View source for Ground proximity warning system

{{Short description|Alert system meant to prevent pilots from flying or taxiing into obstacles}}
{{Use mdy dates|date=October 2011}}
[[Image:GPWSMode2.svg|200px|thumb|A diagram showing two aircraft paths, the lower of which would trigger GPWS]]
A '''Ground Proximity Warning System''' ('''GPWS''') is a system designed to alert [[aircraft pilot|pilots]] if their aircraft is in immediate danger of [[controlled flight into terrain|flying into the ground]] or an obstacle. The United States [[Federal Aviation Administration]] (FAA) defines GPWS as a type of [[terrain awareness and warning system]] (TAWS).<ref name="faa-ac23-18">{{Cite web |url=http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/7ca84861d31651a5862569b2006dbcfe/$FILE/ATTOVW46/AC23-18.pdf |title=Archived copy |access-date=January 15, 2009 |archive-date=January 19, 2016 |archive-url=https://web.archive.org/web/20160119083037/http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/7ca84861d31651a5862569b2006dbcfe/$FILE/ATTOVW46/AC23-18.pdf |url-status=dead }}</ref> More advanced systems, introduced in 1996,<ref>{{cite web|url=http://www51.honeywell.com/aero/technology/key-technologies2/avionics3/egpws.html?c=23 |title=Enhanced Ground Proximity Warning System (EGPWS) – Honeywell Aerospace Engineering |publisher=1.honeywell.com |access-date=October 17, 2011 |url-status=dead |archive-url=https://web.archive.org/web/20111002025011/http://www51.honeywell.com/aero/technology/key-technologies2/avionics3/egpws.html?c=23 |archive-date=October 2, 2011 |df=mdy-all }}</ref> are known as '''enhanced ground proximity warning systems'''  ('''EGPWS'''), a modern type of TAWS.

==History==
{{main article|Terrain awareness and warning system#History}}
In the late 1960s, a series of [[controlled flight into terrain]] (CFIT) accidents took the lives of hundreds of people. A CFIT accident is one where a properly functioning airplane under the control of a fully qualified and certified crew is flown into terrain, water or obstacles with no apparent awareness on the part of the crew.<ref name="faa-historical-ac23-18">Note: Original text copied from U.S. FAA Circular AC23-18 [http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/7CA84861D31651A5862569B2006DBCFE?OpenDocument] {{Webarchive|url=https://web.archive.org/web/20171201043615/http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/7CA84861D31651A5862569B2006DBCFE?OpenDocument|date=December 1, 2017}}. As a work of the U.S. Government, there is no copyright on the work, and it may be freely copied, and is thus included here. Additional or reduced text and formatting, not included in the original, have been added here for clarity and emphasis.</ref>

Beginning in the early 1970s, a number of studies examined the occurrence of CFIT accidents.<ref name="faa-historical-ac23-18" /> Findings from these studies indicated that many such accidents could have been avoided if a warning device called a ground proximity warning system (GPWS) had been used.  As a result of these studies and recommendations from the [[National Transportation Safety Board|U.S. National Transportation Safety Board]] (NTSB), in 1974, the FAA required all large turbine and turbojet airplanes to install [[Technical Standard Order|TSO]]-approved GPWS equipment.<ref name="faa-historical-ac23-18" /><ref>(§§ 121.360 and 135.153) (39 FR 44439, December 18, 1974)</ref>

The UN [[International Civil Aviation Organization]] (ICAO), recommended the installation of GPWS in 1979.<ref>[http://www.flightglobal.com/pdfarchive/view/1999/1999%20-%201940.html Personality ''Flight'' Aerospace Industry Awards 1999]</ref>

[[C. Donald Bateman]], a Canadian-born engineer, developed and is credited with the invention of GPWS.<ref>{{cite web |url=http://www.invent.org/hall_of_fame/231.html |title=Invent Now &#124; Hall of Fame &#124; Search &#124; Inventor Profile |publisher=Invent.org |access-date=October 17, 2011 |url-status=dead |archive-url=https://web.archive.org/web/20110927041453/http://www.invent.org/hall_of_fame/231.html |archive-date=September 27, 2011 |df=mdy-all }}</ref>

