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| Title: | Uncommanded roll, Boeing 737-236 Advanced, G-BGJI |
| Micro summary: | During an engineering test flight, a Boeing 737-236 experienced an uncommanded roll. |
| Event Time: | 1995-10-22 at 1609 UTC |
| File Name: | 1995-10-22-UK.pdf |
| Publishing Agency: | Aircraft Accident Investigation Board (AAIB) |
| Publishing Country: | United Kingdom |
| Report number: | EW/C95/10/4 |
| Pages: | 72 |
| Site of event: | 15 nm north-west of Bournemouth International Airport |
| Latitude/Longitude: | 50°55.72'N 002°12.55'E |
| Departure: | Gatwick Airport, London, England |
| Destination: | Gatwick Airport, London, England |
| Airplane Type(s): | Boeing 737-236 Advanced |
| Flight Phase: | Cruise |
| Registration(s): | G-BGJI |
| Operator(s): | British Airways |
| Type of flight: | Revenue |
| Occupants: | 2 |
| Fatalities: | 0 |
| Serious Injuries: | 0 |
| Minor/Non-Injured: | 2 |
| Other Injuries: | 0 |
| Executive Summary: | The crew reported at 1330 hrs at Gatwick to carry out a post-heavy maintenance check, test flight on the aircraft. The first officer (F/O) completed the external check, while the commander completed the 'Flight Deck Preparation' items of the aircraft checklist. A Standby (STBY) Rudder system check was carried out with no abnormalities noted and during taxi before take-off, the Yaw Damper indicator showed normal response to turns. When the aircraft was in straight and level flight at FL200 with an indicated airspeed of 290 kt, Autopilot and Autothrottle engaged and Yaw Damper ON, the aircraft experienced roll/yaw oscillations. The Flight Data Recorder (FDR) showed that the Autopilot and Autothrottle were disengaged, and the commander reported that the Yaw Damper was switched OFF but the crew were unable to stop the oscillations. A MAYDAY call was broadcast at 1609 hrs. The crew had the impression that the bank angle would have continued to increase had opposite roll control inputs not been applied. A descent was made to around FL75 and as the airspeed was allowed to reduce towards 250 kt the oscillations began to decay rapidly and stopped. The total duration of the roll/yaw event was about seven minutes. A low speed handling check was carried out, and it was found that the aircraft handled well at a speed 150 kt, with Flap 15° selected and with the landing gear down. It was decided to return to London Gatwick Airport in this configuration, and the MAYDAY was downgraded to a PAN. The crew recovered the aircraft to Gatwick without further incident. The investigation identified the following causal factors: (i) Contamination of the connector on the Yaw Damper Coupler, in the Electronic and Equipment Bay, by an unidentified fluid had occurred at some time prior to the incident flight and compromised the function of its pin to pin insulation. (ii) Sufficiently conductive contaminant paths between certain adjacent pins had affected the phase and magnitude of the signals transmitted to the Yaw Damper Actuator, thereby stimulating a forced Dutch Roll mode of the aircraft. (iii) The location of the Electronic and Equipment (E&E) Bay, beneath the cabin floor in the area of the aircraft doors, galleys and toilets made it vulnerable to fluid ingress from a variety of sources. (iv) The crew actions immediately following the onset of the Dutch Roll oscillations did not result in the disengagement of the malfunctioning Yaw Damper system. Four safety recommendations were made. |
| Learning Keywords: | Operations - Maintenance |
| Operations - Upset - Uncommanded or excessive Roll | |
| Systems - Flight Control System |
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