Wednesday, November 3, 1999 Published at 18:23 GMT
Crash rumours focused on 'thrust reversers'
EgyptAir 767: Flight 990 reported reverser fault in Los Angeles
Speculation into what caused the catastrophic loss of EgyptAir Flight 990 focused in the early stages of the investigation on one particular part of the aircraft - the so-called "thrust reversers".
But reports that the aircraft had been suffering a fault with one of its two thrust reversers had initially sparked a flurry of speculation - not least because it was a failure of this one piece of equipment which caused a Lauda Air Boeing 767 to crash in Thailand with the loss of 223 lives in 1991.
A jet's forward thrust is created by accelerating large amounts of air as it passes from the engine intake to the exhaust.
In the engine, the air is compressed, mixed with fuel, burned in a combustion chamber and then expelled with great force from the rear.
By compressing the air to some 30 times its original pressure and then passing it through a narrow exhaust port at great speed, enormous momentum is created which can be used to push a plane, or any other vehicle forward.
In this way, the enormous forward thrust of the jet engine in normal operation is turned into massive braking at a flick of a cockpit switch.
Thrust reversing does not involve changing the direction of the engine itself. This would be as mechanically impossible as putting a car into reverse while going forward at high speed.
Passengers hear and feel the thrust reversers in operation shortly after the plane touches down - noise levels rising as the pilot throttles up the engines to increase the slowing force.
There are several different systems but all work on the same principles.
This equipment means that an average-sized commercial aircraft needs 500 metres less runway to come to a stop.
Thrust reversers can use a variety of safety locks to prevent the system from accidentally deploying, including sensors and switches to check that the aircraft is actually landing at a low speed.
According to the official accident report, as that airliner continued to climb some 45 minutes after take-off from Bangkok, the pilots noticed a warning light alerting them to a thrust reverser problem.
Minutes later, a hydraulic activator pin triggered the reverser to open, creating a catastrophic spin.
With the thrust from the two engines flowing in opposite directions, the pilots could not control the aircraft and it plunged to earth.
After months of investigation, the US Federal Aviation Administration ordered manufacturers to re-engineer thrust reversers on all US-registered Boeing 767s.
Maged Masri, EgyptAir's technical head, told reporters that the Flight 990 aircraft underwent the modification in 1993 at a British Airways workshop.
Could disaster have struck twice?
After the disaster, it was revealed that EgyptAir 767 rolled off the production line at the same time as the Lauda Air aircraft.
Moreover, there were reports that the US air regulators have issued 10 "airworthiness directives" on thrust reversers since the Lauda Air disaster, calling for more changes to the equipment.
But experts say that the EgyptAir circumstances are completely different to the 1991 disaster.
"Investigators have seen no similarities with the Lauda Air crash as of yet, except the fact that both planes disappeared from the skies suddenly," said Jim McKenna of Aviation Week in Washington.
"The initial radar data indicates that the EgyptAir plane descended from 33,000ft in a fairly straight line.
"If a thrust reverse was involved, investigators have told me they would have expected to see the plane turn. They have no evidence of that having happened.
"The problem reported with the EgyptAir flight led to the thrust reverser being deactivated so it could not be used on landing - and could not, theoretically, be deployed during flight."
Jim Ferguson, a British aviation writer, agreed.
"A tremendous amount of work was put into the thrust reversers on this engine to make sure that this could never happen again," he said.