F 
R

1
- 
9 
9

---

Home | Update | LATEST ISSUE | Gallery | FR Profile | Datafiles | FR 1/99

THE ENHANCED GROUND PROXIMITY WARNING SYSTEM

by Christopher Hess

Every year airliners with no technical problems crash into the ground for no obvious reasons. Rarely are there survivors. The reason for the accident in most cases is the loss of situational awareness of the crew. For example either because there was a deviation from the prescribed flightpath or a misunderstanding with the air traffic controller.

The number of controlled flight into terrain (CFIT) accidents has significantly decreased since 1974. Back then, the US aviation authorities had mandated the Ground Proximity Warning System GPWS from AlliedSignal for all transports. Other countries followed and today, 95 percent of the worldwide airliner fleet are equipped with GPWS.

However, GPWS was designed for the aircraft of its time and does not protect the aircraft in all cases. Even today, there is an average hull loss rate of large airliners due to CFIT every year, among them GPWS equipped aircraft.

Often, the crew received little or no warning. The reason is that conventional GPWS warnings are based on radar altimeter readings which are only pointed downwards to the terrain underneath the aircraft. This may not be enough in steep climbing terrain. Another weakness of the old system is that it is disabled in final approach to avoid false alarms when (intentionally) approaching the runway for landing.

For two years now, AlliedSignal has been marketing an improved version ñ called the Enhanced GPWS. EGPWS is mainly taking advantage of the digital cockpit avionics of modern airliners. Large Displays allow the presentation of a terrain map. This gives the crew a visual warning of upcoming terrain threats, along with the still sounding aural alerts.

EGPWS is comparing the know aircraft position and altitude with an internal 20 MB terrain data base. This data base covers approximately 95 percent of the Earth. The Earthís surface is catalogued in 9 by 9 kilometer squares, each of which is given a height index. Near airports the data is refined to 400 by 400 m squares. Based on the altitude data, the EGPWS computer generates the cockpit display.

However, only those obstacles are shown which are within 2000 feet of the aircraft flight path. Terrain reaching just below the aircraft flightpath is colored in green, terrain above the flight path is presented in shades of yellow and terrain more than 2000 feet above the flight path is shown in red.

The virtual map allows the system to warn the pilots at least 60 seconds before the obstacle with an aural and visual warning. At 30 seconds to go, the systems commands a pull-up. The acoustic warning even works when the crew has switched the map display off.

The second most important improvement with EGPWS is that it continues working even when the landing gear and flaps are extended. This is possible because of the terrain data refinement in the areas close by airports. Also the worldwide data base contains the exact threshold locations of the most important airports with a runway length over 3500 feet, allowing the system to discern if the aircraft is really on an approach to a runway.

Depending on the phase of flight, EGPWS operates in seven different modes:

• Mode 1 warns the crew of excessive descent rates, for example due to a navigation error during descent to approach. Also, the system will give a warning if a sinkrate of 1000 ft/min is exceeded on final.
• Mode 2 works as a predictive mode based on the terrain data base as described above.
• Mode 3 sets off if the aircraft goes into an inadvertent descent after take-off or missed approach. The mode is automatically deactivated once the aircraft has reached a safe altitude.
• Mode 4 warns if the aircraft violates a minimum terrain clearance during climbout, cruise or descent and approach. This mode is to prevent modes 1 and 2 not being able to give a timely warning because the aircraft is already flying close to the terrain.
• Mode 5 is supposed to warn from excessive deviations from glidepath during an ILS approach. The mode switches on when the crew selects an ILS frequency and puts the gear down.
• Mode 6 is warning of excessive bank angles. Here the sensitivity goes up with decreasing altitude. (warning at 40 degrees of bank at 150 feet altitude and already at 10 degrees of bank at 30 feet altitude).
• In mode 7, EGPWS is working as a windshear alerting system.

More and more airlines have realized the value of the enhanced GPWS and are updating their fleets. American Airlines, for example, just received FAA certification for equipping the seven different aircraft types of its fleet with EGPWS. American, which is currently operating 634 aircraft, has ordered a total of 700 systems and has already 230 units in service.

In Germany, Lufthansa has committed to equip the Lufthansa Airlines and Lufthansa Cargo fleet with EGPWS. The order includes 175 systems for retrofit on existing aircraft. Also the airline has already A319 and A321 aircraft in operation which came with EGPWS as a standard option. Furthermore, several Lufthansa simulators will be equipped with the system also.

EGPWS is not a mandatory system yet. However, FAA mandates the system on all aircraft with more than six passengers after 2003. Then at the latest there will be a demand for systems which are not aimed at the complex modern airliners. AlliedSignal is already working on other versions of its EGPWS. Along with a stand-alone unit for regional and business aircraft, the manufacturer plans a unit which combines display and computer for small aircraft, along with an EGPWS with an integrated GPS receiver. Also other manufacturers, among them VDO Luftfahrtgeräte in Germany, are getting ready to break AlliedSignalís monopoly market position with new terrain avoidance systems.

From page 72 of FLUG REVUE 1/99