What happened
On May 10, 2012, a Beech-200 King Air, registration EC-KNP, was performing a non-commercial ferry flight from Blois to Paris le Bourget. During the ILS 27 approach, the crew noticed an unusual noise and observed that the nose gear uncommanded extension indicator was illuminated, while the green down-lock light remained extinguished. After an initial go-around, the crew attempted to extend the gear using the emergency extension procedure, but the symptoms persisted.
To avoid obstructing the main runway, the crew opted for an approach on runway 21. During the landing roll, the nose gear unexpectedly retracted, causing the aircraft to come to a halt on the runway. The incident resulted in damage to the propellers and the lower forward fuselage.
The investigation
The investigation focused on the mechanics of the nose gear actuator. Upon disassembly, investigators found that the actuator rod could move freely within the body, accompanied by a metallic noise. The internal components showed metal shavings and grease. Specifically, the threads of the threaded sleeve had been stripped, preventing the rod from being driven effectively. The internal clearance (play) between the rod threads and the sleeve threads was measured at 0.6 mm, exceeding the manufacturer's limits.
Investigators also examined the maintenance history at the Blois maintenance facility. While the facility claimed to use Nyco Grease GN17, spectrometric analysis revealed that the grease found inside the actuator was either an older version of the grease (pre-2003) or from a different manufacturer. Furthermore, the tools used to measure internal clearance were locally manufactured, unapproved, and lacked the precision necessary to detect excessive play.
Findings
- The primary cause of the incident was inadequate maintenance of the nose gear actuator, which led to the accelerated degradation of the sleeve threads.
- The use of unapproved, locally made measurement tools contributed to the failure to detect excessive internal clearance.
- The presence of incompatible or incorrect grease may have compromised lubrication efficiency.
- The actuator rod was an FAA PMA replacement part rather than an original equipment manufacturer (OEM) part.