Event Details

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
Site of event:15 nm north-west of Bournemouth International Airport
Latitude/Longitude:5055.72'N 00212.55'E
Departure:Gatwick Airport, London, England
Destination:Gatwick Airport, London, England
Airplane Type(s):Boeing 737-236 Advanced
Flight Phase:Cruise
Operator(s):British Airways
Type of flight:Revenue
Serious Injuries:0
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
Close match:Uncommanded roll, Serious incident on March 20, 2001 at Frankfurt/Main, to Airbus Industrie A320-200
Excessive right aileron required on Boeing 737-400, EI-BXB, at Dublin Airport
Flight control system failure, Report on the incident to Airbus A320-212, G-KMAM, London Gatwick Airport, on 26 August 1993
Aileron control cable failure on a Boeing 737-3TO on takeoff at Seattle, September 27, 1997
Uncommanded roll to the left, Boeing 737-3B7, August 5, 1995
Uncommanded roll, Douglas DC-8-71F, December 14, 2001
Uncommanded roll, McDonnell Douglas DC-9-15, July 12, 1997
Uncommanded roll during cruise, Airbus A320-211, April 28, 1995
Uncommanded roll and yaw, Douglas DC-9-34, April 30, 1998
Control difficulties, McDonnell Douglas MD-11, October 21, 1998
Reduced roll capability, Boeing 747-422, April 1, 2003
Roll control difficulties, McDonnell Douglas MD-11F, N583FE, January 15, 2003


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