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The TCAS warning says: " DESCEND, DESCEND" and immediately after the crew receives: "TERRAIN, TERRAIN", what would they do?

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    $\begingroup$ Best guess: they would panic $\endgroup$ – kevin Nov 19 '15 at 13:12
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    $\begingroup$ I would prefer to take on the other aircraft rather than the terrrain, but that's just me. $\endgroup$ – Tyler Durden Nov 19 '15 at 18:11
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TCAS (II) does not give the descend RA below 1200 ft AGL while climbing or below 1000 ft AGL while descending.

TCAS Inhibits

Image from FAA Advisory Circular 20-151A Table 2

In case such an unlikely event happens (and is not an equipment malfunction), the best possible maneuver is to climb away from the incoming aircraft i.e. turn and climb to the side of the previous route.

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    $\begingroup$ EGPWS has a "look ahead" feature so can give a terrain warning even if you are not within 1200' AGL.... $\endgroup$ – Lnafziger Nov 19 '15 at 16:16
  • $\begingroup$ @Lnafziger, the table implies that it should switch to TA ("TRAFFIC, TRAFFIC") in that case. $\endgroup$ – Jan Hudec Nov 20 '15 at 14:57
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I would pull up and once in a stable climbs I would turn in either direction in this situation. If I get a warning to descend it is to prevent a conflict with another aircraft, whereas a terrain warning is to prevent impact with the ground. The ground is much bigger than another airplane, so the lower risk is collision with an aircraft. In other words, if I descend I will certainly hit the ground, if I climb I may hit another aircraft, so climbing wins. Turning in either direction will hopefully decrease the collision risk.

It would suspect this is an unlikely situation, TCAS should not give an instruction which would put you into another conflict.

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  • $\begingroup$ Wouldn't turning increase the probability of crashing with the ground and/or the aircraft? $\endgroup$ – Tom Nov 19 '15 at 15:20
  • $\begingroup$ If the aircraft is climbing then contact with the ground shouldn't be a concern if turning. If you fly straight ahead then you will potentially be putting the aircraft into conflict again, so while turning could potentially increase the possibility of collision it has a greater chance of decreasing it. $\endgroup$ – GdD Nov 19 '15 at 15:38
  • $\begingroup$ Makes sense. Also wouldn't it be better to instruct the aircraft above to ascend? what is the likelihood of this ever occurring? $\endgroup$ – Tom Nov 19 '15 at 15:41
  • $\begingroup$ This scenario is pretty unlikely as I don't think TCAS would say to descend if it was going to cause a terrain warning. As for communicating, that's actually a bad idea as it could cause confusion. See the Underlingen disaster as an example of that. $\endgroup$ – GdD Nov 19 '15 at 16:35
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    $\begingroup$ @GdD If the aircraft is climbing then contact with the ground shouldn't be a concern if turning. <-- Depends on the terrain. For example if you got a GPWS alert in a canyon because the floor is rising making a turn (even in a climb) could cause you to impact the canyon wall. I'd wager the climb and the chance of hitting another aircraft would still be preferable to a descent and the certainty of hitting terrain though: You've got a better chance with Big Sky than you do with Big Dirt. $\endgroup$ – voretaq7 Nov 19 '15 at 18:52

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