It’s more than 30 years old, and yet it keeps evolving, gaining new capabilities. Ground proximity warning system (GPWS) technology has come a long way since it was first developed by Don Bateman, AlliedSignal’s chief engineer of Flight Safety Systems. All along, it has entered a growing number of aircraft types and helped significantly in reducing the safety problem of controlled flight into terrain (CFIT).
But now, as an enhanced GPWS (EGPWS)–or terrain awareness warning system (TAWS), as designated by the U.S. Federal Aviation Administration (FAA)–it can do much more.First installed in air transport aircraft only, the latest versions of EGPWS from Honeywell (formerly AlliedSignal) are for rotary-wing aircraft. Now, systems suited for virtually all aircraft types and sizes are available.
Helicopters and GA
The latest Honeywell systems entered flight test in the spring, in Bell 206 and MD900 helicopters. Their FAA certification was expected in October. Honeywell offers two helicopter EGPWSs, the Mark XXI and Mark XXII. The Mark XXI, targeted at the light helicopter market, is a Class B TAWS, while the Mark XXII offers Class A performance, potentially for the larger transport and corporate rotorcraft. Sikorsky Aircraft plans to offer a Class A EGPWS as a factory option on its S-76.
With Class A performance, the EGPWS offers look-ahead alerts, altitude callouts, and terrain display. It also provides a high-resolution terrain database for both on- and off-airport/heliport operation, and gives callouts during an autorotation. It even helps prevent tail strikes during a landing by using inputs from the aircraft’s attitude heading reference system (AHRS).
Systems with the Class B designation offer many of same features as Class A systems. Both have built-in, internal Global Positioning System (GPS) capabilities. However, the Class B system requires no radio altimeter interface, and its database is somewhat limited; for example, it will provide the aircrew with the elevations of runways and heliports, but not of the terrain around them. Also, it does not require a display, though it must be capable of providing display data should the aircraft owner decide to add a display in the future. Without a display, the aircrew relies on aural and annunciator alerts.
The helicopter EGPWS’s introduction followed closely that of the Bendix/King KGP 560 EGPWS, which Honeywell developed for the general aviation marketplace. This Class B system was first installed for initial standard type certificate (STC) approval on a Pilatus PC-12. It received technical standard order (TSO) approval in July.
EGPWS for Everyone
Honeywell now offers nine models of EGPWS, ranging from the digital Mark V for air transport aircraft and costing about $68,000, to the KGP 560, which costs less than $10,000. "Our goal is to have EGPWS in every aircraft," says an optimistic Todd Curtis, Honeywell’s head of business development for GPWS and enhanced products.
With this full line-up, Honeywell claims to have EGPWSs certified in more than 200 aircraft types. It is now standard equipment in all Airbus and Boeing models, and is standard or a standard option in all business-jet models and many regional aircraft. Indeed, the system is factory installed on 80% of the new bizjets, according to Curtis.
Honeywell claims that more than 100 airlines operate with EGPWS and that some 5,000 aircraft have flown more than 30 million hours with the well-proven system on board. That’s a lot of aircraft, but it just scratches the surface of EGPWS’s potential. Honeywell is looking at a general aviation market of more than 100,000 aircraft, representing 520 different types that range from single-engine piston aircraft to twin turboprops. Its KGP 560 is already approved in some 30 aircraft types.
"The best news," says Curtis, "is that you can use FAA form 337 field approval to certify the KGP 560. You don’t need an STC." Further firing up the general aviation market for EGPWS is the fact that several small-airplane manufacturers have decided to offer the system as a factory option by 2002.
New regulations add further fuel to the EGPWS market. Since 1974, the FAA has made GPWS mandatory on Part 121 air transport aircraft, and since 1992 on Part 135 aircraft bearing 10 passengers or more. But with the new TAWS regulation, all turbine-powered fixed-wings operating under Part 91 and configured for six or more passenger seats must have a Class B EGPWS.
