The Ground Proximity Warning System (GPWS) is a flight deck system that gives pilots visual and audible warnings of various conditions that might cause danger to the aircraft and its passengers. It was invented by Don Bateman, a Canadian born engineer. He started work on the GPWS in the late 1960’s after a series of Controlled Flight Into Terrain (CFIT) accidents killed hundreds of people. Ground Proximity Warning System was introduced in air carrier aircraft in 1974. After that time there was a dramatic decrease in these type of accidents. In 2003, the average CFIT risk was less than one aircraft loss every 91 million departures. While before GPWS in 1974, the CFIT risk was about one large commercial jet aircraft loss every 0.8 million flights. This means a reduction in risk of about 100 times over the 30 years. The rate of CFIT accidents is still alarmingly high as a proportion of total accidents though. In air transport jets and turboprops, more than 50% of all accidents involve CFIT.

GPWS relies on a number of various data from the various instruments in the cockpit as well as other data that is available to the aircraft, to calculate if the aircraft is about to enter into a potentially dangerous situation which may lead to CFIT. GPWS is so effective that the International Civil Aviation Organisation (ICAO) has issued an order that GPWS must be installed as standard equipment in all commercial aircraft that fly internationally. This law has been effective since 1998.

Traditional GPWS does have a downside. Since it can only gather data from directly below the aircraft, it can not predict future terrain features. If there is a dramatic change in terrain, such as a steep slope, GPWS would not detect the aircraft closure rate until it is sometimes too late for evasive action. GPWS has also been know to give rise to false warnings. The problem of false warnings is of major concern. If a substantial fraction of the warnings received are evaluated by pilots as being false or unnecessary, they will not trust these systems anymore, even if some of these warnings are correct and could save the aircraft. However since 1974 there has been several updates to the GPWS. The newest addition to the GPWS is the Enhanced Ground Proximity Warning System (EGPWS). In addition to retaining the current GPWS alerting functions, EGPWS is a more advanced system, which offers reduced chance of false alarm and also uses a self-contained worldwide airport digital terrain database and Global Positioning System (GPS) which is used in conjunction with airplane’s position, barometric altitude, and flight path information to determine potential terrain conflicts. This provides the pilot with a birds-eye view of the surrounding terrain and issues an aural alert when the aircraft projected course approaches an obstacle.
There are a maximum of seven different protection situations under which the GPWS operates. These are called modes and they depend on the conditions the aircraft is subject to.

Mode 1 alerts flight crews to excessive rates of descent, when the aircraft is near terrain. Using radar altitude and rate of descent information, the ground proximity warning computer monitors the flight profile when the aircraft is within 2,500 feet of terrain. If there is an excessive rate of descent for the aircrafts current altitude above ground level, the red GPWS warning lamp will illuminate and the voice alert “SINK RATE” will be heard. If the rate of descent continues or worsens a continuous “WOOP WOOP PULL UP” voice warning will be heard. As the aircraft recovers from the excessive rate of descent the red GPWS warning lamp will extinguish and the voice warning will cease.


Mode 2 warns flight crews of excessive closure rates with rapidly rising terrain. Using airspeed, rate of descent and radar altitude GPWS compares terrain below the aircraft to the flight path. If terrain rises significantly within 2000 feet of the aircraft a closure rate is measured. When the closure rate shows danger to the aircraft the red GPWS warning light will illuminate and the voice alert “TERRAIN TERRAIN” will be heard. If the closure rate continues or worsens the voice call out will change to a continuous “WOOP WOOP PULL UP” warning until terrain is no longer a threat to the aircraft. To help ensure sufficient recovery the red GPWS warning lamp will remain illuminated until the aircraft climbs 300 feet above the altitude where the last “PULL UP” voice warning was heard.

Mode 3 warns flight crews of unplanned for descents and loss of altitude after take-off or during a missed approach. GPWS recognises a take-off or missed approach using aircraft speed, radar altitude, barometric altitude and aircraft configuration changes. GPWS then monitors radar altitude and barometric altitude during the initial climb out. Should the aircraft descend, GPWS compares any loss of barometric altitude to highest altitude gained. When this difference is 10% or more, the red GPWS warning lamp will illuminate and the voice alert “DON’T SINK” will be heard. When a positive rate of climb is re-established, the red GPWS warning lamp will extinguish and the voice alert will stop. Once the aircraft has gained 1000 feet above the ground, Mode 3 is disabled. When GPWS again detects a take-off or missed approach situation, Mode 3 protection is automatically enabled.



