The recent AF953 near-miss incident as reported by the BBC describes the ground collision alert warning triggering, preventing a CFIT by the plane.

An initial incident report published by BEA said an automated warning, saying "Pull up", was activated. The plane then ascended quickly from 9,000ft (2,743m) to 13,000ft (3,962m) to avoid crashing.

BEA did not say how close the plane came to Mount Cameroon.

How close does a plane have to come within the ground (as measured by the radar) before the warning triggers? Does this number vary significantly between aircraft models, or is there some legislated fixed value for the distance?

  • $\begingroup$ AFAIR it is fixed time. $\endgroup$
    – Jan Hudec
    May 27, 2015 at 12:55
  • $\begingroup$ I found an graph on wikipedia showing the altitude triggering an alarm depend on the sink rate $\endgroup$
    – Manu H
    May 27, 2015 at 13:18
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    $\begingroup$ This diagram comes from page 37 of TSO-151C and only shows Mode 1. There is not a simple, one size fits all answer to this question. $\endgroup$ May 27, 2015 at 13:57

3 Answers 3


There are 7 modes of warning

Mode 1. Large descent rate near terrain. Gear and flap position not a factor in the warning.

Mode 2. Large closure rate with terrain.

Mode 2A. Occurs for a large closure rate if the flaps are not in the landing configuration and glideslope deviation is more than 2 dots.

Mode 2B. Alerts for a large closure rate if the flaps are in the landing configuration.

Mode 3. Alerts for a large altitude loss during takeoff or during a missed approach with the flaps not in the landing configuration or landing gear not down.

Mode 4. Alerts when the airplane is too close to the terrain and the landing gear or flaps are not in the landing configuration

Mode 4A alert when the landing gear is not down and gives the aural messages TOO LOW GEAR at low airspeeds, or TOO LOW TERRAIN at high airspeeds.

Mode 4B alerts when the landing gear is down and the flaps are not in landing configuration and gives the aural messages TOO LOW FLAPS if the airplane is at low airspeed, or TOO LOW TERRAIN at high airspeeds.

Mode 5 alerts when the airplane goes below the glideslope during approach if the landing gear is down.

Mode 6 gives aural callouts when the airplane descends through set altitudes with the landing gear down. It gives callouts like MINIMUMS or DECISION HEIGHT.

Mode 7 gives windshear warnings.

Every one of these modes is altitude, speed and aircraft configuration dependent.

In the Enhanced GPWS, these triggers are also changed based on the position of the aircraft.

Regulatory requirement 14 CFR 121.354

Technical requirements TSO-151C

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    $\begingroup$ Do you have a source for your answer? Also, your answer does not answer the question at the moment. $\endgroup$
    – March Ho
    May 27, 2015 at 13:12
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    $\begingroup$ The source is Boeing but it is common knowledge. It answers part of the question which is 'How close does a plane have to come within the ground (as measured by the radar) before the warning triggers?'. The answer to which is really, it depends on the situation. $\endgroup$ May 27, 2015 at 13:16
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    $\begingroup$ As for regulatory requirements, 14 CFR 121.354 is a starting point. FAA TSO-151C are the system requirements. $\endgroup$ May 27, 2015 at 13:22
  • $\begingroup$ @SportsRacer I think if it were common knowledge the question would not have exist. And you really should provide links and references to external resources to enhance your answer $\endgroup$
    – Manu H
    May 27, 2015 at 13:24
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    $\begingroup$ @ManuH its common knowledge if you operate an airplane with EGPWS. $\endgroup$
    – casey
    May 27, 2015 at 13:54

The GPWS doesn't use radar but rather a terrain database and a GPS position. Alert are given based on time before collision rather than a fixed distance, since a distance that provides inadequate reaction time for a fast aircraft might be unreasonably large for a slow one, and it is important with warning systems like this that all alerts be valid ones, not false alarms that one gets in the habit of ignoring.

Times can vary by system, but I think it's generally about 1 minute away you get a "caution" message, and 30 seconds away, a "pull up" warning.

Link to Wikipedia article -- gives some GPWS theory but doesn't address algorithms: http://en.m.wikipedia.org/wiki/GPWS

  • $\begingroup$ Enhanced GPWS (EGPWS) uses a terrain database and GPS, however the most basic system and even EGPWS use the aircraft's radio altimeter (often called a Radar Altimeter) to do it's calculations. $\endgroup$ May 27, 2015 at 13:27
  • $\begingroup$ This is a better link for information. boeing-727.com/Data/systems/infogpws.html $\endgroup$ May 27, 2015 at 13:30

It depends on the type of aircraft and the avionics systems onboard. A STOL aircraft may not give you any warnings at all. Garmin G1000 systems are entirely configurable to the pilots settings. I personally have mine set to alert me when I'm at 500ft from any terrain.


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