to ensure lubrication penetration and to check for corrosion.