Incident Description
On 10 May 2012, a Beech 200 King Air (EC-KNP) operated by Air Taxi and Charter International was conducting a non-commercial ferry flight from Blois to Paris Le Bourget. The two-pilot crew (captain as PF, copilot as PNF) performed an ILS approach to runway 27. During landing gear extension, an unusual noise was heard, the nose gear red transit light illuminated, and the green lock light remained off. The air traffic controller reported the nose gear appeared down. The crew initiated a go-around and retracted the gear. The red light remained on, so they performed the emergency extension procedure with the same indications. They then carried out an approach to runway 21 to benefit from a headwind and avoid blocking the main runway. During the landing roll, the nose gear retracted. The aircraft came to a stop on the runway. Damage was found to the lower forward fuselage and both propellers.
Investigation Findings
Examination of the nose gear actuator revealed that the threaded sleeve's threads had been torn away, preventing the actuator rod from being driven. Prior to the tear, the threads had experienced progressive wear due to friction between the sleeve and the threaded rod. The clearance between the rod and sleeve threads measured 0.6 mm. The actuator rod was not a manufacturer original part but an FAA-approved PMA replacement part. Grease found inside the actuator was analyzed by spectrometry and found to have a thickener different from the Nyco Grease GN17 used by the maintenance shop. It could have been an older version of Nyco Grease GN17 (whose thickener changed in 2003) or grease from another manufacturer. No water was detected in the grease.
Maintenance Issues
The last internal clearance check on the actuator was performed on 19 November 2010, with 210 cycles remaining before the next due check per the maintenance manual. The manual does not require recording clearance values, and the shop stated that each check found clearance below 0.40 mm. However, the lubrication task associated with the clearance check was not listed on the work order, and no lubrication was recorded. The tool used to measure internal clearance was locally made, not listed in the approved tooling of the maintenance organization, and not approved by the oversight authority. A demonstration showed the measurement was difficult and the tool's precision was insufficient for the required tolerances.
Similar Events
The investigation identified five similar cases of torn threads in the actuator's threaded sleeve. Three had probable causes determined: C-GISH (18 May 2002) from lack of lubrication; G-BYCP (24 March 2007) involved an FAA PMA actuator with water-contaminated grease; and F-GFDJ (9 May 2011) from inadequate maintenance on lubrication.
Probable Cause
The serious incident was due to inadequate maintenance of the nose gear actuator, leading to accelerated degradation of the threaded sleeve. The use of a locally designed unapproved tool may have contributed to the non-detection of excessive internal clearance.