On February 4th 2023, there was a near miss at the airport in Austin TX, where a 767 performing an auto land in foggy conditions nearly collided with a 737 that was taking off on the same runway. The Tower Controller mistakenly cleared the 737 for takeoff with only a 3 mile final for the 767 and resulted in a near miss. The 767 initiated a go-around after noticing the error and barely avoided disaster.

My question is if the TCAS of the 767 would have issued an alert with the 737 on the ground? Does TCAS only work for aircraft in the air or can it also issue alerts for runway incursions such as this one?

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    $\begingroup$ Please rephrase your question to ask about the general case, not a particular incident. The answer by BowlOfRed will remain valid. Questions speculating on accidents/incidents still under investigation are specifically off-topic on Av.SE. $\endgroup$
    – Ralph J
    Commented Feb 7, 2023 at 4:42

1 Answer 1


Per the FAA booklet: Introduction to TCAS II, it does not alert against ground targets.

The ground level estimate is then subtracted from the pressure altitude received from each Mode C equipped nearby aircraft to determine the approximate altitude of each aircraft above the ground. If this difference is less than 360 feet, TCAS considers the reporting aircraft to be on the ground. If TCAS determines the intruder to be on the ground, it inhibits the generation of advisories against this aircraft.

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  • $\begingroup$ This has me wondering ... the altimeter has the local air pressure set, and will always have some error ... The radar altimeter less so, but I'm sure it still does. How is estimated elevation of ground calculated, and/or why not simply use the radar altimeter value in the first place? I wonder what the algorithm/calculation actually is. $\endgroup$
    – Steve
    Commented Feb 9, 2023 at 2:56
  • $\begingroup$ After thinking a bit more, is TCAS using the radar altimeter, and that chart is actually just saying the error between the RA and the baro altimeter is called "estimated elevation of ground"? (I'm not a pilot) $\endgroup$
    – Steve
    Commented Feb 9, 2023 at 3:03
  • $\begingroup$ RA isn't always available, so pressure altitude is normally used. Local air pressure isn't used. So it doesn't know how far another plane is above the ground, just approximate MSL (which is fine for collision detection). It has to guess if it's on the ground based on the local height (which may be very different some distance away). $\endgroup$
    – BowlOfRed
    Commented Feb 9, 2023 at 4:05

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