Terrain Avoidance Technology: What Lies Ahead?

FAA’s March 2005 mandate required all U.S.-registered, turbine-powered aircraft operating under Parts 91,121 and 135 to be equipped with terrain awareness warning systems (TAWS). Compliance levels within these three groups were determined by specific numbers of seats and types of operation. Generally, large airline aircraft were required to install Class A systems, while smaller commuter types could install the slightly less sophisticated, Class B systems. Separately, FAA authorized the voluntary use of less advanced, Class C, TAWS units in smaller turbine- and piston-engine aircraft. Many other nations have adopted rules reflecting FAA criteria.

Now that covered U.S. aircraft are compliant, one may ask how effective the mandate has been. The only practical answer is to note that since these rules came into force, the number of controlled flight into terrain (CFIT) accidents within the affected groups appears to have dropped to virtually zero. So the TAWS regulations appear to have worked, since CFIT avoidance was their prime purpose. To many, however, the very notion that an otherwise airworthy aircraft, crewed by licensed pilots, could be destroyed on impact with the ground while under "controlled flight" seems a contradiction in terms. Yet such accidents have occurred over the history of aviation, and their causes generally boil down to a lack of situational awareness in reduced visibility conditions.

In some cases, in fact, the crews had a reasonably good idea of where they were and knew of high ground in the vicinity. But they assumed, after programming the automatic flight control system (AFCS), that it would take them on a safe descent path, only to crash following human factors errors during the busy descent phase. Two accidents stand out, both of which reportedly occurred after the crews received changes in their originally planned descent procedures. In 1992, an Airbus A320 landing at Strasbourg, France, struck a mountain following inadvertent selection of flight path angle instead of the adjacent descent rate control, causing the AFCS to produce a steeper, though not immediately noticed, descent path. In 1995 a Boeing 757 hit high ground near Cali, Columbia, after selecting an incorrect, but identically named and distant, non-directional beacon listed in the flight management system’s database. In these two cases, as with virtually all other CFIT accidents, TAWS would have saved lives.

First, GPWS

In the early 1970s, following a number of high-profile CFIT accidents, Bendix engineer Don Bateman coupled a radio altimeter with a voice recorder preprogrammed with a number of pilot warnings–including "Terrain, Terrain," and "Pull up, Pull up"–that were sequentially triggered at preset altitudes, as an aircraft came closer to the ground. This was the beginning of the ground proximity warning system (GPWS). In 1973 FAA mandated GPWS in transport aircraft, and the move to eliminate CFIT was under way.

Yet GPWS still had a long way to go. While it had many built-in safety and alerting features, its design inhibited alerts when the undercarriage was lowered. This decision reflected the not unreasonable assumption that at that point in a flight, the pilot would either have the runway in sight or would be correctly flying an approach procedure which would lead him to it, and GPWS warnings would simply be distractions.

But that wasn’t always the case. In 1997, when the ILS at Guam was out of service for a short period, the crew of an arriving Boeing 747 was forced to fly a VOR-based, non-precision approach, a procedure they had rarely used. With gear down and thus no GPWS warnings of either its closing proximity with the ground or its higher than normal descent rate, the aircraft struck a hill not far from the airport, killing all on board. Ironically, the aircraft was accurately positioned on the centerline of the runway approach path, and it crashed close to the VOR station it was using for landing guidance.

Equally ironically, Bateman and his team had, one year earlier, developed a greatly improved version of the original concept. Taking advantage of the vastly increased computing power which had become available, they linked the radio altimeter and other onboard sensors to GPS, and then added the key element, the digitized terrain map. Before that time, terrain maps had been highly classified products of the Cold War and were strictly off-limits to the civil community. With the technology’s public release, the ingredients for the enhanced ground proximity warning system (EGPWS) were in place.

The system now could take a major step forward with its ability to provide pilots a "look-ahead" capability. It shows what is in front of the aircraft rather than simply telling pilots, almost in hindsight, that they are passing over high ground, which might get a lot closer to the airplane if they keep going in the same direction. Unlike the earlier GPWS, therefore, the enhanced system issues terrain warnings well before the aircraft reaches the higher ground, allowing pilots to take avoidance actions earlier.

In those early days, Bendix flew its King Air, equipped with both GPWS and EGPWS, to demonstrate the difference between the two. On one trip, the aircraft flew at a few hundred feet over the water off Canada’s west coast towards an island with steep cliffs, which appeared from several miles out as a blocky red object on the weather radar display. On drawing nearer, the EGPWS voice alert announced, "Terrain, Terrain," followed shortly afterwards with its "Pull up, Pull up" command. The pilot immediately executed a full power climb and, as altitude was gained crossing the shore line several seconds later, the earlier GPWS unit issued its "Pull Up" command. At that location, in similar circumstances but in reduced visibility, the GPWS warning might have been too late.

EGPWS and TAWS

Much has changed since those early years. Technology advances have yielded system improvements, including the widespread introduction of high-definition, TAWS-compatible, multifunction displays available even for relatively small aircraft. One reported consequence of this is that the demand for Class B TAWS, for which cockpit displays are not required, has been less than expected, with the vast majority of purchasers opting for Class A systems.

