What happened
On 15 December 2014, a Saab 2000, registration G-LGNO, was performing a scheduled flight toward Sumburgh Airport in the Shetland Islands. While flying approximately 7 nautical miles east of the airfield, the aircraft was struck by lightning.
Following the strike, the commander attempted to make nose-up pitch adjustments. However, the aircraft did not respond as the crew anticipated. The aircraft subsequently entered a steep descent, reaching a minimum altitude of 1,100 feet above sea level. During this period, the aircraft pitched to a 19-degree nose-down attitude and accelerated to a speed exceeding its maximum operating limit (VMO) by 80 knots, with a peak descent rate of 9,500 feet per minute. The aircraft eventually began to climb back to its intended altitude.
The investigation
The AAIB investigation focused on the interaction between the flight crew and the aircraft's automated systems. Investigators examined the flight control system, the autopilot, and the Air Data Computer. The investigation included a review of the aircraft's maintenance history and an analysis of the autopilot's certification requirements regarding flight crew override capabilities.
Data from the flight recorders revealed that while the pilots believed they had taken manual control, the autopilot remained engaged. The investigation established that the crew's manual nose-up inputs were being actively countered by the autopilot's pitch trim function. This function was applying nose-down trim in an attempt to maintain the previously selected altitude.
Findings
- The aircraft was struck by lightning while flying in weather conditions involving heavy rain and snow.
- The flight crew's manual pitch inputs were met with opposing nose-down trim from the autopilot.
- The autopilot remained in an engaged state despite the pilot's physical inputs on the controls.
- The aircraft exceeded its maximum operating speed (VMO) by 80 knots during the descent.