The UK Air Accidents Investigation Branch (AAIB) has recommended that regulators review certification requirements for autopilot systems and order modifications to Saab 2000 systems, based on the probe of an inflight incident two years ago.
The Dec. 15, 2014, incident began when a Flybe Saab 2000, on approach in instrument conditions with the autopilot engaged, was struck by lightning while following a pilot’s input to vector away from the approach to Scotland’s Sumburgh Airport. The captain—assuming the lightning strike had disengaged the autopilot—attempted to climb via nose-up inputs, but the aircraft, programmed to maintain a 2,000-ft. altitude, counteracted the manual commands with nose-down pitch trim inputs. The flight crew finally regained control of the aircraft, but not until it descended to 1,000 ft. and the autopilot was disengaged when the system detected invalid data from an air data computer.
“The commander applied and maintained full aft control column (nose-up elevator) input; however, the autopilot’s nose-down elevator trim authority exceeded the commander’s elevator nose-up authority and the aircraft pitched nose-down and descended, reaching a peak descent rate of 9,500 ft./min.,” the AAIB found. “If the autopilot system had been designed to sense pilot-applied control forces and to disengage when it sensed a significant force . . . the autopilot would have disengaged shortly after the aircraft climbed above 2,000 ft., in response to the aft column inputs applied by the commander.”
The Saab 2000 is unusual in that it combines three attributes: Override forces do not disengage the autopilot; engaging main pitch trim switches has no effect on the autopilot; and the autopilot can input trim that is opposite of manual control column input. Other models with similar attributes, including the Airbus A300 and Fokker 70/100, underwent system redesigns.
The AAIB recommends Saab’s system undergo a similar redesign. It also urges the FAA and EASA to review autopilot system design “to ensure that the autopilot does not create a potential hazard when the flight crew applies an override force to the flight controls.”
Saab issued an operator’s message in February 2015 highlighting autopilot disengage procedures. Flybe briefed its pilots and now puts new pilots through a comparable simulator scenario.