In March 2000, the U.S. [[Federal Aviation Administration|FAA]] amended operating rules to require that all U.S. registered turbine-powered airplanes with six or more passenger seats (exclusive of pilot and copilot seating) be equipped with an FAA-approved TAWS.<ref name="faa-historical-ac23-18" /> The mandate affects aircraft manufactured after March 29, 2002.<ref name="FAR">{{cite web |url=http://rgl.faa.gov/Regulatory_and_Guidance_Library%5CrgFAR.nsf/0/465764AB441B0864862568C8006451A9?OpenDocument |title=Sec. 121.354 – Terrain awareness and warning system |access-date=April 29, 2007 |publisher=[[Federal Aviation Administration]] |archive-date=December 8, 2016 |archive-url=https://web.archive.org/web/20161208151802/http://rgl.faa.gov/Regulatory_and_Guidance_Library%5CrgFAR.nsf/0/465764AB441B0864862568C8006451A9?OpenDocument |url-status=dead }}</ref>

== Effects and statistics ==
Prior to the development of GPWS, large passenger aircraft were involved in 3.5 fatal CFIT accidents per year, falling to 2 per year in the mid-1970s. A 2006 report stated that from 1974, when the U.S. FAA made it a requirement for large aircraft to carry such equipment, until the time of the report, there had not been a single passenger fatality in a CFIT crash by a large jet in U.S. airspace.<ref>{{Cite web | last = Sabatini | first = Nicholas | title = Downward Pressure on the Accident Rate | publisher=[[Federal Aviation Administration]] | url = http://www.faa.gov/news/speeches/news_story.cfm?newsId=7170 | access-date =March 19, 2009}}</ref>

After 1974, there were still some CFIT accidents that GPWS was unable to help prevent, due to the "blind spot" of those early GPWS systems.  More advanced systems were developed.

Older TAWS, or deactivation of the EGPWS, or ignoring its warnings when an airport is not in its database,<ref>{{cite web|url=http://www.tvn24.pl/12690,1653907,0,1,lotniska-w-smolensku-moglo-nie-byc-w-bazie-gpws,wiadomosc.html |title="Lotniska w Smoleńsku mogło nie być w bazie GPWS" – Polska – Informacje – portal TVN24.pl – 27.04.2010 |publisher=Tvn24.pl |date=April 27, 2010 |access-date=October 17, 2011}}</ref> still leave aircraft vulnerable to possible CFIT incidents. In April 2010, a [[Smolensk air disaster|Polish Air Force Tupolev Tu-154M aircraft crashed near]] Smolensk, Russia, in a possible CFIT accident<ref>{{cite web|author=Wacław Radziwinowicz, Moskwa, mich |url=http://wyborcza.pl/1,76842,7786163,Sledztwo__Jak_doszlo_do_katastrofy.html |title=Śledztwo. Jak doszło do katastrofy |publisher=Wyborcza.pl |date=April 19, 2010 |access-date=October 17, 2011}}</ref> killing all passengers and crew, including the President of Poland [[Lech Kaczyński]].<ref name="Levin">{{cite news| url=https://www.usatoday.com/travel/flights/2010-04-13-fog-plane-crash-poland_N.htm | work=USA Today | title=Device spurs questions in Polish crash | first=Alan | last=Levin | date=April 13, 2010}}</ref><ref>{{cite web|url=http://www.tvp.info/informacje/swiat/tu154-mial-system-ostrzegania-nie-zadzialal/1663905 |title=Tu-154 miał system ostrzegania. Nie zadziałał? |publisher=Tvp.Info |date=April 15, 2010 |access-date=October 17, 2011}}</ref><ref>{{cite web|url=http://www.fakt.pl/Tu-154-prezydenta-nie-mial-prawa-sie-rozbic-,artykuly,69621,1.html |title=Smolot prezydenta nie miał prawa się rozbić! |date=April 19, 2010 |publisher=Fakt.pl |access-date=October 17, 2011}}</ref><ref>{{cite web|url=http://www.wprost.pl/ar/192532/Nie-bylo-naciskow-na-zaloge-Tu-154-zapis-czarnych-skrzynek-bedzie-upubliczniony/ |title=Wprost 24 – Nie było nacisków na załogę Tu-154, zapis czarnych skrzynek będzie upubliczniony |publisher=Wprost.pl |date=April 16, 2010 |access-date=October 17, 2011}}</ref> The aircraft was equipped with TAWS made by Universal Avionics Systems of Tucson.<ref name="Levin"/> According to the Russian Interstate Aviation Committee, the TAWS was turned on.<ref>{{Cite web |url=http://www.mak.ru/russian/info/news/main_frame.html |title=Untitled Page |access-date=April 24, 2010 |archive-url=https://web.archive.org/web/20100325181802/http://www.mak.ru/russian/info/news/main_frame.html |archive-date=March 25, 2010 |url-status=dead }}</ref> However, the airport where the aircraft was going to land (Smolensk (XUBS)) is not in the TAWS database.<ref>{{cite web |last= |first= |title=Universal Avionics - Error Page |url=http://www.uasc.com/documents/support/usable_datums.pdf |url-status=dead |archive-url=https://web.archive.org/web/20110717130644/http://www.uasc.com/documents/support/usable_datums.pdf |archive-date=July 17, 2011 |access-date=April 24, 2010 |website=www.uasc.com |publisher=Universal Avionics Systems Corporations}}</ref><ref>{{cite web |last= |first= |title=Universal Avionics - Error Page |url=http://www.uasc.com/documents/support/Unknown_Datum_Codes.pdf |url-status=dead |archive-url=https://web.archive.org/web/20170918021247/http://www.uasc.com/documents/support/Unknown_Datum_Codes.pdf |archive-date=September 18, 2017 |access-date=April 24, 2010 |website=www.uasc.com |publisher=Universal Avionics Systems Corporations}}</ref> In January 2008 a [[2008 Polish Air Force C-295 Mirosławiec crash|Polish Air Force Casa C-295M]] crashed in a CFIT accident near Mirosławiec, Poland, despite being equipped with EGPWS; the EGPWS warning sounds had been disabled, and the [[Pilot in command|pilot-in-command]] was not properly trained with EGPWS.<ref>{{cite web|url=http://aviation-safety.net/database/record.php?id=20080123-0 |title=ASN Aircraft accident CASA C-295M 019 Miroslawiec AB |publisher=Aviation-safety.net |access-date=October 17, 2011}}</ref>