Mode 4 provides minimum terrain clearance protection in all phases of flight. GPWS will alert the flight crew to insufficient terrain clearance based on airspeed, altitude, rate of descent and aircraft configuration. Mode 4 protection is especially valuable when an aircraft is on a normal flight path, when excessive descents or closure with terrain are not a factor. Just after take-off at the same time as Mode 3 is monitoring descent rates, Mode 4 begins to build a ‘protective floor’ below the aircraft. This floor will continue to build under the aircraft until reaching 800 feet above the ground, where it will remain throughout the flight until the aircraft begins initial approach at the destination. If the aircraft penetrates this ‘protective floor’ during the flight, the red GPWS warning light will illuminate and the voice alert “TOO LOW TERRAIN” will be heard. Once the aircraft is safely back above the ‘protective floor’, the red warning lamp will extinguish and the voice alert will end. During the approach and landing GPWS monitors air speed and aircraft configuration. The minimum terrain clearance floor is automatically adjusted as the airspeed is decreased. If landing gear is not down and locked at 500 feet above ground the red GPWS warning lamp will illuminate and the voice alert “TOO LOW GEAR” will be heard. If flaps are not set to landing configuration within 170 to 250 feet of the ground the red GPWS warning lamp will illuminate and the voice alert “TOO LOW FLAPS” will be heard. Pilots may elect to disable the flap alerts by engaging the flap alert override function either prior to or during a flap alert annunciation. This allows landings in configurations other then normal landing flaps, with no GPWS flap alert.

Mode 5 provides protection for inadvertent descent below a glide slope while on an approach using the Instrument Landing System (ILS). GPWS monitors the flight path in relation to the glide slope centre line while on the ILS final approach corse. The Mode 5 glide slope alert is enabled when a navigational receiver is tuned to an ILS frequency and receiving a glide slope signal, the landing gear is down and the radar altitude is less then 1000 feet. Should the aircraft descend a preset amount below the glide slope, the amber GPWS glide slope alert lamp will illuminate and the voice message “GLIDE SLOPE” will be heard. Glide slope voice messages are initially softer and less frequent then other GPWS alerts and warnings, but become louder and more frequent as the aircraft deviation below glide slope worsens. Once the pilot makes a correction and the aircraft is returning to the glide slope centre line, the amber glide slope alert lamp will extinguish and the glide slope voice alert will cease. To allow repositioning below glide slope when visual conditions are achieved or on localiser only approaches, pilots may elect to disable the Mode 5 glide slope alert function. This is done by depressing the amber glide slope alert lamp or switch either prior to or during a glide slope alert annunciation.

Mode 6 contains altitude and bank angle call out functions designed to increase situational awareness for the flight crew. These call out are selected by the airline when GPWS initially installed in the aircraft. The decision height minimum call out is a function of radar altitude. The pilot sets the radar altitude *BUG or Descision Height (DH) manually and GPWS provides the call out “MINUMUMS” at the selected altitude on the approach to landing. Altitude call outs can be chosen from a large menu of possible combinations. These call outs are provided to pilots as a means to confirm aircraft position during approach. An example of one combination is “500…300…100…50”. Bank angle call outs are designed to alert flight crews to the possibility of over-banking, which may lead to serious flight path deviation or loss of control. These call outs occur at preset maximum limits tailored to the aircraft type. If the aircraft exceeds the preset maximum a “BANK ANGLE” call out will be heard. Near the ground radar altitude is used to advance the bank angle alert so call outs are given sooner providing additional time for pilot corrective action. The voice warning will cease when the aircraft is no longer exceeding the maximum limits.

Mode 7 protection offers detection and warning for encounters with wind shear up to 1500 feet above ground. GPWS uses airspeed, radar altitude, barometric rate, angle of attack, air temperature, vertical and longitudinal acceleration 500 feet above the ground and aircraft specific performance information to detect encounters with severe wind shear. When an aircraft first encounters the strong head winds, decreasing tail winds and vertical updraughts associated with the outflow of microburst activity, the amber GPWS wind shear alert lamp will illuminate and remain on as long as the condition persists. Some systems will also annunciate the voice alert “CAUTION WIND SHEAR”. If the aircraft enters the area of microburst activity where head winds rapidly decrease, tailwinds increase and vertical downdraughts become severe the red GPWS wind shear warning lamp will illuminate, a short two tone siren will be heard followed by the voice warning “WIND SHEAR”. GPWS lamps and voice warning will cease when the aircraft is no longer in wind shear condition. Recovery should always continue to a safe altitude above terrain. The Mode 7 GPWS protection is only offered in the Mk V and Mk VII GPWS.


