Setback over Pittsburgh black box

The Pennsylvania black box was found in a crater

The cockpit voice recorder recovered from the crash site of the hijacked airliner which came down in Pennsylvania has been sent to the manufacturer to try to extract information.

Federal Bureau of Investigation officials had hoped to gain valuable clues into how the hijackers took over United Airlines Flight 93, saying the recorder had been found in "fairly good condition."

However, initial attempts to extract information from the tapes have proved fruitless, and the unit has been sent to the manufacturer, a Justice Department official told reporters in Washington.

Black boxes from the Pentagon attack were badly burned

Investigators had hoped that the recorder from the Pennsylvania crash might yield the most clues, as it was the only one from the four downed airliners which was not subjected to a prolonged fire.

All 45 passengers and crew members on board Flight 93 that took off from Newark, New Jersey, bound for San Francisco were killed.

Investigators hoped that the voice recorder, and the data recorder, recovered a day earlier, could reveal whether passengers tried to gain control of the airliner before it crashed.

Both black boxes from the hijacked aircraft which crashed into the Pentagon have also been recovered and turned over to the Federal Aviation Administration.

However, according to a report in the Washington Times, investigators have so far failed to extract information from the Pentagon attack voice recorder because it was so badly damaged.

Click here to see a diagram of a black box

Despite this, it is hoped that the data recorder could provide details of the final moments of the American Airlines plane, which was hijacked shortly after it left Dulles International Airport on a flight to Los Angeles.

It crashed into the headquarters of the US military with 64 passengers and crew on board.

Black boxes: Important advance in air safety

Black boxes - an aircraft's flight data recorder (FDR) and cockpit voice recorder (CVR) - are two of the most important contributions to air safety since the beginning of the era of commercial flights.

The data collection devices - which are actually orange - are mounted in the tail of an aircraft.

Under internationally agreed regulations, commercial aircraft must carry the equipment to record the performance and the condition of the aircraft in flight.

Strong and insulated

The recorders are housed in immensely strong materials, such as titanium, and insulated to withstand a crash impact many times the force of gravity and temperatures of more than 1,000 degrees Celsius.

The recording material is itself insulated against accidental deletion.

Modern black boxes record up to 300 factors of flight including:

• speed and altitude
• aircraft pitch
• cockpit conversations
• radio communications.

The safety precautions are designed to ensure that accident investigators will be able to recover the recorders, compile a full picture of an aircraft's last moments from the recordings - and then accurately explain what went wrong.

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