Bendix, which over the intervening years became Allied Signal and was subsequently acquired by Honeywell, has sold close to 40,000 terrain warning systems. But Honeywell has experienced increasing competition from ACSS, Universal Avionics Systems, Chelton, L-3 Communications, Garmin, Sandel and others. Since EGPWS was virtually synonymous with Honeywell’s product, however, FAA’s installation mandate adopted the more generic TAWS acronym, although both systems met the agency’s specifications.

Increased competition has spurred the development of TAWS-related features well beyond those spelled out in FAA’s specifications. Three noteworthy ones are Honeywell’s Runway Awareness and Advisory System (RAAS), ACSS’ Ground Collision Avoidance Module (GCAM) and Universal’s suite of flightdeck terrain displays.

For its Runway Awareness and Advisory System, Honeywell developed an add-on software module for later-model EGPWS units aimed at reducing the risks of runway incursions, incorrect runway use and other potential surface hazards. RAAS uses airport data stored in the EGPWS database, coupled with GPS and other onboard sensors, to monitor the aircraft’s movements around the airport in low-visibility conditions with voice announcements at critical points, e.g., "Approaching Runway 24 Left." Other warning messages include confirmation when the aircraft is lined up on the runway prior to takeoff: for example, "On Runway 24 Left, 5,600 feet remaining." But should the pilot inadvertently line up on a parallel taxiway and commence a takeoff, it will issue an "On Taxiway, On Taxiway" warning as soon as the aircraft exceeds 40 knots. Conversely, after touchdown, it will continue to announce the stopping distance remaining to the end of the runway.

Had RAAS been available in 2000, it could have prevented the loss of a Boeing 747 that, in very low visibility, mistakenly entered a runway undergoing major resurfacing, instead of taxiing farther to a parallel runway that was fully operational. The aircraft crashed when the pilot attempted to take off. While the RAAS database is updated periodically, short-term revisions of this type for all the world’s airports would be impractical and are covered instead by Notices to Airmen (NOTAMs).

GCAM

The ACSS Ground Collision Avoidance Module contains advanced predictive algorithms to enhance the basic TAWS responses in order to provide pilots with more complete information about their current situation and thereby guide optimal responses. Described as a "performance-based" system, the GCAM accesses data from the airplane personality module (APM), which permanently stores the aircraft’s performance characteristics under varying conditions of power, weight, altitude, temperature and other inputs. With this information, the GCAM determines whether, in a conventional TAWS "Pull Up" situation, the aircraft can actually climb rapidly enough to avoid impacting the terrain ahead. If not, pilots receive an "Avoid Terrain" command, and the terrain display shows alternate directions to fly the aircraft to safer areas.

To avoid nuisance warnings, the ACSS unit limits its look-ahead terrain search and alerts to those obstructions which lie in a narrow sector of 1.5 degrees to either side of the aircraft’s projected track. It also displays Terrain Advisory Lines within 30 degrees to either side of track to visually alert the crew of potential obstructions. In an "Avoid Terrain" situation, where a turn is made away from an obstruction, the system switches to a wider sector and leads the rate of turn while searching the terrain ahead to as far around as 90 degrees from the original track. Terrain that the aircraft will be able to climb above is then shown with discrete hatch marks on the pilot’s display.

Cross-Section View

Universal has developed a range of display formats that break away from the conventional planview of terrain ahead. These include a vertical cross-section of the terrain far along the aircraft’s future flight plan track and well in advance of the standard TAWS alerts. These displays clearly show the pilot where the aircraft would be in relationship to the underlying terrain. Another format builds on the company’s synthetic vision technology and depicts a computed cockpit view of the terrain ahead, with the topography layered in standard TAWS colors of red, yellow and green, respectively denoting terrain above, at or below the aircraft’s present altitude.

The company offers a terrain planview presentation, but links it to a predictive trend vector which continuously shows the aircraft’s position 30 seconds ahead. The planview is only displayed when threatening terrain falls within the 30-second flight time envelope–more than adequate for an avoidance maneuver–and otherwise leaves the display clear for other crew needs. As an added feature, the system displays a flashing symbol of the actual terrain encounter point well in advance, thereby allowing the crew to plan an avoidance maneuver.

Future Directions

FAA and international mandates have brought about the installation of TAWS in a very large portion of the world’s aircraft. However, two main communities are not included in these rules, i.e., non-turbine, fixed-wing aircraft, particularly those that are privately owned, and helicopters. Both of these categories fall under the voluntary Class C TAWS arrangement. While no firm indications of general aviation’s adoption of Class C equipment are available, a slowly growing demand is discernible, industry observers say. They feel that FAA’s proposed move to nationwide ADS-B implementation could accelerate TAWS adoption within this group.

On the other hand, the U.S. National Transportation Safety Board (NTSB) recently recommended the use of TAWS in helicopters performing emergency medical service, which often involves low-level operations at night or in low visibility. These operators have suffered a significant number of fatal CFIT accidents. Reportedly, relatively few U.S. commercial helicopters have been equipped with TAWS. But it is understood that FAA is considering NTSB’s concern, and this could result in a TAWS mandate for certain types of helicopter operations.

Honeywell also is looking at an "assisted recovery" concept, where the aircraft AFCS automatically would perform the pull-up maneuver following a TAWS alert or, alternatively, turn the aircraft away from prohibited airspace. This is certainly feasible in today’s fly-by-wire aircraft and could save valuable seconds in certain situations.