Air accident investigators have released their first information about the events that led to the crash-landing of BA flight 038 at Heathrow.
The force of the impact embedded the engines in the ground
The plane landed short of the runway but stayed largely intact, and all 152 passengers and crew on board escaped.
A final report will be released once a much more extensive inquiry has been carried out.
LOSS OF POWER
An initial assessment from the Air Accidents Investigation Branch (AAIB) said that the Boeing 777 failed to respond to demands for extra thrust during its final descent towards Heathrow.
"At approximately 600ft and two miles from touch down, the autothrottle demanded an increase in thrust from the two engines but the engines did not respond," the preliminary report said.
"Following further demands for increased thrust from the autothrottle, and
subsequently the flight crew moving the throttle levers, the engines similarly
failed to respond."
The lack of thrust meant the speed of the aircraft reduced, and it came down on the grass short of the runway, the report added.
The findings confirm early theories that the pilots were pitched into a struggle to control the crucial final moments of descent. Only hours after the crash, an airport worker had told the BBC that one of the pilots had described how the aircraft "just shut down".
What remains to be revealed is why both engines failed to respond.
A simultaneous failure of the highly-reliable engines seems unthinkable, so investigators will focus their attention on the plane's systems.
The initial report says that "the range of aircraft systems that could influence engine operation" will now be examined in greater detail. Information from various modules within the plane will be gathered and studied.
The plane's fuel supply, hydraulics, electrical and computer systems are all likely to be examined in this phase.
Theories floated earlier relating to a loss of power also included:Fuel contamination or starvation - if water got into the system, or some blockage occurred, it would explain loss of power, although not why both engines were hit at the same timeRunning out of fuel - although this continues to look unlikely in itself, as the AAIB report says that a "significant amount" of fuel leaked from the aircraft, and in any case that would not explain why no early warning of fuel shortage was apparently given to the crew Bird strike - if 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 - although the engines are put through rigorous testing to ensure they can withstand this Wind shear - rapidly-changing winds in very squally conditions can cause planes to stall - but no reports from the area at the time suggest significant wind shear conditions
Key to any investigation is the action of pilots and co-pilots. In some cases, even if they were not responsible for an initial problem, their responses can avert or spark a tragedy.
The picture which has emerged so far from Heathrow suggests that the pilots did a heroic job in getting their stricken plane into the airport grounds and crash-landing without loss of life.
Senior First Officer John Coward was handling the plane at the time, and received warm praise from Captain Peter Burkill for the way the crisis was dealt with.
"We had an outstanding team on board," Capt Burkill said in a brief statement the day after the accident. "I am proud to say every member played their part expertly."
He said everyone had followed procedures as they had been trained to, and that First Officer Coward had done "the most remarkable job".
He added that it was not possible to make any public comment about the circumstances while the investigation was under way.
Air accident investigators will be able to listen to the cockpit conversations between the men, which will have been on one of the plane's two flight recorders. This will help them assess exactly how the crew handled the crisis.
Until the Heathrow incident, no Boeing 777 had ever crashed, in 10 years of service.
The plane quite simply is seen as one of the most reliable in the world.
Boeing, together with Rolls Royce, which makes the engines, will offer the investigation team any help needed.
From the initial accident report, it appears that the performance of the engines themselves is not in question, but the systems operating them are being closely examined.
The plane involved in the incident is relatively new, at about six years old, and is one of 43 in the BA fleet.
The fact that the plane withstood the crash-landing so well was also key to saving lives.
Some airlines are known to have question marks over their safety records and maintenance standards, but this is highly unlikely to be a factor in an incident involving a major European airline like BA.
An error by an individual technician can never be ruled out, and has sometimes been a key part in a chain of events which brought down a plane, but in this case the fact that the plane had almost completed its long flight from China seems to make it less likely to be a factor.
A major question after any airline crash is whether deliberate attack involving a device on a plane, or sabotage, is a factor. The authorities were quick to make clear there was no indication at Heathrow that any deliberate action was involved.
At the time, weather conditions were favourable and visibility good, although there was a gusty wind which could conceivably have played a role.
Wind shear has been a factor in previous tragedies. If a strong enough gust of wind catches the plane during the critical phase of final descent it can prove catastrophic. But no specific reports suggest conditions at Heathrow were severe enough to trigger this.
GETTING THE FULL PICTURE
The task of investigators from the Air Accidents Investigation Branch (AAIB) will be proving much easier than in many cases, as the flight recorders and wreckage have been easily accessible and not deep under water or damaged by fire.
As well as the recording of cockpit conversations, the inquiry team have accessed the plane's technical data from the second so-called "black box" recorder.
The fact that all crew and passengers survived 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.
Most accidents are caused by a chain of events rather a single catastrophic one, so investigators will be keen to explore all aspects of the incident before reaching their final conclusion.
A fuller report is due to be released within 30 days.