What happened
On 18 March 1998, a BAe ATP, registration G-MANG, was performing a scheduled passenger flight from Southampton to Manchester. The aircraft, operated by a commercial carrier, arrived at Manchester Airport with 58 passengers and 4 crew members on board. Following a normal touchdown on Runway 24, the aircraft began its landing roll.
Approximately nine seconds after the commander took control of the aircraft, a violent vibration began to intensify. The oscillation was so powerful that items on the flight deck were displaced and overhead locker lids in the cabin burst open, ejecting contents. The commander, believing a tyre had failed, attempted to steer the aircraft toward a runway exit. However, the nosewheel steering authority was lost, and shortly thereafter, the nose landing gear collapsed. The aircraft came to a halt with the forward fuselage resting on the runway.
During the evacuation, the aircraft's nose-down attitude made the use of rear slides steep. While most passengers evacuated safely via overwing exits and the rear slides, one passenger sustained 1 serious injury after jumping from the wing edge.
The investigation
The AAIB investigation focused on the cause of the intense vibration and the subsequent structural failure. Examination of the runway showed a 'chevron' pattern over 250 metres, characteristic of severe torsional oscillation, known as shimmy. The investigation found that the nose gear pintle mounting structure had been torn from the fuselage due to extreme overload.
Investigators examined the nose landing gear steering system and discovered that the steering cables were badly worn. Furthermore, a detailed strip-down of the nose landing gear at the manufacturer's facility revealed that the 'weight-on-wheels' microswitch had failed in the 'AIR' position. This failure prevented hydraulic pressure from being ported into the steering system, which is essential for providing damping against oscillations.
Findings
- The primary cause of the structural failure was a destructive nosewheel shimmy (torsional oscillation).
- The oscillation became uncontrollable because the hydraulic damping system was ineffective due to a lack of fluid pressure.
- The steering system failed to pressurise upon landing because the air/ground microswitch was stuck in the 'AIR' position.
- This microswitch failure was caused by moisture contamination that led to corrosion of the actuator secondary plunger, which eventually caused the internal mechanism to fail.
- The moisture ingress was likely due to a damaged 'O' ring seal during the assembly of the plunger.
- The presence of worn steering cables likely acted as a contributing factor to the instability of the landing gear.