Because the overwhelming majority of Controlled Flight Into Terrain (CFIT) accidents occur near an airport and the fact that aircraft operate in close proximity to terrain near an airport. The terrain database contains higher resolution grids for airport areas. Lower resolution grids are used outside airport areas for aircraft en-route altitude make CFIT accident less likely and terrain feature detail is less important to the flight crew. With the use of accurate GPS or *FMS information, the EGPWS is provided present position, track and ground speed. With this information the EGPWS is able to present a graphical plan view of the aircraft relative to the terrain and advise the flight crew of potential conflict with the terrain or obstacle. Conflicts are recognised and alerts provided when terrain violates specific computed envelope boundaries on the projected flight path of the aircraft. Alerts are provided in the form of visual light annunciation of a caution or warning, audio annunciation based on the type of conflict and colour enhanced visual display of the terrain or obstacle relative to the forward look of the aircraft. The terrain display is provided on the weather radar indicator *ETHAS display or a dedicated EGPWS display and may or may not be displayed automatically. The terrain is displayed relative to the aircraft altitude in three distinct colours using a dot pattern of varying density. Light density green coloured dots indicate terrain 1000 to 2000 feet below the aircrafts current altitude. Medium density green dots indicate terrain at a safe altitude below the aircraft down to 1000 feet below. Medium density yellow dots indicate terrain from approximately 500 feet below the aircrafts altitude depending on phase of flight up to 1000 feet above the aircraft. High density yellow dots indicate terrain 1000 to 2000 feet above the aircraft. And high density red dots indicate terrain 2000 feet or more above the aircrafts current altitude. As an aircraft approaches significant terrain EGPWS looks ahead and provides alerts and warning to the flight crew. At one minute from dangerous terrain a voice alert “CAUTION TERRAIN” is heard twice and the threat is displayed as a solid bright yellow colour. This alert will repeat is the threat is still present after seven seconds. If no action is taken at thirty seconds the voice warning “TERRAIN TEARRAIN PULL UP PULL UP” and the terrain threat is displayed as a solid bright red colour.

EGPWS warns of terrain 60 seconds prior to encountering the terrain. This is approximately 30-45 seconds more than that of traditional GPWS allowing the pilot flying the aircraft more time to react. In addition to the features previously mentioned, the new system also does not rely on landing gear or wing flap positions to determine if the aircraft is landing. Instead, it uses its database of airport locations to determine if the airplane is approaching the runway. This function of the EGPWS addresses the "No Warning" category of CFIT accidents that have occurred because a traditional GPWS is de-activated when the landing gear is down. A study of 15 such CFIT accidents estimated that 45 out of 77 (58%) of lives lost could have been avoided through the use of EGPWS in the 15 crashes.























GLOSSARY:

GPWS: Ground Proximity Warning System. A radar-based flight deck system to give pilots audible warning by means of horns, hooters, taped or synthetic voices of terrain close beneath an aircraft's flight path.

EGPWS: Enhanced Ground Proximity Warning System. A new and improved form of Ground Proximity Warning System.

ILS: Instrument Landing System. Consists of the localizer, the glide slope and marker radio beacons (Outer, Middle, Inner). It provides horizontal and vertical guidance for the approach.

ICAO: International Civil Aviation Organisation

GPS: Global Positioning System (Navstar). Navigational system using orbiting satellites to determine the aircraft's position on the Earth. Developed at first for military use, then widespread on commercial and private aircraft, it is now expected to replace the ground-based navigational systems for its accuracy and reliability.

CFIT: Controlled Flight Into Terrain. It describes an accident whereby an airworthy aircraft, under complete control of the pilot(s), inadvertently flies into terrain (or an obstacle, or water). The pilots are generally unaware of the danger until it is too late.

DH: Descision Height. The preset height at which the pilot must decide if he is landing or doing a go-around.





BIBLIOGRAPHY:

1. http://en.wikipedia.org/wiki/GPWS

2. http://www.egpws.com/general_information/general_information.htm

3. http://www.boeing-727.com/Data/systems/infogpws.html

4. http://www.pilotfriend.com/flight_training/aircraft%20instruments/Miscellaneous
%20Instruments1.htm

5. http://www.airodyssey.net/reference/glossary.html

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