What happened
On 24 July 2001, a Cessna 421C, registration N6315X, was conducting a private flight at Oxford Airport when the pilot experienced significant instrument failures during the climb. While ascending through 600 feet, the airspeed indicator dropped to zero, and the vertical speed and altitude indicators became unresponsive. The pilot subsequently requested a priority landing.
During the approach, the pilot attempted to extend the landing gear but noted that the right main landing gear red light remained illuminated, indicating the gear was not locked. Despite performing emergency procedures—including aircraft yawing, applying positive 'g' loads, and utilizing the 'blow down' system—the gear failed to lock. Although air traffic control provided visual confirmation that the gear appeared down, the cockpit indication persisted.
Upon landing on Runway 20, the aircraft initially touched down on both main wheels. However, during the landing roll, the right main landing gear leg began to retract. This caused the right propeller, engine, wing lower skin, and flap to contact the runway, forcing the aircraft to veer off the runway onto the grass. There were no injuries to the pilot or the observer.
The investigation
Investigators examined the aircraft's maintenance history and found that the right main landing gear hydraulic actuator had been stripped and reassembled following a previous accident in 1998. Post-accident testing on jacks revealed that while the gear locked correctly under hydraulic pressure, the right actuator failed to lock when manually pulled into position, unlike the left side.
Upon disassembly of the right actuator, investigators found a metal sliver within the mechanism and evidence of metal transfer (pick-up) from the bronze locking segments onto the ram surface. Microscopic examination showed significant wear on the non-chamfered edges of the locking segments and the groove in the ram.
Findings
- The right main landing gear failed to lock because one of the bronze locking segments had been installed in the incorrect orientation during a previous maintenance overhaul.
- The incorrect installation prevented the locking sleeve from moving fully into position, which compromised the stability of the lock.
- The improper orientation caused increased contact pressures, leading to metal pick-up and wear on the locking components.
- The difficulty in identifying the correct orientation of the segments during maintenance was exacerbated by the presence of oil on the components.