What happened
On 3 November 2000, a Fokker F28 Mark 0100, registration G-BYDN, was performing a scheduled passenger flight from Newcastle to Paris Charles de Gaulle. During the descent to 11,000 feet, the aircraft began to oscillate in pitch. Upon the crew disconnecting the autopilot, the pilot found the control column was nearly immobile, with only minimal fore and aft movement available. The commander managed to regain some control by applying significant force, but the aircraft continued to experience pitch instability.
As the situation deteriorated, the crew declared a Mayday. During the high-workload period, autopilot 1 was inadvertently engaged. This caused the aircraft's flight control system to actively oppose the crew's manual inputs, as the system attempted to maintain a pre-selected flight path. To assist with the pitch-up tendencies, the commander instructed passengers to move to the forward seats to shift the aircraft's center of gravity. After managing the control difficulties and performing a radar-vectored approach, the aircraft landed safely on Runway 2/7 Right without injuries to the 71 passengers or 5 crew members.
The investigation
The AAIB examined the aircraft's flight recorders and the physical components of the flight control system. Investigators found that the elevator servo cables were slack and noted excessive friction in the manual trim wheel. Testing of the No 2 elevator servomotor revealed an insulation failure caused by carbon dust accumulation from worn motor brushes.
Analysis of the flight data showed two distinct periods of pitch instability. The investigation also looked into the possibility of ice accumulation on the elevator servo capstans, a known issue in certain temperature and humidity conditions that could impede cable movement. While the exact presence of ice could not be confirmed, the symptoms were consistent with previous reports of such occurrences.
Findings
- The initial restriction of elevator movement was likely caused by ice accretion on the elevator servo capstan.
- The inadvertent engagement of autopilot 1 significantly increased the crew's workload and exacerbated the control difficulties.
- The crew's preoccupation with the primary control issue prevented them from noticing the visual alert on the flight display indicating that autopilot 1 had been engaged.
- The aircraft's stabilizer attempted to override the crew's manual inputs because the autopilot's control authority is greater than that of the pilot's manual elevator inputs.