After worries that modern cockpits can confuse pilots, the latest aids promise clearer guidance

David Learmount/LONDON

Highly automated "glass" cockpits were designed to reduce pilot workload and save the flightcrew from their own mistakes. In service, however, it became clear that expectations had been too high, according to the US Federal Aviation Administration's 1996 study of the man/ machine interface in modern flightdecks.

Another study, by the University of Miami/NASA Ames from 1993 to 1997, found that nearly 70% of pilots converting from "classic" to glass cockpits believed that "automation does not reduce total workload". A poll of the same pilots after a year's line service found that their opinions had not changed.

One reason for pilots' problems with highly automated cockpits was that, although the technology makes it easier to fly and navigate accurately, the systems behind the interface are complex. The number of flight management system modes available had raised the possibility of "mode confusion" - introducing an entirely new cause of confusion into the cockpit. Also, a profusion of high-quality data was suddenly available instantly. Pilots like having the information there for their use, but little of it is presented intuitively.

Given the pilots' concern about systems complexity, and the studies that show it has caused fatal accidents through mode confusion error, for example, it might seem foolish to introduce yet more systems. The early glass cockpits did not have terrain awareness warning systems (TAWS), global navigation satellite systems (GNSS), or aircraft collision avoidance systems (ACAS), but they do now. The next few years will show whether these innovations, despite their apparent potential to improve safety, have negated some of their benefits by adding to the cockpit's complexity and the crew's remoteness from the real world.

Concerns abound about bringing a new piece of flightdeck equipment into line service. Topping the list of worries are dispatch reliability in the line environment and whether pilots will always use the equipment within its limitations and for its design purpose. For example, pilots might be tempted to use the TAWS terrain display, rather than the designated approach aids, to navigate along valleys during an approach to an airport in hilly territory; or to employ the global positioning system (GPS) as a primary navigation aid where it is not approved as such; or to use ACAS to maintain separation from other aircraft rather than as the final resort manoeuvre warning device that it is designed to be.

Senior pilots in major airlines using TAWS, including the most advanced version of the equipment - Honeywell's Enhanced Ground Proximity Warning System (EGPWS) - have no doubt that it brings considerable benefits in line service. Capt Brent Blackwell, American Airlines' manager for international operations, says of the EGPWS: "It's marvellous, fabulous, and has almost universal acceptance among the line crews."

American was the first airline to commit to fleet-wide voluntary fitting of the equipment when it was still feeling the corporate after-shock of its Boeing 757 controlled flight into terrain (CFIT) accident on approach to Cali, Colombia, in December 1995. Three years after American started the EGPWS equipment programme, the system is now fitted to almost all of its approximately 700 aircraft, says Blackwell - including the American Eagle regional airline fleet. Since the equipment has been installed, there have been no terrain alerts, says Blackwell, "probably because the EGPWS is there". He says it makes the pilots so much more terrain-aware that the likelihood of getting too close to the ground unintentionally is massively reduced.

In a modern TAWS-equipped cockpit, the pilots are provided, on their navigation display (ND), with colour-coded terrain contours when the aircraft is 2,500ft (760m) or less above the ground. The colour coding changes according to the terrain's height relative to the level that the aircraft is at - whether it is above, at the same height as, or below the aircraft's level. In a "classic" cockpit, EGPWS would have its own display, which can be integrated with the weather radar display, although the modes have to be selected one at a time.

The imagery is generated from a terrain database - the core of the EGPWS - which is integrated with the aircraft's navigation system to enable accurate depiction of the aircraft's location relative to the terrain. Flight management systems (FMS) choose the navigation aids with the strongest signals to determine accurate position in any given area, and it is these aids and the terrain database that provide the TAWS overlay with the usual navigation and track information on the ND.

This display is the primary provider of terrain awareness, but the basic EGPWS display is also linked to a warning system that provides alerts based on horizontal or vertical proximity to terrain, or a terrain closure rate or sink rate that is too high. Options can extend the information the display can show and the cautions that can be provided. The terrain database uses WGS84 geodetic references, and is designed to be accurate to 1m within 92km (50nm) of all the surveyed airfields in it.

