AT 11 minutes past nine on the evening of October 20, 1948, a Lockheed Constellation took off from Schiphol Airport at Amsterdam, bound for New York via Prestwick with 30 passengers and a crew of 10. It never made it. At 26 minutes to midnight, while trying to land at Prestwick, it crashed on Auchinweet Farm, a mile-and-a-half east of Tarbolton and four miles from the airport. Everyone on board perished.

What went wrong? What caused - if we exclude Lockerbie, which was an act of sabotage - Scotland's worst ever air accident?

The facts are in themselves simple.

It was a typical October night in Ayrshire - rain, low cloud, and a south-westerly blowing. The captain of the aircraft was Dirk Parmentier, one of the most experienced pilots of his generation - someone who knew Prestwick Airport well.

About 70 miles from Prestwick, First Officer Kevin O'Brien made radio contact with the airport. The plane would be landing on Prestwick's longer runway, which ran in a north-westerly direction.

But while still about 100 to 200ft above it - and therefore below the cloudbase - the Constellation turned south and started climbing. Because of the crosswind - the south-westerly - it would land on the shorter runway, which ran in a north-easterly direction. The plane then turned east, intending to get into position to make a new approach.

At 28 minutes to midnight the crew radioed that the aircraft had hit something and was on fire. As they wrestled with the controls, the aircraft passed over Tarbolton.

The last message from the flight crew was an anxious: ''Have you any idea where we are?'' It then ploughed into the ground at Auchinweet Farm, a mile-and-a-half north-east of Tarbolton. Thirty-four people were killed outright, with the remaining six dying in hospital.

An examination of the wreckage established what the ''something'' the aircraft had hit were power cables. They stretched across the countryside on pylons 1000 yards apart. The Constellation had gone through them where they crossed Langlands Farm, almost due west of Tarbolton. But these cables were no more than 40ft above ground, which meant that the Constellation had been flying incredibly low.

A public inquiry was held at Ayr Sheriff Court the following January. It sat for 10 days, hearing evidence from aviation and crash experts, as well as eye witnesses from Tarbolton.

The most obvious explanation for the accident - technical failure - was dismissed. Everything in the plane seemed to be working perfectly, and it had been properly maintained since it entered service in 1947.

The inquiry failed to come up with one overriding reason for the crash. It had to content itself with what it called in its report ''the coincidence of a number of adverse circumstances''.

At his pre-flight briefing, Captain Parmentier had been told that the cloud base at Prestwick was between 800 and 1000ft, with a moderate south-westerly blowing.

The Constellation had been due to take off at 9.30pm, but instead took off at 9.09. Five minutes before this, a fresh weather report from Prestwick stated that the cloud base was now down to 600-1000ft. The flight crew never received the information.

As they headed north-west over England, the situation at Prestwick worsened. The cloud base was now 300ft, and the south-westerly was blowing at an estimated 50mph. Even when the aircraft made radio contact with the airport, this information was not passed on, because it wasn't ''normal procedure'' to do so.

The low altitude at Tarbolton was soon explained. On charts used by KLM pilots, the height of the land around the village was given as 45ft above sea level, when in fact it was nearer 450. This error had first been made on American Air Force charts, and KLM had copied the figure without checking. So while the crew thought they were flying at about 400ft over Tarbolton, the aircraft was actually 40ft above the ground.

The cause of the fire was the 132,000 volts which went through the plane as it hit the cables. It set the fuel tanks alight, and as the plane passed over Tarbolton just above roof-top height, eye witnesses stated that it was on fire.

KLM's flight manual forbade pilots to land on Prestwick's shorter runway when the cloud base was below 700ft. Yet Captain Parmentier must have known the height of the cloud base was 300ft, even if it hadn't been reported to him. He had dropped below it on his first approach. So why was he going to attempt a landing on the shorter runway? The inquiry considered various explanations, but never reached a conclusion. One later theory was that, while climbing away from the first approach, he noticed something wrong with an engine, and decided to make a landing no matter the risks involved.

Certainly, the position of the fire extinguisher selection switch in the cockpit had been moved to the number two engine position. Not only that, the blades on this engine had been so coarsely pitched by the pilot that its power was reduced. By the time this theory was put forward, the engines had been scrapped, so it couldn't be tested. The main argument against this was that the crew never reported any malfunction. Then again, they might have been too busy dealing with it to do so.

The inquiry report made three main recommendations. All ''navigational instructions'' about UK airports held by airlines should be submitted to the Ministry of Civil Aviation for checking.

Deteriorating weather conditions should be passed from meteorologists to air traffic control staff, who would pass them on to flight crews.

And meteorologists should be trained in flying techniques so that they knew the kind of information required by aircraft pilots.

But the main lesson learned was that accidents need not happen because of one catastrophic mechanical failure or pilot error. They could be due to a catalogue of minor happenings which in themselves are relatively unimportant, but which taken together cause death and destruction.

That's probably what happened on that dirty October night at Tarbolton 50 years ago.