What happened
On 15 December 2014, a Saab 2000, registration G-LGNO, was performing a commercial flight near Sumburgh Airport, Shetland. While flying on a southerly heading following a decision to abort an approach due to heavy rain and snow, 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 using nose-up pitch and trim inputs. However, the pilots perceived that the aircraft was not responding to their controls. As the aircraft reached 4,000 ft amsl, it entered a significant nose-down pitch, reaching a maximum descent rate of 9,500 ft/min and exceeding the maximum operating speed by 80 KIAS. The aircraft reached a minimum altitude of 1,100 ft amsl before the crew managed to initiate a climb and divert to Aberdeen.
The investigation
The AAIB investigation examined flight data, ATC communications, and the aircraft's physical condition. Inspectors found minor damage to the radome and APU exhaust consistent with lightning. While the crew initially suspected a failure of the fly-by-wire elevator controls, subsequent functional tests of the flight control and autopilot systems revealed no technical malfunctions.
Findings
- The aircraft was struck by lightning, which caused only minor structural damage.
- The autopilot remained engaged despite the crew's belief that it had disconnected following the lightning strike.
- Because the autopilot was still in altitude tracking mode, it interpreted the pilots' manual nose-up inputs as a disturbance and applied prolonged nose-down pitch trim to attempt to maintain the selected altitude.
- The autopilot only disengaged once invalid data from an air data computer caused a system disconnect during the descent.