What happened
On 17 April 2017, a Slingsby T67M MkII Firefly, registration G-BNSP, was conducting a private flight involving local aerobatics and circuit testing at Turweston Aerodrome. Following a successful landing, the pilot applied brakes and began a 180-degree turn while taxiing at a walking pace. During this maneuver, the nose of the aircraft suddenly dropped, causing the aircraft to come to an immediate and abrupt stop. Upon exiting the cockpit to inspect the damage, the pilot discovered that the nosewheel had detached from the landing gear fork and was lying on the runway.
The investigation
Investigators examined the nosewheel and fork assembly, determining that the axle had migrated out of position, which allowed the wheel to separate from the fork. The retaining K-nut, which is intended to secure the axle, was missing and could not be located. The aircraft had undergone its annual service only 16 flying hours prior to the event, during which time the nosewheel assembly had been disturbed.
Technical analysis of the maintenance manual revealed that instructions for fitting the nosewheel were not highly specific. The investigation considered several contributing elements, including the possibility that the axle had been installed from the left side, creating a rotational torque that could loosen the nut during steering. The condition of the K-nut, the presence of grease on the axle bolt, and the potential for a worn nut were also scrutinized.
Findings
- The nosewheel axle migrated out of the fork, leading to the loss of the nosewheel assembly.
- The retaining K-nut was missing from the scene.
- The installation of the axle from the left side may have created a tendency for the nut to undo during wheel rotation.
- The presence of grease on the axle bolt and the potential for a worn K-nut may have compromised the locking capability.
- While no official data supports a widespread trend of this specific failure, some maintenance organizations had previously implemented a safety split pin to prevent similar occurrences.
Safety action
Following an assessment of the incident, the Type Certificate holder is evaluating a modification to provide more positive locking for the axle assembly. They intend to issue a Service Bulletin and will consult with the CAA regarding the potential for a mandatory Airworthiness Directive.