But unlike Samuel L. Jackson's 2006 fictional Hollywood blockbuster in which a nest of vipers causes death and destruction on a jet, this reptile was concerned only with self-preservation.

QF191 was about 20 minutes into its 6.15am flight from Cairns to Port Moresby on Thursday when a woman pointed outside the plane and told cabin crew: ''There's a snake on the wing … there's its head and if you look closely you can see a fraction of its body.''

While some passengers scoffed in disbelief, she was correct.

Rick Shine, a snake expert at the University of Sydney, said the specimen was a ''very uncomfortable'' scrub python, the longest snake in Australia.

''There's no way it could be anything else,'' he said. ''They're common in north Queensland. They're ambush predators and if there are rodents anywhere nearby, they'll most likely be in the vicinity. They often find their way into tight ceiling spaces in houses, although I've never heard of one on a plane until now.''

One passenger, Robert Weber, a website designer in Cairns, said: ''The people at the front were oblivious to what was going on but the passengers at the back were all totally focused on the snake and how it might have got onto the aircraft.

''There was no panic. At no time did anyone stop to consider that there might be others on board.''

Mr Weber said initially the snake was tucked away ''quite neatly'' but then the wind caught the last 30 centimetres of its tail, ''pulling him straight out''.

He said that from that moment, everyone watched on as the trip became ''a life and death struggle for the snake''.

''I felt quite sad for it, really. For the remainder of the flight, he was trying to pull himself back into the plane, even though he was fighting against 400km/h winds. The cabin crew told us that at cruising altitude, it was minus 12 degrees outside - but not even that was able to finish him.''

Mr Weber, who videoed the struggle, said both pilots took it in turns to visit the rear of the plane and watch as, several times, the snake hauled itself to safety, only to be dragged out again. As it slowly lost its grip, the wind repeatedly whipped it against the side of the plane, spraying blood across the engine.

''At that point, the pilot turned to us and said: 'He should be dead'. Yet even on descent, the snake was fighting to find safety. ''Until we landed, I looked out the window and the thing was still moving.''

The president of the Australian Licensed Aircraft Engineers Association, Paul Cousins, said: ''It appears as though the snake has initially crawled up inside the landing bay, maybe housed himself in there, and then crawled into the trailing ledge flap assembly.''

The snake would have been comfortable there but after the plane took off and the flaps moved back, it was probably shaken by the noise and vibration. Once it moved, it was caught in the wind. Mr Cousins said it would not have been possible for the snake to reach the cabin from its original location.

A Qantas spokeswoman said: "We have never heard of this happening before.''