What happened
On 7 October 2014, a BAE ATP, registration G-BTPF, was performing a night cargo positioning flight from Bournemouth Airport to London Stansted. While flying manually at FL70, the commander experienced two sudden, violent, uncommanded inputs into the aileron controls. The first event caused the aircraft to roll left with enough force to pull the control wheel from the pilot's hands, requiring significant physical effort to correct. Shortly after, a second uncommanded input caused a roll to the right.
During the descent back to Bournemouth, the crew noted that the cockpit's fluorescent floodlights were flickering. Upon landing and parking the aircraft, the crew discovered an acrid electrical smell and observed smoke emanating from the forward lower belly hatch. No injuries were reported, and there was no damage to the aircraft.
The investigation
The AAIB investigation examined the aircraft's primary flight controls, the Aileron Standby Control System (SCS), and the electrical distribution systems. Although the flight data recorder (FDR) showed fluctuations in aileron position and roll attitude, the 1 Hz sampling rate was too slow to capture the rapid, high-force movements reported by the pilot.
Subsequent maintenance inspections revealed that some aileron cable tensions were slightly outside the manufacturer's recommended limits. Additionally, investigators found a chafed wire and corroded contacts on the left aileron position synchro transmitter. However, these components were not part of the system that would have triggered the SCS warnings or caused the reported uncommanded inputs without alerting the crew.
Findings
- The investigation could not identify any mechanical or electrical fault capable of causing the reported uncommanded aileron inputs.
- No specific cause for the smoke or the acrid smell could be established, as inspections of the forward lower fuselage area and electrical components showed no evidence of heat or smoke damage.
- The aircraft's electrical systems, including the No 1 TRU and No 2 Inverter, were replaced as a precaution, but testing of overhauled components showed no faults.
- The reported fluctuations in normal acceleration during the event were consistent with a period of turbulence, though the crew reported no significant turbulence at the time.