GPWS, Meet GPS

Location enhances ground-proximity warnings

October 1, 1997

What's the single biggest safety-of-flight threat facing turbine aircraft year after year? It isn't even a contest. Controlled flight into terrain (CFIT) accidents overshadow all other categories of mishaps. From 1988 to 1995, for example, CFIT accidents have been responsible for most commercial aircraft fatalities. Add up all the turbine aircraft accidents during that period blamed on either midair collisions, in-flight fires, icing, wind shear, or fuel exhaustion, and CFIT — at 17 accidents — beats them by a long shot.

The Flight Safety Foundation estimates that today this category sents nearly three-quarters of the total fatal turbine aircraft accident risk. Typically, four to eight large jet aircraft and several times that many turboprops and business jets around the world are lost or damaged each year in CFIT accidents. As sobering as these figures are, it used to be much worse. The CFIT accident rate decreased sharply in the United States after the airlines introduced the ground proximity warning systems (GPWS), beginning in the mid-1970s. In the United States, GPWS is required equipment on all commercial-use turbine aircraft having 10 or more passenger seats. Many other nations also mandate its use.

As the statistics demonstrate, however, there is plenty of room for improvement. The typical CFIT accident is usually the result of a series of small errors that culminate in an otherwise mechanically sound aircraft's being flown into the ground. Misread approach charts, incorrectly set altimeters, and misunderstood ATC instructions often play a role. Most occur during the approach and landing phases, and often they involve a nonprecision instrument approach that has one or more step-down fixes. Many happen at night and in IMC conditions. (Relatively few CFIT mishaps involve precision approaches, which most often employ constant 3-degree glideslopes.) The one similarity these accidents share is the crews' loss of situational awareness — or, more specifically, their loss of vertical situational awareness. Most such accidents occur with the aircraft lined up on or near the final approach course but at too low an altitude.

GPWS is designed to improve a crew's situational awareness by supplying a wake-up call — so to speak — whenever an aircraft strays too close to terrain. Early GPWS units used inputs from the aircraft's barometric and radar altimeters to supply terrain warnings. Unfortunately, the rate of false warnings was quite high, with pilots tending to ignore a GPWS alert until they could verify for themselves that it was real. As one accident report after another revealed, this approach sometimes leads to tragedy. In a typical crash, the GPWS sounds for less than 10 seconds prior to impact, while average pilot reaction time to the warning is about 5.5 seconds. In mountainous terrain, that short delay often made the critical difference, since many of these accidents happened within 300 feet of a mountaintop or ridge line.

Over the years, GPWS design has been refined and the false warning rate has been greatly reduced. The first-generation units, called Mk Is, have given way to more sophisticated devices that generate earlier warnings. The newest (Mk VII) GPWS blends altitude, airspeed, and aircraft configuration inputs with mathematical algorithms that define various "alert envelopes." If the aircraft penetrates an alert envelope, a warning is triggered. All of the alert envelopes expand or contract according to changes in these inputs. GPWS can warn of conditions such as excessive aircraft descent rate too close to the ground, insufficient terrain clearance, descent below glideslope, inadvertent descent shortly after takeoff, and excessive bank angle. It can even advise when the aircraft has flown into a microburst if the unit is equipped with the latest reactive wind shear warning capability.

Each alert mode includes specific voice annunciations to advise the crew as to the nature of the threat, a feature early GPWS units did not have. For example, approaching too low to the ground with gear retracted will trigger a "Too low, gear" warning. An inadvertent descent after takeoff will cause "Don't sink" to sound, while flying excessively low beneath an electronic glideslope will elicit a "Glideslope" call-out. The GPWS can also be programmed to supply advisory altitude calls during an approach, including a "Minimums" call at DH or MDA.

Despite the increased sophistication of newer GPWS installations, average warning time is still quite short — about 15 seconds or so, depending upon the alert mode that triggers it. Pilot reaction time, therefore, remains a serious limitation to its effectiveness. A completely new approach to GPWS design promises to change all that, however. AlliedSignal Aerospace has recently marketed what it calls Enhanced GPWS. EGPWS is to terrain warning what the Jacuzzi is to bath time — a true quantum leap in design.

EGPWS starts with a current-generation GPWS and adds three never-before-seen features: GPS positioning, a worldwide terrain database, and a worldwide runway database that includes most public-use and military runways of 3,500 feet or longer. Together with existing alert modes, these new features enhance a crew's awareness of surrounding terrain dramatically. EGPWS supplies greatly increased warning times — as much as one full minute prior to computed impact in certain warning modes. For the first time, pilots will also see a visual representation of threatening terrain on the aircraft's radar or EFIS screens.

During a recent 2-hour demonstration flight in AlliedSignal's Beech King Air C90, Pilot put EGPWS through its paces over the rugged Cascade Mountains east of Seattle.

