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Friday, 16 November, 2001, 12:42 GMT
Turbulence theory in Airbus crash
Tail fin of Airbus
The tail fin is now the focus of the investigation
By BBC News Online's Peter Gould

What caused the tail fin of the American Airlines plane to shear off, sending it into a fatal dive?

That is the key question now facing investigators trying to explain the crash of Flight 587 in New York.

The latest theory is that the Airbus A300 flew into the wake of a Japan Airlines 747 that took off just ahead of it from JF Kennedy airport.


We'll be looking very carefully at how the tail failed

George Black
NTSB investigator

The cockpit voice recorder, recovered from the wreckage of the Airbus, captures two loud rattling sounds shortly before the crew lost control.

In between the two noises, the Captain is heard referring to a "wake encounter".

Distance between planes

This is how pilots describe the air disturbance caused by a plane flying ahead. The spirals of air created by the wingtips of large jets can create turbulence for any following aircraft.

The potential hazard to all aircraft has long been recognized, and air traffic controllers have to ensure there is a safe distance between any two aircraft, particularly when the plane in front is a large jet like a Boeing 747.

Wreckage of engine
There was no sign of internal damage to the engines

Flights are normally kept two minutes apart, but it appears that Flight 587 took off only one minute and 45 seconds after the Japan Airlines jumbo jet.

What is crucial is the distance between the two aircraft. Crash investigators have indicated that the planes were the required distance apart as they climbed away from the airport.

However, weather experts will now be trying to see if weather conditions on the morning of the crash might have increased the risk of wake turbulence.

"We have never had a modern civilian jetliner come apart in flight," said Mary Schiavo, former inspector general at the US Transportation Department.

"It is so unbelievably catastrophic."

Unexplained

Aviation experts say the puzzling circumstances of the New York crash recall the loss of two Boeing 737s in separate incidents in the United States during the 1990s.



Flight 587 timeline
  • Departed: JFK Airport 0913 EST (1413 GMT) for Dominican Republic
  • 107 seconds after take-off rattling noise heard by pilots
  • 114 seconds: Captain reports "wake encounter"
  • 121 seconds: Second airframe rattling sound
  • 127 seconds: Control of the plane is lost
  • 144 seconds: Recording ends as plane crashes

  • A United Airlines plane crashed in Pittsburg, and a US Air jet went down in Colorado.

    "As in the case of the Airbus, both accidents involved aircraft flying in good weather, which suddenly plunged to earth," says Kieran Daly, of the Air Transport Intelligence online news service.

    "Investigators never really got to the bottom of what went wrong.

    "The Airbus crash is a peculiar incident. If, as now seems likely, there was no problem with the engines, then clearly the circumstances are very odd indeed."

    What is clear is that the A300 was coming apart before it hit the ground. The tail fin was recovered from Jamaica Bay, half a mile from the fuselage. And both engines were ripped off the wings, also suggesting severe stress on the airframe.

    The same aircraft is reported to have been shaken by air turbulence in 1994, in an incident that left a number of passengers injured.

    Repairs

    In the history of commercial aviation, there is only one other case of an airliner losing its vertical stabilizer. In 1985, a Japan Airlines 747 crashed into a mountain with the loss of 520 lives, after the fin fell off during a flight from Tokyo to Osaka.

    The accident was eventually blamed on inadequate repairs to the plane, following a "hard" landing.

    Wreckage of Flight 587
    Air Transport Intelligence's Kieran Daly: "A peculiar accident"
    In New York, crash investigators have been looking at the way the fin was secured to the fuselage. It has emerged that one of the six mounting points was reinforced during a repair 13 years ago, before the aircraft was delivered to the airline.

    But it is also clear that the fin sheared off above the mountings, raising questions about the strength of composite materials used in the tail structure. An expert from the Federal Aviation Administration has been called in to examine the wreckage.

    Meanwhile, the aircraft's flight data recorder is still being analysed, and crash investigators also have the accounts of eyewitnesses, who described how the A300 appeared to "wobble" before entering a steep, spiral dive.

    No-one doubts that the loss of the tail fin would have made it impossible for the pilots to keep control of the Airbus.

    The key to the investigation is establishing just how it came off.

     WATCH/LISTEN
     ON THIS STORY
    The BBC's Nick Higham
    "Increasingly the investigation is focusing on the plane's tail section"
    Marion Blakely, Chairwoman, NTSB
    "We are launching a major investigation"
    Aviation industry analyst Robert Hewson
    "Inside the engines they'll be looking for signs of damage"

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