The black boxes in all commercial transport aircraft are, in fact, bright orange, so they can easily be seen amidst wreckage. This has caused confusion. After a crash in the Sahara in the Sixties, local troops were sent off to scour the accident scene for the black boxes. They returned empty-handed after two days of searching before anyone remembered they hadn't told them the true colour of the boxes.

Despite their importance in the popular mind, black boxes do not recount the full causes of an accident. They merely complement the evidence of air traffic control conversations, radar tracks and, above all, the wreckage itself. At Lockerbie, the Air India crash into the Irish Sea in 1985, and TWA 800 in July, the black boxes were of little help.

Housed in the tail, the part of a plane least damaged in most crashes, the two black boxes consist of a Cockpit Voice Recorder, which records flight-deck conversation and radio traffic on a loop-tape, and a Flight Data Recorder, which takes readings from the instruments on speed, acceleration, altitude, heading, pitch, thrust, and the position of the ailerons, elevators and so on.

The FDR parameters can range from the first five of the above in the oldest planes to over 500 in some of the newest. They are equipped with "pinger" radio beacons so they can be located underwater. Encased in quarter- inch thick stainless steel containers for crash resistance, they are also designed to withstand a heat of 1,100C for half an hour, and submersion to a depth of 20,000 feet.

The FDR started life shortly after World War II as a film camera pointed at a set of duplicate cockpit instruments inside a box. But undeveloped film is a sensitive medium and few survived the heat or impact of a serious crash.

The CVR has only been compulsory since 1974. Pilots didn't like the idea of second-guessing a dead colleague, but after a BFA Trident crashed to the ground at Staines in 1972, further resistance was useless. The wreckage and the FDR told how flaps had been retracted, and warning devices ignored.

The captain's autopsy showed he had probably suffered a massive heart attack just after take-off. But nobody could ever work out why the danger signals had been ignored and overridden. After the Air Accident Investigation Branch had recommended CVRs for every large plane the Civil Aviation Authority made them compulsory.

It was only to be expected that CVRs would become the subject of ghoulish interest. On a web-site called "The Aviation Disaster Page" you can review a selection of major accident CVR transcripts.

Pilots tend to say similar things as their last moments approach. "What happened?" usually marks the beginning of the emergency. A resigned "shit" - in whatever language - is usually the last word on the tape. Air crash investigators used to publish entire transcripts of the CVR in their official reports, but pressure from pilots' unions has made this much rarer. Only relevant excerpts may now be quoted, like the remarks of the co-pilot on the Kegworth Boeing 737 when he was asked which engine was acting up. "It's the le... the right one."

Investigators are always pushing for better black boxes, and many would like a video recorder installed in the cockpit, too. Today's computerised systems are so complex that not even the latest FDRs can provide a true record of what the pilots are actually seeing on their instruments.

Black boxes have been so successful in aviation that they have spread to transport systems. Ships have had them since 1987 and all new trains now have them. This was not generally known until August, when one of the black boxes on the train that crashed at Watford, killing one and injuring 70, turned out to have no tape in it.

Although lorries have their tachographs, there are no serious moves afoot to put a black box in the family Toyota, although the growing use of electronics in cars makes it increasingly feasible. It is not too hard to imagine the day when an accident investigator will be listening to 30 minutes of your choicest insults against other drivers, culminating in the "shit" that immediately precedes an ugly prang.

Andrew Weir worked on `Black Box', a four-part documentary series on safety in the air, which starts on Channel Four at 9pm on Tuesday 26 November