== Commercial aircraft ==
{{More citations needed section|date=May 2022}}
[[Image:FAA excessive sink rate graph.svg|thumb|right|The FAA specifications<ref>{{cite web|url=https://drs.faa.gov/browse/excelExternalWindow/689C6CCC0EAA329A862581920072014B.0001|title=Technical Standard Orders (TSO) - TSO-C151d.pdf|website=drs.faa.gov}}</ref> have detailed requirements for when certain warnings should sound in the cockpit.]]

The system monitors an aircraft's height above ground as determined by a [[radar altimeter]]. A computer then keeps track of these readings, calculates trends, and will warn the flight crew with visual and audio messages if the aircraft is in certain defined flying configurations ("modes").

The modes are:
#Excessive descent rate ("SINK RATE" "PULL UP"){{#tag:ref|On older GPWS, a noise often referred to as a "whoop" may sound twice before the "PULL UP" aural warning.|group="Note"}}
#Excessive terrain closure rate ("TERRAIN" "PULL UP")
#Excessive large obstacle closure rate ("OBSTACLE" "PULL UP")
#Altitude loss after takeoff or with a high power setting ("DON'T SINK")
#Unsafe terrain clearance ("TOO LOW – TERRAIN" "TOO LOW – GEAR" "TOO LOW – FLAPS")
#Excessive deviation below [[Instrument landing system glide path|glideslope]] ("GLIDESLOPE")
#Excessively steep bank angle ("BANK ANGLE")
#[[Downburst#Danger to aviation|Windshear]] protection ("WINDSHEAR")

The traditional GPWS does have a blind spot. Since it can only gather data from directly below the aircraft, it must predict future terrain features. If there is a dramatic change in terrain, such as a steep slope, GPWS will not detect the aircraft closure rate until it is too late for evasive action.

In the late 1990s, improvements were developed and the system is now named ''"Enhanced Ground Proximity Warning System"'' (EGPWS/[[Terrain awareness and warning system|TAWS]]). The system is combined with a worldwide digital terrain database and relies on [[Global Positioning System]] (GPS) technology. On-board computers compare current location with a database of the Earth's terrain. The Terrain Display gives pilots a visual orientation to high and low points near the aircraft.

EGPWS software improvements are focused on solving two common problems: no warning at all, and late or improper response.

===No warning===
The primary cause of CFIT occurrences with no GPWS warning is landing short. When the landing gear is down and landing flaps are deployed, the GPWS expects the airplane to land and therefore, issues no warning. However, the GPWS can also malfunction because of a short circuit. On September 26, 1997, [[Garuda Indonesia Flight 152]] crashed into a hilly area, killing all 222 passengers and 12 crew on board. Despite the fact that the plane was nearing terrain, the GPWS did not activate, even though the landing gear and landing flaps were not deployed. EGPWS introduces the Terrain Clearance Floor (TCF) function, which provides GPWS protection even in the landing configuration.