Aircraft operating under Part 135 with six to nine passenger seats also must have a Class B system, and with more than nine seats they must be fitted with a Class A system, joining aircraft operating under Part 121. This applies to all new aircraft produced after March 29, 2002, and to all older aircraft needing retrofit by March 29, 2005.
Honeywell’s EGPWS has been advancing steadily in the aviation marketplace. But equally evolutionary is the system’s growing list of performance capabilities. And Honeywell officials will tell you that, in terms of greater situational awareness, courtesy of EGPWS, there is much more to come.
"We have the data [for a worldwide terrain database]," says Curtis. "Now it is only a matter of what all we can do with it.
"Our intent is make the EGPWS a module for our Epic avionics suite," he adds. "And we’re working with other [avionics] manufacturers to develop modules for them as well."
With integration in Honeywell’s flight management system (FMS), the EGPWS can acquire flight plan data, plus inputs from various sensors. One feature that can result from this combination, says Curtis, is a kind of "intent bus," which will indicate where the aircraft is going in relation to terrain. "Now the flight crew can utilize the terrain database in the EGPWS to check the accuracy of their flight plan," he explains. "But first we must develop a display for this."
Another new development at Honeywell would have the EGPWS interface with the weather radar. In this case, weather information would be overlaid on EGPWS display so that, with a vertical situation display, the crew can see the terrain profile and what weather is above it.
"The vertical situation display is certified in EGPWS today," says Curtis. "It will be evolving [as an added feature] into aircraft soon. How soon depends on the capabilities of their displays."
Honeywell even believes its EGPWS can help prevent runway overruns, such as the one on March 5 by the Southwest Airlines B737 in Burbank, Calif.
"We know where the runway is and we know how long it is from our database," says Curtis. "We also know from the on-board sensors, our altitude, airspeed, flight path and trajectory. So the display can show where I am, where I should land, and where I actually will land if I proceed with my course."
The elements remaining to gain this full capability, he adds, are a pictorial to display the information for a safe landing and data on runway conditions–which likely will come eventually via a data link.
The more data fed into the EGPWS, the more capable the system becomes. Consider, for example, inputting traffic information through the enhanced traffic alert collision avoidance system (ETCAS). Now, when making an approach, the flight crew of an aircraft in trail can be made aware of oncoming turbulence from the lead aircraft vortices. Honeywell is working on this feature, too.
The Here and Now
While Honeywell engineers work on these futuristic enhancements, crewmen today enjoy an EGPWS that far exceeds the early ground-prox systems, which were prone to nuisance and false alarms because their inputs derived only from barometric and radio altimeters. Today’s system employs geometric altimetry, which merges field elevations and GPS altitude, as well as the radio altimeter.
The system prioritizes the use of the three sources, depending on the phase of flight, to assure the most accurate altitude reading. For example, the field elevation predominates while the aircraft is on the ground; the radio altimeter during takeoff; and the GPS during cruise. The system continually calculates the mean sea level (MSL), and it maintains position accuracy of from 50 to 75 feet. With the KPG 560 model, pilots have the option of checking their radio altimetry against the GPS reading, which can be beneficial in case of a frozen pitot tube and lost pressure altitude.
With such multisensor accuracy, plus a massive terrain database, the EGPWS can provide seven modes of capability:
Excessive descent rate, offering alerts and warnings for excessive descent with respect to altitude above ground level (AGL);
Rising terrain, to prevent impacting hills and mountains;
Descent after takeoff, to prevent "sinking" after initial climb;
Terrain clearance, to assure the aircraft simply remains safely above ground;
Excessive glideslope, to assure the aircraft does not approach the runway too high or too low;
Advisory callouts, for when the aircraft descends through predefined altitudes below 2,500 feet AGL or a decision height set on the radio altimeter, or when bank angles become too steep; and
How It Works
Basically, a Class A EGPWS works as follows: It uses data from the GPS and other navaids, the FMS, and the air data sensors to determine the aircraft position both longitudinally and vertically (lat/long). Then it adds that information to data from the terrain/runway database to form a display showing the terrain elevations around the aircraft.
