The automated ground collision avoidance system (Auto GCAS) on the F-16 is set up for only 5G's.

If the system predicts an imminent collision, an autonomous avoidance maneuver—a roll to wings-level and +5g pull—is commanded at the last instance to prevent ground impact.

(Lockheed Martin)

Why is that? Is it enough to pull out from the dangerous attitude? Because of the max speed or minimum level in peace time?

  • 1
    $\begingroup$ "Roll wings level and pull up at 5g" isn't going to save you if you are already pulling a 5g overbanked turn at 250 feet AGL in a winding narrow valley, like this training exercise in the UK: youtube.com/watch?v=kT7qrYi8R_M $\endgroup$
    – alephzero
    Aug 17, 2021 at 22:19

1 Answer 1


It's a tradeoff, as explained in the USAF patent:

The value of the gain function (N3) depends on the load factor capability of the aircraft. A reasonable value for N3 is between 50 and 75 percent of the maximum load factor capability of the aircraft, (i.e., a 9 g aircraft would have between a 5 g and 7 g recovery load factor). The actual value chosen will be a tradeoff on the part of the aircraft operator and may depend on aircraft mission. A 50 percent value for N3 will allow the pilot more capability to override the recovery maneuver while a 75 percent value for N3 lowers the auto recovery initiation altitude and helps avoid nuisance commands for recoveries and thus makes the auto recovery system more transparent to the pilot while maneuvering close to a floor altitude. [emphasis added]

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— Bice, Gregory W., Mark A. Skoog, and John D. Howard. "Aircraft ground collision avoidance and autorecovery systems device." U.S. Patent No. 4,924,401. 8 May 1990.

The patent also explains if the terrain slope increases (when the selected floor is below the terrain), then additional g's will be commanded.

(By the way, the patent is based on the F-16, but is applicable to any airplane and the commanded g's takes a few factors into account, i.e. if GCAS is installed on say a KC-135, then it most certainly won't be 5 g.)

Below are two frames from two videos. The first shows Auto GCAS saving an unconscious pilot; the second a testing of the system. In the first the g-reading reached 9.1 g's (unclear when the pilot recovered and took over but judging by the breathing, soon after system activation); in the second it was a constant 5 g.

enter image description here enter image description here

  • $\begingroup$ Thanks for your answer, so terrain mapping is the reason for the max G(flatter - low G, but can also provide 7 G's for recovery. Guess while low value of G's pulls give the pilot less stress for neck injury and time to getting better faster $\endgroup$
    – George Geo
    Aug 17, 2021 at 19:20
  • 10
    $\begingroup$ ‘Only 5G’ is, I suspect, a statement from somebody who hasn’t experienced it. $\endgroup$
    – Frog
    Aug 17, 2021 at 21:02
  • 3
    $\begingroup$ One use-case for the GCAS is when the pilot is unconscious, having G-LOC'd himself already. (Rare, but it happens.) In that case, the pilot isn't going to be performing any sort of anti-G strain, nor controlling where his head is. If the default answer were "well just pull 9 G's" then you probably risk some pretty serious neck injuries from the system. Frog is correct -- 5 G's is a lot -- probably enough to G-LOC most people in the general population. $\endgroup$
    – Ralph J
    Aug 17, 2021 at 21:58

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