EGPWS works in a couple of ways. In its primary mode, it simply relates the aircraft's barometric altitude, heading, and airspeed to a slice of space 2,000 feet above and below the aircraft. If this space contains terrain, it will be displayed to the pilot at any range available on the aircraft's radar or EFIS display. Various alerts and warnings will occur if the aircraft's projected path will bring it too close to the terrain. The system also builds a terrain clearance floor (TCF) around all runways in its database. The TCF starts at 500 feet above all terrain within a half-mile of the runway and slopes upward to 800 feet above all terrain at a distance of 25 miles. Whenever the aircraft penetrates the TCF, a "Too low, terrain" warning will sound. EGPWS determines penetration of the TCF by comparing GPS-computed position to topographic information contained in the terrain database. This allows the terrain warning to occur during a nonprecision (lacking electronic glideslope) approach, even if the aircraft is fully configured for landing with flaps and landing gear down, something current GPWS cannot do. This feature alone should greatly reduce the incidence of CFIT mishaps. Like most other warning modes, this one is modulated according to aircraft speed and configuration, a feature designed to reduce false warnings as the aircraft maneuvers for a normal landing.

Some airports have unusual terrain features that might cause an unacceptably high number of false warnings. EGPWS allows operators to alter the alert envelope control logic to compensate for these peculiarities without affecting how the system operates at other airports.

An exceptionally valuable feature of EGPWS is the terrain awareness and display system (TADS). TADS displays terrain within 2,000 feet of the aircraft's altitude as a pattern of yellow or green dots on the radar or EFIS display. Green dots indicate that the terrain is below the aircraft's altitude; yellow means it is above. The density of the dot pattern changes from light to heavy to indicate relative altitude of the terrain, ranging from well above the aircraft (more than 2,000 feet), to well below. If EGPWS calculates that the aircraft's present course and speed will result in a collision with terrain, it announces "Caution, terrain" 60 seconds prior to projected impact. At this point, the most threatening terrain on the visual display will change to a solid yellow block. If no action is taken by the pilot, the block will turn to solid red 30 seconds from projected impact and the voice annunciation will command "Terrain, terrain, pull up."

AlliedSignal is currently flight-testing a man-made obstacle detection mode that it plans to incorporate in production models once a reliable obstruction database is constructed. Unlike natural terrain, which tends not to change very much, man-made obstructions are built and removed with regularity, so timely database updating is a must. During our recent demo flight — the first one AlliedSignal conducted for the media with this mode functional — a tall antenna atop a mountain ridge elicited an "Obstacle!" voice warning from the unit, along with a yellow and then a red icon to mark its location on our radar screen as we approached it. AlliedSignal believes it necessary to differentiate the obstacle warning from the terrain warning so that a pilot flying near such an antenna in pancake-flat Kansas, for instance, won't ignore it in the belief that it is a false terrain warning.

Pilot Marcus Howard, who has flight-tested AlliedSignal's various GPWS designs for more than nine years, demonstrated how "smart" this newest offering is during our jousting match with the Cascades' peaks. For example, flying parallel to a mountain ridge a mile or so off our wing produced no voice warning. When Howard banked more than 5 degrees towards the ridge, however, the unit immediately suggested that continuing the turn was a really bad idea. Maneuvering for a circling approach to Pangborn Memorial Airport in Wenatchee, Washington, with gear and flaps extended, the unit was intelligent enough not to give nuisance warnings — even though the field is crowded on most sides by nearby towering terrain. Perhaps the best demonstration of EGPWS's early warning value occurred when we intentionally flew at cruise speed toward the top of a mountain with a steeply sloped face, aiming just above the ridge line. It is the kind of situation in which earlier GPWS designs are most deficient, since they depend on the radar altimeter to detect the rapidly rising terrain. As expected, EGPWS dutifully provided ample warning a full minute — or more than 4 miles — away. "Now," said Howard, "we'll turn off EGPWS and see what kind of warning we get from the earlier-generation system." An eternity best describes how we waited, and waited, for some indication that there was a mountain directly in front of us. Perhaps 8 seconds away, the "Terrain, terrain, pull up" warning finally blared. I won't mention the vastly increased pucker factor and sweaty palms that this demonstration induced in the observer, but Howard's point was well made.

There are limitations to EGPWS that pilots need to be aware of. For instance, terrain database coverage does not encompass certain countries — including some in Asia and South America — that choose not to release topographic data to the public; coverage is nonetheless quite extensive. Also, to conserve memory storage space, the most detailed terrain resolution is used only within a 16-mile radius of runways in the database. At greater distances, the terrain contouring becomes progressively less detailed. For the most part, however, there is no good reason for an aircraft to be venturing close to terrain when far from airports anyway.

At a list price of around $68,000, about 25 percent more expensive than the previous-generation GPWS, AlliedSignal's newest offering is a steal if it prevents just one CFIT accident. With generous price discounts available for multi-unit purchases, EGPWS should find ready acceptance among turbine aircraft operators. It represents a huge step forward in the battle to eliminate CFIT accidents.