Editing Ground proximity warning system (section)

== 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}}