Early GPWSs simply viewed the terrain below the aircraft, providing a 30-second (at best) alert prior to possible impact. But look-ahead algorithms provide data to predict possible incursions with terrain at up to two minutes in advance.
The EGPWS provides both horizontal and vertical look-ahead. With the horizontal look-ahead, the airplane can "see" at least a quarter mile on each side of the aircraft. So, if the aircraft enters into a bank turn, the EGPWS can "anticipate" the turn and warn against possible CFIT. This is in addition to the advisory callout heard when the bank angle is too steep–a feature tailored to either air transport aircraft or business jets.
The crewmen receive from the EGPWS an aural alert and a visual warning from a multicolor image. The green color on the image indicates terrain safely below the aircraft. Yellow represents a cautionary alert 60-seconds prior to the predicted time of impact and is accompanied by a "caution terrain" aural message. And red indicates terrain that the aircraft could impact within 30 seconds; it is accompanied by an aural "terrain, terrain, pull up."
To enhance the crew’s situational awareness, Honeywell has added as options more color variations to the EGPWS display. For example, working with British Airways, the company added a "Peaks Mode" to the system, which is beneficial when an aircrew is flying at a safe elevation but, in case of an emergency, still would like to know about the terrain below. Airlines flying over, say, the Himalaya Mountains might welcome this feature.
What Peaks Mode offers is a depiction of the terrain below in various shades of green to denote ground elevations. The crew can view the terrain on their display even while 2,000 feet above the terrain. This feature is available for all EGPWS models and is standard in the KGP-560.
Oceanic water, too, appears on the EGPWS display, in a cyan color. This was an easy feature to develop from the Honeywell database, says Curtis, because all ocean surfaces are, obviously, at one elevation: zero MSL. (Inland bodies of water, including the Great Lakes are not yet painted in cyan, Curtis adds, because they are not at zero MSL.) For transoceanic crews, the cyan color benefits by making them aware of when they cross a shoreline.
The feature that perhaps most allows expanded capabilities to enter the EGPWS is Honeywell’s proprietary terrain/airport/ obstacle database. It represents a five-year project, gathering data from multiple sources worldwide.
For air transport customers, the database includes all runways with hard surfaces that are 3,500 feet or longer, and for non-air transport pilots, all runways 2,000 feet or longer, regardless of the surface. All told, Honeywell’s database includes more than 25,000 runways (factoring in various approaches to each one) in 10,000 airports worldwide, and more than 70,000 obstacles in most of North America that are 100 feet AGL or more. And the worldwide data gathering continues at Honeywell.
The obstacle database is being expanded to include other parts of the world. In addition, there are updates. An EGPWS can include 20 to 50 MB PCMCIA cards, depending on the area of coverage required. With a single card, costing $100, operators can update multiple EGPWS units. Once offered every four months via a service bulletin, the updates are now issued about twice yearly. The more-frequent updates were needed initially, according to Curtis, because Honeywell was still gathering new data in different parts of the world. "Now, it’s pretty solid," he adds, of the database, "so the frequent updates are less critical."
This continuously growing database, combined with geometric altimetry allows the EGPWS to establish, for example, a terrain/runway clearance floor. This compares the aircraft position, heading, and the height above field (HAF) against the database to provide a protective, seamless envelope around the aircraft during the approach and down until the wheels touch the pavement.
Honeywell also was able to develop a "runway picker" capability, in which the EGPWS’s sophisticated algorithms help assure that the terrain/runway clearance floor is directed to the desired runway–a benefit for pilots flying to multirunway airports and/or to areas where more than one airport is in close proximity. Should the aircraft approach the wrong runway, the system will give the crewmen an alert.
What more can EGPWS do? It does have other features, and as it becomes further integrated into avionics suites such as the Honeywell Epic, the system undoubtedly will offer even more.