Accurate navigation information is crucial for descent or departure in a mountainous area - as it would be with or without EGPWS - and GPS is now widely used to back up conventional navigation aids. Don Bateman, Honeywell Flight Safety Systems Group senior engineer and effectively "father" of the EGPWS, says that most airlines, even those that used to be sceptical about GPS, now use it as a back-up navigation aid because of its high accuracy in almost all circumstances.

Bateman says more widespread GPS use by the airlines from the early 1990s has restored the faith that carriers had nearly lost inFMS navigation displays. This loss of confidence was caused by the high incidence of display "map-shift" events, which could be detected only by crews using raw navigation data to check their position. GPS use has now virtually eliminated map-shift, says Bateman, and has contributed to the EGPWS display's accuracy and reliability. He admits that GPS may not have the integrity to be a primary aid for precision navigation, but points out that it adds safety anyway: "It is a superb monitor. It has more than adequate integrity for a warning system." Bateman adds: "Just about every operations department has been able to produce a business case now for GPS."

Accident highlights urgency

Bateman says that although AlliedSignal Avionics (now part of the Honeywell group) had been working on the EGPWS for some time, the Cali accident highlighted the urgent need for such a system. The company set up field trials with co-operative airlines as well as conducting its own tests. British Airways was the first airline to join the AlliedSignal tests programme by installing EGPWS in one of its Boeing 747-400s in 1996.

Asked whether his airline has worked towards the same goal with other manufacturers, BA's general manager for flight technical and training, Capt David Fleming, says: "There is no competition." Honeywell is the only provider of a working enhanced TAWS and looks likely to remain alone for several years, adds Fleming. BA is aiming for fleet-wide fitment by the end of next year.

One of the early products of BA's testing, says Bateman, was a request by the airline to be able to use the database to provide terrain mapping even when the aircraft was at cruising altitude and well clear of the ground. So an EGPWS option called "peaks mode" was born. Peaks mode shows the terrain in shades of green, highlights the tallest mountains and indicates the height of the lowest and highest terrain in the display. BA uses it, and American is having it retrofitted.

Fleming says this mode is particularly useful when a route takes the aircraft over a wide area of high ground such as that in western China, because it alerts the pilot to the safest options if a pressurisation failure were to force an emergency descent. Blackwell says that "the most dramatic response" comes from his crews flying 757s and 767s on the South American routes at night. They find peaks mode "very comforting" to have. "It takes the stress out of flying over invisible high ground," Blackwell explains.

Peaks mode cannot be used at the same time as weather radar on the navigation display and a pilot has the choice of either flicking between modes on his/her own display, or setting weather on one pilot's display and peaks on the other, says Fleming.

BA also has an EGPWS option known as geometric altitude (GA). Because basic EGPWS works on barometric altitude above sea level, it is useless for airport approaches in Russia and China where air traffic control provides the QFE altimeter setting only (the setting that gives height above touchdown for the runway in use). Once QFE is set on the altimeters, BA's standard operating procedures (SOPs) dictate that an EGPWS, unless it has GA, is switched off during descent.

GA uses a combination of radio altitude integrated with the terrain database to work out terrain height relative to the aircraft, and it can also be checked against GPS height. A secondary benefit of GA, says Fleming, is that it can alert a pilot to crew failure to set their altimeters correctly during descent.

Another addition to the database shows obstacles close to airports. Some airlines work with Honeywell to update this information frequently by downloading data captured in the non-volatile flash memory of an EGPWS during approaches to and departures from airports. Blackwell says obstacle data are rarely essential, but it could be reassuring in the event of an engine failure on take-off from a hot and high airport, where the crew might be unable to climb clear of some obstacles. The ability to avoid them laterally would add to safety, he explains.

