What happened
On 15 December 2014, a Saab 2000, registration G-LGNO, was performing a flight toward Sumburgh Airport in the Shetland Islands. While flying approximately 7 nautical miles east of the airport, the aircraft was struck by lightning. Following the strike, the pilot attempted to make nose-up pitch inputs to maintain control. However, the aircraft responded unexpectedly, pitching down to a minimum of 19 degrees. This maneuver caused the aircraft to exceed its maximum operating speed (VMO) by 80 knots, reaching a peak descent rate of 9,500 feet per minute. The aircraft eventually stabilized and began to climb after descending to 1,100 feet above sea level.
The investigation
The investigation focused on the interaction between the flight crew's manual controls and the aircraft's automated systems. Investigators examined the flight data and cockpit voice recordings to determine why the aircraft failed to respond to the pilot's nose-up commands. The inquiry also reviewed the aircraft's maintenance history, the meteorological conditions involving thunderstorms, and the certification requirements for autopilot systems regarding flight crew override capabilities.
Findings
- The investigation established that the autopilot remained engaged throughout the event, despite the pilots' belief that they had taken manual control.
- A critical finding was that the autopilot pitch trim function actively countered the pilot's nose-up inputs by applying nose-down trim in an attempt to maintain the previously selected altitude.
- The lightning strike occurred during a period of significant weather, including heavy rain, snow, and high winds.
- The aircraft's descent was characterized by a significant excursion beyond the structural speed limits of the airframe.
Safety action
Following the analysis of the autopilot's behavior during the crew override, five safety recommendations were issued. These recommendations specifically address the design of autopilot systems and the regulatory certification requirements to ensure that automated trim functions do not create hazardous flight conditions when pilots attempt to manually override the flight guidance system.