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Wednesday, November 3, 1999 Published at 18:23 GMT


Sci/Tech

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".

The loss of flight 990
Subsequent examination of the flight data recorder from the plane - retrieved in the days after the crash - showed no evidence that a thrust reverser had mistakenly deployed.

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.

Click here to see a thrust reverser in action.

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.


[ image: Jim Hall: US air safety chief has rubbished rumours]
Jim Hall: US air safety chief has rubbished rumours
Thrust reversers are mechanical devices mounted near the engine exhaust, either internally or externally, which block the airflow and turn it around in the opposite direction.

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.

Safety systems

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.


[ image: Salvage operation: Teams will look for engines]
Salvage operation: Teams will look for engines
But following the Lauda Air disaster, investigators found that the reverser on one of the Boeing 767's Pratt and Whitney PW4000 engines had deployed - something the manufacturers said was theoretically impossible.

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.


[ image: 1991: Lauda Air 767 suffered 'impossible' crash]
1991: Lauda Air 767 suffered 'impossible' crash
And previous pilots of the Flight 990 aircraft reportedly told investigators that there had been a problem with a thrust reverser prior to its deactivation in Los Angeles.

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.


Jim McKenna of Aviation Week: "This scenario is unlikely"
"Since Lauda Air, mandatory design changes have been put in place to prevent an in-air thrust reverser deployment.

"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.


[ image:  ]

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Pratt and Whitney Engines

Pratt and Whitney PW4000 engine

Boeing: The 767

National Transportation Safety Board

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