A Sumburgh-bound aircraft carrying 30 passengers nose-dived after being struck by lightning because efforts by the crew to lift the aeroplane’s nose and climb were overruled by the onboard autopilot system.
An air accident report into the December 2014 incident has found the Loganair Saab 2000 fell rapidly after being struck on its journey from Aberdeen.
Passengers spoke at the time of hearing a bang like a “gunshot” along with a big white flash.
The plane quickly lost altitude and reached a peak descent rate of 9,500 feet per minute. However, the aircraft started to climb after reaching a minimum height of 1,100 ft above sea level.
It flew back to Aberdeen where it made a safe landing.
The Air Accident Investigation Branch has made a series of safety recommendations concerning the autopilot system.
In a 74-page report released today, the AAIB says: “During the approach phase of a routine flight the aircraft was struck by triggered lightning. Procedures intended to prevent flight in areas where lightning may be encountered do not protect against triggered strikes.”
The report adds that the lightning caused only minor damage. Functional tests and inspections carried out after the aircraft had landed did not reveal any faults.
However, it states evidence from the manufacturer’s simulation work indicated the aircraft had responded “as expected” to the recorded control deflections.
“The commander’s actions following the lightning strike were to make manual inputs on the flying controls, which appear to have been instinctive and may have been based on his assumption that the autopilot would disconnect when
“However, the autopilot did not disconnect and was attempting to maintain a target altitude of 2,000 feet amsl [above mean sea level] by trimming nose-down while the commander was making nose-up pitch inputs.
“The control forces felt by the commander were higher than normal because the autopilot was opposing his inputs and he may have attributed this to a flight control malfunction caused by the lightning strike.”
The report adds that the commander applied and maintained “full aft control column (nose-up elevator) input”. However, the autopilot’s “nose-down elevator trim authority exceeded the commander’s elevator nose-up authority and the aircraft pitched nose-down and descended, reaching a peak descent rate of 9,500 ft/min”.
“The autopilot then disengaged … and this allowed the commander’s nose-up pitch trim inputs to become effective. “The aircraft started to pitch up just before reaching a minimum height of 1,100 ft above sea level.
“If the autopilot system had been designed such that operating the pitch trim switches resulted in autopilot disengagement, the autopilot would also have disengaged early in the sequence of events.”
The main thrust of the five safety recommendations are that the design of the Saab 2000’s auto-pilot system be reviewed and modifications made to ensure the system “does not create a potential hazard when the flight crew applies an override force to the flight controls”.
The autopilot did not disconnect and was attempting to maintain a target altitude of 2,000 feet.
Loganair’s director of flight operations Andy Thornton said: “We welcome the findings of the report published by the AAIB and take on board its content.
“Immediately following this incident, Loganair issued enhanced guidance in consultation with the aircraft manufacturer to all company pilots flying the Saab 2000.
“We also amended our training syllabus and worked with our third-party provider to augment and refine training procedures to include a scenario similar to that experienced by our flight crew in December 2014.
“Our pilots regularly go through extensive training and Loganair prides itself on the excellent standards and quality of its pilot workforce.”