THE MYSTERY OF FLIGHT BAO38!

By Tom Symonds - Transport correspondent, BBC News.

Mystery still surrounds the crashed British Airways Boeing 777. Why did a modern airliner, with an experienced crew on board, suddenly lose power in the seconds before landing? It is a question that many in the aviation industry simply can't stop asking. The mystery of flight BA038 intrigues them in so many ways.

The Air Accident Investigation Branch is admired around the world as the gold standard in crash investigation. Its experts are either experienced pilots or engineers.

Their task: to produce a highly detailed report of the crash that explains, rather than blames. Pulling together information from a variety of sources, including the AAIB's initial reports, it's possible to describe the last minute of the flight.

BA038 had been descending gradually into Heathrow, the autopilot and the automatic throttle system controlling the aircraft. As the handling pilot, first officer John Coward would have been preparing to take manual control below 1,000 feet. The trouble started two miles out at 600 feet, as the plane was slowing down in its landing configuration. At this point the engines would have required more power to keep the plane from sinking below the glideslope - an invisible three degree path down to the runway, generated by radio waves.

The plane had not run out of fuel, and there is no mention of birds being sucked into the engines, or violent blasts of wind throwing it off course. When the automatic throttle demanded more power, the engines initially responded. Then first the right engine, followed eight seconds later by the left, powered down - to a level below the thrust needed.

Warnings would have flashed up on engine monitoring screens in the centre of the control panel, showing the power was below that required. A lower screen would have shown more detailed information about the flow of fuel around the aircraft. The primary displays would show the likely height the plane would descend to in the next minute.

Faced with the knowledge that a disaster was in the making, the crew had around 40 seconds to save their aircraft. It's understood the captain Peter Burkill quickly reduced the amount of wing flaps deployed.

This was as important as the skilful manipulation of the control column by John Coward, in saving the aircraft. It cuts drag, speeds the plane up a little, and when a pilot has speed, he can maintain altitude. But it would only delay the inevitable -the plane would have been losing both speed and height, a potentially catastrophic situation.

The 150 tonne Boeing just cleared the busy A30, the airport perimeter fence, and a radio mast before crashing to the ground in a stall - where the plane can simply fly no longer. There would have been further warnings in the cockpit, including the stick-shaker, where the controls vibrate to alert the pilots.

But the quick actions and training of the crew had saved many lives. The latest information from the investigation has removed some of the possible causes from the list. The plane had not run out of fuel, and there is no mention of birds being sucked into the engines, or violent blasts of wind throwing it off course.

In fact, both engines were turning as the plane hit the ground. Significantly the AAIB has said "the autothrottle and engine control commands were performing as expected", suggesting no failure of a data link between the automatic systems and the engines. Instead the investigators specifically mention the plane's fuel system.

The Boeing 777 has three fuel tanks - one in the centre that is used up first, and one on each wing, which would have been in use during the later stages of the flight. There are six different pumps to push the fuel to the engines. If any pump fails, fuel can be routed via an alternate. If an engine fails the fuel can be 'balanced' between the wings to take account of the one-sided thrust, and the fact that the tank on one wing will empty faster than the other.

Keeping everything working is an 'electronic engine control', part of a system called the FADEC which monitors the power needed. This takes into account a range of variables including: the configuration of the aircraft, the condition of the outside air, the state of the engines themselves, and of course, the position of the plane's throttles. This system knows the limitations of the engines and stops them being damaged by heat or pressure. Crucially it is supposed to work independently of the plane's autopilot, to make sure the engines function properly.

It is this collection of computers, tanks, pumps, sensors and their backups, which the investigators are examining closely. But they will also be examining the fuel. It might have been contaminated. Or fuel 'waxing' may have occurred. This results from partial freezing, and pilots say the outside air temperature at some altitudes en route to the UK was down to minus 70 degrees that day - some of the coldest readings they could remember. There are heating systems to bring the fuel up to the temperature required. Perhaps these failed.

The US Federal Aviation Administration has identified previous seven incidents involving Boeing 777s where ice and melting water clogged up the sensors and pressure lines of the FADEC system, preventing it from controlling the engines properly. The biggest concern was that this could happen on both engines simultaneously, a scenario eerily similar to last week's crash.

The FAA's findings only related to General Electric GE90 aircraft engines. The pair on the British Airways Boeing were Rolls Royce engines. Maybe they suffered a similar problem.

At least modern airliners provide investigators with the maximum amount of information possible. Every aspect of the plane's performance was recorded, including the words of the crew. It could be weeks before the mystery of flight BA038is solved. The airline industry is waiting, with great anticipation, for answers.
BBC NEWS REPORT.