An interim report by the Air Accident Investigation Branch said that just moments later, a ball of lightning appeared in the cockpit and a bolt struck its nose, travelling the full length of the plane before leaving at its tail.
But a misunderstanding involving the autopilot system meant that the commander and co-pilot then struggled to regain control of the aircraft as it descended at high speed.
The report said that they wrongly believed the system had disengaged. As the co-pilot declared a Mayday, they aggressively tried to gain height, whilst every move was countered by the autopilot.
The more aggressive his movements on the control column, the less effect it had.
When it reached 4,000ft, the plane suddenly pitched nose down and started falling at 9,500ft a minute, giving the crew barely 20 seconds to act.
At 1,100ft, as "pull up" alarms sounded in the cabin, the captain applied full power and the aircraft finally started to climb.
The plane diverted to Aberdeen where it landed safely, with only minor damage, and shaken passengers disembarked.
The report said the crew may have thought the lighting strike had disabled the autopilot because it had knocked out some of the other controls.
But in fact, it was still operating and trying to adjust in order to fly at the level it has been instructed.
Only when the computers became overloaded with faulty data during the plunge did it disengage itself and give the pilot seconds to save the flight.
After the incident, which occurred on the evening of December 14, passenger Shona Manson said it was only after they landed in Aberdeen, when the shaken captain came out of the cockpit to speak to passengers, that she realised how potentially serious the incident could have been.
"It was really, really bumpy," she said. "If it was someone who's a bad flyer, it'd be their worst nightmare.
"We were on descent and I said to my partner, we're going back up again, and just as we started to go up again there was an almighty bang and a flash that went over the left wing.
"Then we were really ascending, and at that point there were a few folk looking around going 'oh my God, what's happening?' The poor guy across the aisle from me just had eyes like rabbits in headlights."
She said that one Glaswegian man was so shaken that he decided not to get back on the plane the next day and headed home instead.
The AAIB report said: "Although the pilots' actions suggested that they were under the impression the autopilot had disengaged at the moment of the lightning strike, recorded data showed that it had remained engaged."
The report said no technical problems were found with the plane, which is now back in service, and that pilot training now included simulations of this incident.
The AAIB investigation is continuing, looking at crew training, autopilot design, and any "human factors".
- Daily Telegraph
