Dutch roll in a British Airways Boeing 737-200Adv on flight test in October 1995 has been attributed to fluid penetrating connector pins associated with the yaw damper coupler, according to a UK Air Accident Investigation Branch (AAIB) report. The full inquiry had been prompted by fears of uncommanded "rudder hard-over" events generated by two fatal 737 accidents in the USA.

The AAIB report says: "Contamination of the connector on the yaw damper coupler, in the electrical and equipment [E&E] bay [beneath the floor near the doors, galleys and toilets] by an unidentified fluid-compromised the function of its pin to pin insulation. Sufficient conductive contaminant paths between...adjacent pins had affected the phase and magnitude of the signals transmitted to the yaw damper actuator," stimulating Dutch roll.

Recommendations include checks on the ingress of fluid to the E&E bay in all 737s, and Boeing design action to prevent it.

Source: Flight International