===Late warning or improper response===
The occurrence of a GPWS alert typically happens at a time of high workload and nearly always surprises the flight crew. Almost certainly, the aircraft is not where the pilot thinks it should be, and the response to a GPWS warning can be late in these circumstances. Warning time can also be short if the aircraft is flying into steep terrain since the downward-looking radio altimeter is the primary sensor used for the warning calculation. The EGPWS improves terrain awareness and warning times by introducing the Terrain Display and the Terrain Data Base Look Ahead protection.{{Citation needed|date=March 2019}}

== Incidents ==
In commercial and airline operations, there are legally mandated procedures that must be followed should an EGPWS caution or warning occur. Both pilots must respond and act accordingly once the alert has been issued. In Indonesia, the captain of [[Garuda Indonesia Flight 200]] was charged with manslaughter for not adhering to these procedures.<ref>{{cite web|last=Thomas |first=Geoffrey |url=http://atwonline.com/operations/yogyakarta-crash-captain-arrested-charged-manslaughter |title=ATW: Yogyakarta crash captain arrested, charged with manslaughter |publisher=Atwonline.com |date=February 6, 2008 |access-date=April 29, 2019}}</ref>

In 2015, [[Air France]] Flight 953 (a Boeing 777-200ER aircraft) avoided controlled flight into terrain after the EGPWS detected [[Mount Cameroon]] in the aircraft's flight path. The pilot flying immediately responded to the initial warning from the EGPWS.<ref>{{cite web |last1=Hradecky |first1=Simon |title=Incident: Air France B772 near Douala on May 2nd 2015, GPWS averts controlled flight into terrain at FL090 |url=http://avherald.com/h?article=4867f2bd&opt=0 |website=The Aviation Herald |access-date=21 April 2022}}</ref>

== General aviation ==
{{main article|Terrain awareness and warning system#TAWS types}}

TAWS equipment is not required by the U.S. [[Federal Aviation Administration|FAA]] in piston-engined aircraft, but optional equipment categorised as '''TAWS Type C''' may be installed. Depending on the type of operation, TAWS is only required to be installed into turbine-powered aircraft with six or more passenger seats.

A smaller and less expensive version of EGPWS was developed by [[AlliedSignal]] (now merged with [[Honeywell]]) for general aviation and private aircraft.<ref>{{cite web |url=http://www.aeroworldnet.com/3tw05179.htm |title=AeroWorldNet(tm) – AlliedSignal Develops New EGPWS, Display for General Aviation Market [May 17, 1999&#93; |publisher=AeroWorldNet |date=May 17, 1999 |access-date=October 17, 2011 |url-status=dead |archive-url=https://web.archive.org/web/20120106021157/http://www.aeroworldnet.com/3tw05179.htm |archive-date=January 6, 2012 |df=mdy-all }}</ref>

== Fast military aircraft ==
For fast military aircraft, the high speed and low altitude that may frequently be flown make traditional GPWS systems unsuitable, as the blind spot becomes the critical part. Thus, an enhanced system is required, taking inputs not only from the [[radar altimeter]], but also from [[inertial navigation system]] (INS), [[Global Positioning System]] (GPS), and [[Aircraft flight control system|flight control system]] (FCS), using these to accurately predict the flight path of the aircraft up to {{Convert|5|mi|km}} ahead. [[Computer cartography|Digital maps]] of terrain and obstacle features are then used to determine whether a collision is likely if the aircraft does not pull up at a given pre-set [[G-force|g-level]]. If a collision is predicted, a cockpit warning may be provided. This is the type of system deployed on aircraft such as the [[Eurofighter Typhoon]].<ref>[http://www.eurofighter.com/news/article61.asp Eurofighter Typhoon – BAE SYSTEMS delivers first Eurofighter Typhoon Ground Proximity Warning] {{webarchive |url=https://web.archive.org/web/20080203123456/http://www.eurofighter.com/news/article61.asp |date=February 3, 2008 }}</ref>  The U.S. FAA has also conducted a study about adapting 3-D military [[thrust vectoring]] to recover civil jetliners from catastrophes.<ref>"Multiaxis Thrust Vectoring Flight Control Vs Catastrophic Failure Prevention", Reports to U.S. Dept. of Transportation/FAA, Technical Center, ACD-210, FAA X88/0/6FA/921000/4104/T1706D, FAA Res. Benjamin Gal-Or, Grant-Award No: 94-G-24, CFDA, No. 20.108, December 26, 1994; "Vectored Propulsion, Supermanoeuvreability, and Robot Aircraft", by Benjamin Gal-Or, Springer Verlag, 1990, {{ISBN|0-387-97161-0}}, 3-540-97161-0.</ref>

