Air accident investigators will piece together the events leading to the crash-landing of BA flight 038 before being sure what lay behind the drama.
The force of the impact embedded the engines in the ground
But as the full facts emerge, it may well be that the 152 people on board - and perhaps many more on the ground - are shown to have had an extraordinary escape from tragedy.
Early information suggests the plane suffered some sort of drastic technical failure, pitching the pilot into a struggle to control the final moments of descent.
Key to the accident inquiry will be information from the plane's flight recorders, witnesses and the pilot himself.
External factors including the weather will also be under the spotlight.
The pilot has himself described losing all power, an airport worker has told the BBC's Angus Crawford.
"He just told me... the aircraft just shut down... He lost all his power and everything," the unnamed worker said.
"He said to me he had no warning, absolutely nothing at all. It's just suddenly 'Boom'. It's just lost absolutely everything. It's a miracle, the man deserves an absolute medal as big as a frying pan."
The pilot, he said, had been forced to lift the plane's nose and glide the remaining short distance to Heathrow.
The account echoes that of eyewitness Martin Green, who works at the airport.
"It came in at a very high angle and just dropped like a stone. It seemed to be flying fairly slow and it had a very high angle of attack. The nose was high up in the air, which is very unusual," he told Sky News.
Witness Neil Jones - who holds a general aviation pilot's licence - said his attention was attracted by the louder-than-normal noise the plane was making as it banked low over houses.
"You could see the pilot was desperate, trying to get the plane down," he told BBC News 24. "The aircraft hit the grass and there was a lot of dirt. The pilot was struggling to keep the plane straight. I think he did a great job."
At the time, weather conditions were favourable and visibility good, although there was a gusty wind which could conceivably have played a role.
The task of investigators from the Air Accidents Investigation Branch (AAIB) will be much easier than in many cases, as the flight recorders and wreckage are easily accessible and not deep under water or damaged by fire.
The fact that all crew and passengers survived and can be interviewed will also help build up a detailed picture of exactly what happened and over what timescale.
Already it is clear that passengers were given no early warning of a problem with the plane.
If the pilot's actions did avert a tragedy, the fact that the Boeing 777 plane then withstood the crash-landing so well was also key to saving lives.
It is the first crash anywhere in the world involving one of the planes in the more than 10 years they have been in service, says air transport expert David Learmount.
Mr Learmount also suspects investigations may focus on a sudden, late loss of power.
"I can't believe that this is a straight error of judgement by the pilots. BA pilots don't make error of judgements of that type, especially not at the home base, let alone anywhere else in weather conditions that were not particularly demanding in an aircraft that has an absolutely first class safety record," he said.
"So in the end it makes me wonder if the pilots, at a fairly late stage in the approach, had a major technical problem on their hands that they were fighting with," he said.
"The fact that the aircraft was so far short, in such good visibility, gives some kind of indication that the plane was suffering very late in the approach from a lack of power. We don't know," Mr Learmount added.
Another aviation expert, Sean Maffett, told BBC News the fact such a thing could happen to a "large, sophisticated new aircraft with an excellent safety record" was "extraordinary".
He suggested the cause could be either fuel starvation or "wind shear", where the winds change rapidly in very squally conditions. This can cause planes to stall or "fall out of the sky", he explained.
He added: "I'm afraid the final possibility is that the crew made a big mistake of some sort."
Investigators will certainly want to establish whether both engines were working at the time of the crash - although even if one had failed, the plane should still have been capable of a "normal" undramatic landing.
Rolls Royce, which makes the plane's engines, will offer the investigation team any help needed.
The plane is relatively new, at about six years old, and is one of 43 in the BA fleet.
As to what might have sparked a loss of power in a plane as reliable as the Boeing 777 - and maintained by an airline of BA's standing - birds will be among the suspects.
If, for example, a flock of geese or other large birds had been hit, the damage could conceivably have been sufficient to knock out or seriously compromise both engines.
Some speculation has also focused on a possible fuel shortage but even if levels had run low, ample warning should have been given to the crew.
There has been nothing to suggest sabotage or attack.
The cause, for now, remains a mystery.