Allan Winn/SINGAPORE Kieran Daly/LONDON
TWO MAJOR AIRLINES are about to begin trials of AlliedSignal's enhanced ground-proximity warning system (EGPWS). By the end of February, the system will be flying on a British Airways Boeing 747-400 and, in May, United Airlines is to begin a three-month revenue-service trial of the system on its Airbus A320 fleet. The BA aircraft will be used to undertake dedicated test flights over the mountains of Wales.
AlliedSignal has been demonstrating a computer simulation of the system, using its actual digital-terrain database, to show how its use could help to prevent accidents such as the December 1995 loss of an American Airlines Boeing 757 near Cali in Colombia. A simulation of the aircraft's trajectory at Cali shows that the EGPWS would have identified the terrain as a threat 40s before impact. Typical EGPWS warning times are 45-60s, says AlliedSignal, compared with around 10s on current systems.
The EGPWS differs critically from the current GPWS, giving much longer warning times of impending collision. In the EGPWS, a worldwide digital-terrain database is used to enhance the performance of a conventional radar- altimeter-based GPWS by displaying predicted terrain conflicts up to 18km (10nm) from an aircraft's position. The terrain database has been compiled from several sources, including the digital files of the US Defense Mapping Agency (DMA).
AlliedSignal Commercial Avionics Systems business-development manager Brian Pulk says that there may be inaccuracies of up to 150m (500ft) in some areas because of the differences in data collection, but the resulting database has been compiled using worst-case assumptions.
Pulk says that the DMA data are accurate to one arc-second, but the data used in the database have been simplified: in the area close to an airport (out to about 55km), they are accurate to six arcs, while further out they are progressively coarsened until, at 220km, the accuracy is 30 arc-s.
With these levels of accuracy, the AlliedSignal worldwide terrain database occupies 15Mbytes of computer memory, and can be loaded into the EGPWS box through a standard PCMCIA "smart card".
The EGPWS is still in an early form, and Pulk concedes that there is more to be done with the database. In the meantime, the priority is to get the EGPWS certificated on the Boeing 737-400 and 757, and the Airbus A300-600R (in addition to the A320 and 747). It has already been selected as standard equipment for the Bombardier Global Express and Gulf-stream V ultra-long-range business jets and the McDonnell Douglas MD-95 regional airliner.
Source: Flight International