On May 5, 2016, a military GPWS called [[Automatic Ground Collision Avoidance System]] (Auto-GCAS) equipped aboard an [[General Dynamics F-16 Fighting Falcon|F-16]] was activated after a trainee pilot lost consciousness from excessive G forces during basic fighter manoeuvre training. In an approximately 55 degree nose down attitude at {{convert|8,760|ft|m|abbr=on}} and a speed of {{convert|750|mph|km/h|abbr=on}}, the Auto-GCAS detected that the aircraft was going to strike the terrain and executed an automatic recovery, saving the pilot's life.<ref>{{cite web|url=https://aviationweek.com/defense-space/budget-policy-operations/auto-gcas-saves-unconscious-f-16-pilot-declassified-usaf|title=Auto-GCAS Saves Unconscious F-16 Pilot—Declassified USAF Footage|website=aviationweek.com}}</ref>

== See also ==
{{Portal|Aviation}}
* [[Acronyms and abbreviations in avionics]]
* [[Air New Zealand Flight 901]]
* [[Airborne collision avoidance systems]] (ACAS)
* [[Airport surveillance and broadcast systems]]
* [[American Airlines Flight 965]]
* [[Ansett New Zealand Flight 703]], a CFIT accident associated with GPWS malfunction
* [[Fly-by-wire control systems]]
* [[Santa Barbara Airlines Flight 518]]
* [[South African Airways Flight 228]]
* [[TERPROM]]
* [[Traffic Collision Avoidance System]] (TCAS)
* [[TWA Flight 514]]
* [[Voice warning system]]

== Notes ==
{{Reflist|group="Note"}}

== References ==
{{reflist}}

== External links ==
* [https://www.invent.org/inductees/c-donald-bateman C. Donald Bateman at the National Inventors Hall of Fame]
*[http://atwonline.com/news/story.html?storyID=11641 Garuda Captain ignores EGPWS warnings]
*[https://web.archive.org/web/20120106021157/http://www.aeroworldnet.com/3tw05179.htm AeroWorldNet – 1999 EGPWS costs]
*[http://www.faa.gov/news/speeches/news_story.cfm?newsId=7170 Downward Pressure on the Accident Rate]—speech delivered by Nicholas A. Sabanti, FAA Associate Administrator for Aviation Safety
*[https://web.archive.org/web/20080203123456/http://www.eurofighter.com/news/article61.asp Eurofighter Typhoon – BAE SYSTEMS delivers first Eurofighter Typhoon Ground Proximity Warning<!-- Bot generated title -->]
* [http://www51.honeywell.com/aero/Products-Services/Avionics-Electronics/Egpws-Home3/GeneralInformation.html#Product%20Literature3 EGPWS Video Clips] {{Webarchive|url=https://web.archive.org/web/20140701123702/http://www51.honeywell.com/aero/Products-Services/Avionics-Electronics/Egpws-Home3/GeneralInformation.html#Product%20Literature3 |date=July 1, 2014 }}—Several professional video clips demonstrating enhanced GPWS.
* [https://web.archive.org/web/20070928041803/http://www.flitepartners.com/Enhanced-Ground-Proximity-Warning-System-EGPWS.htm FlitePartners page on EGPWS installations by STC]
* [http://www.boeing-727.com/Data/systems/infogpws.html Boeing 727 GPWS explained with clear diagrams]
* [https://web.archive.org/web/20140628153549/http://www51.honeywell.com/aero/Products-Services/Avionics-Electronics/EGPWS-Home.html Honeywell Aerospace EGPWS Website]
* [https://web.archive.org/web/20080212002743/http://www.planecrashinfo.com/sounds.htm Cockpit Warning Sounds]—planecrashinfo.com

{{Satellite navigation systems}}

{{DEFAULTSORT:Ground Proximity Warning System}}
[[Category:Avionics]]
[[Category:Aircraft collision avoidance systems]]
[[Category:Warning systems]]