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As a for instance, let's say there are 2 jets going at low airspeeds (close to $V_S$) that get too close to eachother and a TCAS Resolution Advisory callout is announced.

Would TCAS issue an RA to order one of the jets to climb, resulting in loss of airspeed and a stall?

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The FAA publishes Advisory Circular 20-151A, which says in section 2-17(a):

Because TCAS II can only accept a limited number of inputs related to airplane performance, it’s not possible to automatically inhibit CLIMB and INCREASE CLIMB RAs in all cases where it may be appropriate to inhibit such RAs. In these cases, TCAS II may command maneuvers that may significantly reduce stall margins or result in stall warning

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  • $\begingroup$ Seems like a "hard left, hard left now!" RA would be good to have in this situation... $\endgroup$ – Danny Beckett Jan 31 '15 at 13:38
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    $\begingroup$ @DannyBeckett: In a turn you have higher wing loading and therefore higher stall speed. So, no, it probably wouldn't. $\endgroup$ – Jan Hudec Jan 31 '15 at 16:37
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    $\begingroup$ @DannyBeckett: Of course the main reason why there is no turn left/right advisory is that the current TCAS is based on Mode C transponder and that only encodes altitude (and distance is calculated from response time). When they switch to ADS-B (that contains speed, heading and position), it will become possible. $\endgroup$ – Jan Hudec Jan 31 '15 at 16:51
  • $\begingroup$ It should be noted, though, that in the hierarchy of command maneuvers, warnings, and aural alerts, TCAS generally ranks lowest in order of priority. In other words, if EGPWS is issuing a "Pull Up" alert, it may suppress a TCAS RA to rapidly descend. Obviously, this sort of situation would be pretty strange, but I do recall there being an entire hierarchical list of all caution and warning systems that have priority over one another. TCAS was pretty far down on that list. $\endgroup$ – Frank Jun 2 '17 at 20:20
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    $\begingroup$ @JanHudec: Turning RAs were proposed and rejected over 25 years ago. The reason they were discarded is that vertical RAs create the needed separation faster than a turn. It takes time to get enough roll angle to get the turn rate up and lateral separation means you need more distance to accommodate the wing span. Also, all TCAS II systems use Mode S transponders which are capable of 25 foot altitude reports and ADS-B out. ADS-B is used by newer TCAS, but not in the RA calculation. It's used to reduce interrogations of distant aircraft. TAs and RAs are all based on interrogation data. $\endgroup$ – Gerry Jun 3 '17 at 3:06
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There is no more risk of stalling an aircraft from a TCAS alert than from being instructed verbally by ATC to climb to a higher altitude for traffic avoidance. Or, (in the case of slow flight at approach speeds) to execute a go-around from an approach to land.

An aircraft can only be stalled if the critical angle of attack is exceeded. A TCAS alert to climb is just that, it does not command the pilot to do anything other than climb to a safe altitude. It tells the pilot where to go, but not how to get there.

Initiating a climb at low speed does not automatically result in a stall as the question would seem to imply. Proper pilot technique is to add power and adjust attitude to maintain safe airspeed and AOA. The aircraft can only be stalled if the pilot pitches the nose up, fails to add sufficient power in the climb, allows airspeed to decay, and holds the attitude until a stall is reached.

So, the short answer is no.

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    $\begingroup$ Welcome! This is an interesting remark about pilot role, but regarding the question, this doesn't add to the selected answer. This should be a comment, not an answer. $\endgroup$ – mins Jun 1 '17 at 23:29
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    $\begingroup$ Or re-edit the existing one and let people vote with the up/down arrows? $\endgroup$ – Michael Hall Jun 2 '17 at 17:24
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    $\begingroup$ You can edit your question as many times as you want, but keep in mind this needs to be an answer to the original question (so you are competing "against" existing answers). For criticizing an existing post without fully answering, you can only comment. Users will vote if you get it correctly, each vote is 10 reps, so 5 votes and you can comment any post. There are also many questions that don't have yet an answer. $\endgroup$ – mins Jun 2 '17 at 17:32
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    $\begingroup$ It's a reasonable answer IMHO. I upvoted it. $\endgroup$ – Terry Jun 2 '17 at 18:29
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    $\begingroup$ Thanks Lnafziger. The way I read the original question it's kind of like asking "would your car's GPS ever command you to turn into the path of an oncoming truck?" Yes, it might, but as driver you are expected to exercise sound judgment. $\endgroup$ – Michael Hall Jun 5 '17 at 17:12
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I think this is possible. TCAS generates Resolution Advisories based on various factors, and two aircraft equipped with TCAS transponders then coordinate their RAs. Each aircraft knows only the bearing, altitude, and distance of the other aircraft; it gets its data from Mode S transponders, which don't send things like stall speed or maximum climb/descent rate. That's all that the basic TCAS algorithm uses. It doesn't necessarily even know its own plane's performance information.

Separate from the basic RA system, an installation can be configured to not issue "Climb" or "Increase Climb" RAs under certain circumstances (each TCAS RA is sufficient to avoid a collision if the other aircraft does nothing, so "Maintain Climb" or "Reduce Descent" or "Do Not Descend" may be enough). This doesn't change the general sense of an RA (climb/descend), but rather modifies the actual RA issued to its own aircrew. However, it does not have to be configured in such a way, and the configuration won't necessarily catch all stall situations. So a TCAS system is certainly capable of issuing an RA that will stall your plane.

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