An AAIB report shows a Saab 2000 passenger plane suffered altitude deviations when the pilot attempted to climb using the flight controls while the autopilot was still engaged in altitude tracking mode. The aircraft had just been struck by lightning during an aborted approach.
On December 15, 2014, a Flybe Saab 2000, operated by Loganair, operated a scheduled passenger service from Aberdeen, Scotland, to Sumburgh Airport, Shetland Islands. Weather at Sumburgh was poor with heavy rain and snow and strong winds. The captain, who was Pilot Flying, descended to 2,000 ft amsl on the approach and established on the localiser approximately 9 nm east of the airport. The aircraft’s weather radar showed a convective cloud cell immediately west of the airport, and the captain decided to discontinue the approach, informed the controller, and turned the aircraft onto a southerly heading. The autopilot remained engaged with heading select and altitude tracking modes selected.
As the aircraft rolled out on the heading, it was struck by lightning, which entered the airframe at the radome and exited at the APU exhaust (in the tail).
The captain began making noseup pitch inputs, which he augmented with nose-up elevator trim inputs using the pitch trim switches on the control yoke. The co-pilot transmitted a MAYDAY to ATC, and the controller offered the flight crew “all options” for an approach or diversion.
The aircraft climbed, but the captain perceived that his increasingly aggressive control column inputs did not appear to be having the expected effect. The co-pilot also applied nose-up pitch inputs and pitch trim inputs, but similarly perceived that the aircraft was not responding as expected. Pitch and roll mis-trim indications were presented on the primary flight displays (PFDs). Both pilots considered the possibility that they had lost control of the aircraft, perhaps because of a failure of the fly-by-wire elevator controls following the lightning strike.
As the aircraft reached 4,000 ft amsl, the pitch attitude tended towards nose-down and a descent began. Invalid data from one of the air data computers then caused the autopilot to disengage. The pitch trim was, by this time, almost fully nose-down, and the aircraft continued to pitch nose-down and descend; full aft control column inputs were made. The peak rate of descent was 9,500 feet per minute at 1,600 ft amsl, pitch attitude reached 19° nose down, and the highest recorded speed was 330 KIAS (VMO, Maximum Operating Speed, was 250 KIAS).
The pilots maintained nose-up pitch inputs and the aircraft began pitching nose-up. Nearing the minimum height achieved of 1,100 ft amsl, the ground proximity warning system generated ‘sink rate’ and ‘pull up’ warnings. The captain applied full power, and the aircraft began climbing. He was still under the impression that elevator control response was not normal, and instructed the co-pilot to select the pitch control disconnect. The co-pilot queried this instruction, because the pitch control did not appear to be jammed, and the commander selected the disconnect himself. This
disconnected the two elevator control systems from each other; each control column remained connected to its respective (on-side) elevator.
The climb continued and the aircraft diverted to Aberdeen. The flight crew ascertained that the aircraft responded to pitch inputs made on either or both control columns. The diversion and landing were uneventful.
The investigation continues; exploring crew training, autopilot design requirements, the human-machine interface, including the autopilot system and other human factors of relevance to the incident.