History of Flight
The crew operated the second sector of a split-duty nightstop, returning from Glasgow to Birmingham. The captain conducted pre-flight external checks and found nothing unusual. During takeoff, a slight nose wheel shimmy was felt but not considered alarming. The flight continued without incident. Due to reduced visibility, a CAT IIIB autoland was performed on Runway 15. After landing and taxiing to the stand, the dispatcher alerted the crew that the left nose wheel was missing.
Nose Wheel Installation
The nose landing gear (NLG) consists of two independent wheels, each secured by a wheel retaining nut. An abutment ring, which has a chamfered inner diameter on one side, should be installed with the chamfer facing inboard to abut the axle flange without fouling the blend radius. Two locking bolts are inserted into slots in the axle to prevent the nut from unscrewing.
Aircraft Examination
Examination found the right wheel attached but the left axle had only the inboard bearing and the abutment ring. The abutment ring was installed with the chamfer facing outboard, creating a gap between it and the inboard flange. This allowed the retaining nut to be torque-tightened but prevented the locking bolts from engaging the axle slots. Consequently, the nut could unscrew over time due to wheel rotation forces. The missing left nose wheel was found on grass about 150 m to the right of Runway 15; the retaining nut, locking bolts, and outboard spacer were not recovered.
Maintenance History
The nose wheels were changed on 4 February 2003 outdoors on the ramp. Two licensed engineers and a technician performed the task; one engineer changed the left wheel. He had performed this task only once before and referred to the Aircraft Maintenance Manual (AMM). He recalled that the abutment ring stayed on the axle during removal, so he did not check its orientation. He concluded that the ring was likely fitted incorrectly before his work. After the 4 February change, the aircraft completed 32 landings. A subsequent fleet check found no other incorrectly fitted abutment rings.
Previous Incidents
Two previous incorrect abutment ring installations on BAe 146/RJ aircraft resulted in left nose wheel detachment: September 1994 (detached on landing) and July 1997 (detached on takeoff). In both cases, the wheel had been changed two days prior. After the 1994 event, the AMM was amended with explicit instructions and a diagram. After 1997, the operator painted red the chamfered face of the abutment ring and the corresponding axle area as a visual indicator, and placed a warning placard on the NLG strut.
Analysis
The wheel detached due to incorrect assembly of the abutment ring on the axle. The design allows the ring to be fitted backwards, preventing the locking bolts from securing the nut. The locking bolts could still be installed and wire-locked without appearing faulty. The direction of right wheel rotation tends to tighten the nut, so only left wheel incidents have occurred. The three known instances suggest that maintenance error probabilities, though small, are real.