Moog, the aircraft system and equipment manufacturer, is investigating the highly unusual failure of an elevator feel computer on an Scandinavian Airlines (SAS) Boeing 767-300ER which forced the crew to improvise a solution before returning to land shortly after take-off from Singapore Changi Airport on 13 July.

The computers, which control the aircraft's elevator artificial feel system, are required because the aircraft's flight control surfaces are powered by hydraulic actuators. 767s have two Moog-supplied computers: one primary computer for manual flight and the second for the autopilot system. Moog's preliminary tests showed "no fault" and the company is beginning a tear-down for full analysis.

Boeing says the computer "does not cause a direct position input to the elevator, but monitors resistance and provides feedback to the crew". It says the SAS failure is "an anomaly, and is not something we've seen before".

SAS confirmed that the crew had control problems soon after take-off and, having climbed on autopilot to a safe altitude, found that the manual control problems persisted. The aircraft then landed using autopilot/autoland, and the faulty computer was changed.

SAS' 767 fleet chief pilot (technical) Capt Curt Cronerud checked the aircraft's handling and, finding no problems flew it to Bangkok.

Cronerud confirms that there is no checklist or flight manual procedure for recognition of a feel system failure, so the crew had to find out by trial and error what worked and what did not. With a physical control restriction, however, it is not normal procedure to use the autopilot because it could trip out unexpectedly at a critical moment because of sudden forces on the controls.

Cronerud says consideration is being given to whether the crews need to be better informed about the feel system, and whether appropriate check procedures should be developed.

Boeing says the elevator feel computer was eliminated as a cause of the October 1999 Egyptair 767 accident "fairly early on" in the investigation because it does not apply forces directly to the elevator.

Source: Flight International