What happened
On 15 December 2014, a Saab 2000, registration G-LGNO, was performing a commercial flight near Sumburgh Airport, Shetland. While the crew was executing a diversion due to heavy rain, snow, and thunderstorms, the aircraft was struck by lightning. The strike entered through the radome and exited via the APU exhaust.
Following the strike, the crew attempted to maintain altitude by applying nose-up pitch and trim inputs. However, the pilots perceived that the aircraft was not responding to their manual controls. As the aircraft reached 4,000 ft amsl, it entered a significant descent, reaching a peak rate of 9,500 ft/min and exceeding its maximum operating speed by 80 KIAS. The aircraft eventually reached a minimum height of 1,100 ft amsl before the crew managed to initiate a climb and divert to Aberdeen.
The investigation
The AAIB examined flight data, cockpit voice recordings, and ATC communications. The investigation focused on why the aircraft failed to respond to the pilot's nose-up inputs. Physical inspections of the aircraft revealed only minor damage to the radome and APU exhaust, with no technical malfunctions found in the elevator or autopilot systems during subsequent testing.
Findings
- The lightning strike caused only minor structural damage.
- The autopilot remained engaged despite the crew's belief that it had disconnected.
- Because the autopilot was in altitude tracking mode, it interpreted the pilots' nose-up manual inputs as a disturbance and responded by applying prolonged nose-down pitch trim to maintain the selected altitude.
- This automated compensation created the illusion of a flight control failure or jammed elevator.