Unknown to the flight crew, Captain Ray Cockerton and first officer Andrew Redknapp, a torch had been left in the right-hand wing. During take-off which, unusually, was at full power because of the poor conditions, the torch lodged in the aileron, the flap which is used to turn the plane.
It had been left there by an engineer at Luton the day before and had not caused any problem in three previous flights.
Mr Redknapp was flying the aircraft on auto-pilot at around 4,000 feet, soon after take-off, when he tried to turn left. The controls jumped out of the auto-pilot mode. After a second failed attempt, Captain Cockerton took over the controls with the auto-pilot disengaged.
"Having controls which don't respond is the classic pilot's nightmare," he said. He used the rudder, which is not normally operated during flights, to reach the right heading, get away from the mountains and over the sea.
The two flight crew then tried to find out what was wrong. Mr Redknapp went into the cabin but could see nothing amiss on the wings.
They went through the check list and contacted the Monarch engineers who had serviced the plane at Luton. The emergency check-list procedure requires both pilots to push as hard as possible to free the controls. When they did, the plane went into a violent lurch, banking at 60 degrees.
Captain Cockerton considered his options but bearing in mind the bad weather in the Canaries, which made any landing impossible, decided to press on to the UK.
"At this stage, I thought we had a 50 per cent chance of getting down safely," Captain Cockerton said.
He began to consider which airport in Britain to land at and declared a full emergency. He chose Manchester because the direction of the runway and the wind was most appropriate to his situation and wind direction and he knew that the airport had good emergency services. Over the Welsh hills, he and his first officer both once more tried to dislodge the blockage but the plane again lurched violently .
The two thought that the problem lay with a jammed aileron at the end of the right wing but had no idea what had caused it. While the six cabin crew had been informed, they did not tell passengers - to avoid causing panic.
Captain Cockerton managed to bring the plane in at a small angle to the runway to allow for the wind but the aileron was still jammed at touchdown.
However, the report produced by the company for the Civil Aviation Authority suggests that emergency action had solved the problem, resulting in a normal manual landing: "I don't think the report conveys the gravity of the situation," he said.
After landing, according to the CAA's report, "the crew demonstrated the problem, at which point a large metal torch was forced through the wing below the right hand aileron".
The passengers were never told about what had caused the incident, but 30 refused to take the plane provided by Airtours to take them back to East Midlands and travelled by land.
At his home in the east Midlands, Captain Cockerton wrote a four-page report and was told by the company that there would be a debriefing session with all the crew. This never materialised.
Apart from a thank you letter from Airtours managers, the incident was barely mentioned again and within a week he was back on duty.
Captain Cockerton said that was a mistake: "I felt occasionally tearful but otherwise I was apparently fine for three or four months, then I started getting flashbacks, insomnia and irritability."
Eventually, in June, he felt that his mental state warranted reporting sick and he sought medical advice He was diagnosed as suffering from post- traumatic stress syndrome and referred to a specialist for successful counselling sessions. Captain Cockerton resigned a few days later and now flies for another leading airline.
As for the engineer who left the torch, he put up a big notice in his mess at Luton asking if anyone had seen his torch. Last night, Monarch refused to comment on whether he was disciplined.