Fleming and Bateman are confident that pilots do not use the EGPWS display as a navigation aid, and emphasise: "It is not a navigation aid, it is an advisory aid." For a start, says Fleming, it is a single system that cannot self-check by comparing its computations with another, and the FMS does not use any EGPWS data as a navigation input. Bateman says that American's crews are still required to check their ND/EGPWS displays against raw navigation data from ground-based beacons or GPS.

Also, there are occasional circumstances on the 747 Classic fleet, where the EGPWS screen cannot be guaranteed to provide accurate terrain position information. One example is arrival at Mumbai in India, after crossing the Arabian Sea, because the inertial navigation system (INS) has been the primary aid in the oceanic area and needs updating. The signals from Mumbai's navigation beacons are not thought to be enough to provide a reliable update, which means the EGPWS screen may suffer map shift. BA 747 classic pilots are therefore directed to inhibit the EGPWS during a Mumbai approach.

Navigation temptation

During a descent into an airport surrounded by high terrain, it might seem tempting to use the EGPWS terrain display to navigate, but Blackwell says the terrain display is not visually compelling in that way. The ND shows the usual magenta line for the aircraft's projected track, and unless that line clearly conflicts with "red" or "yellow" terrain, there is no reason not to follow it, he says. Also, Blackwell points out, the EGPWS display does not attempt to give an impression of visual reality, but provides coarsely textured colour contouring which nevertheless creates terrain proximity awareness.

A pilot might use the EGPWS information to choose a track or direction, says Fleming, but only in an emergency - for example, if the aircraft were forced down towards high terrain at night or in cloud by loss of power or pressurisation. In such an emergency the information provided by the EGPWS would be infinitely better than no information at all, points out Fleming, indicating a solid confidence in the quality of the terrain database by adding: "If your position can be verified, there is no reason why EGPWS information should not be accepted as spot-on."

Honeywell already makes EGPWS variants for light general aviation aircraft. Bateman says it is working on one for helicopters. But with the unique close-to-terrain work that helicopters often perform, he admits there might be more of an issue about the human factors of using EGPWS. Perhaps helicopter pilots might be lured by the visual information on the display to risk flying, or continuing a flight, in marginal visibility and cloudbase conditions where they might not have done so without it. The EGPWS will certainly give the pilot improved warning of terrain, which has the potential to save lives, but trials must determine whether the system provides temptations to bad practice that outweigh the advantages of timely terrain warnings.

Boeing has produced a cockpit display that draws on the EGPWS to provide crews with a vertical situation display (VSD). The VSD uses the bottom section of the ND to show a display of the aircraft's descent profile in the vertical sense, as well as a simple, coarse-pixel vertical profile of the terrain below it. Fleming is cautiously optimistic about its potential to contribute to safe approach and departure management. He says its judicious use "could prevent rushed approaches by indicating whether you are going to make it or whether it will be difficult or impossible".

Bateman says crews will need to be trained well to make good use of the VSD. While the ND is intuitive in the lateral sense, and the VSD is intuitive in the vertical sense, the VSD can show only the flight profile from left to right or vice versa, which means it cannot be directionally intuitive. The pilot is left with the task of mentally integrating the information from the two displays - hence Bateman's call for careful VSD training.

Using traffic information

As for using the traffic alert and collision avoidance system (TCAS) for managing flight relative to other aircraft rather than waiting for resolution advisories before acting, Blackwell says there are situations in a terminal area in which it would be silly not to use the traffic information on the screen. One such situation, he says, is if your own aircraft is positioning for final approach, and you can see on the TCAS display that you are overtaking the aircraft cleared for approach ahead of you: "You don't have to wait to be told to slow down."

On the whole, pilots say that because the information on the TCAS is intuitive, it is a positive aid to traffic awareness. Rather than encouraging a "heads-down" attitude in the flightcrew, it tends to help look out when the visibility is good because there is a natural desire to try to make visual contact with the "real" aircraft revealed by the display.

If there is any overall message from the success of more recent additions to the modern flightdeck, it is that if a device provides accurate information in an intuitive manner, it will help the crew rather than confuse them.

Source: Flight International