If the other traffic is at the same level, and neither climbing or descending. Will the RA say "CLIMB" or will it say "DESCEND"? How will it be decided and what are the possibilities?


  • Mode S is not available
  • Only one aircraft has TCAS

Will it be random?

  • 1
    $\begingroup$ @ymb1 that concept makes sense to me, if it is the case that the "coordination" mentioned is not the same as the "negotiation" that is apparently only available in later versions of TCAS. I'm not clear on that. Also, if the two aircraft are truly level at the exact same altitude, how does the more-authoritative aircraft (lower Mode S) determine what the solution is? Does it, for example, decide to dive (an easier maneuver) and force the other one to climb? I could imagine that being the case but I do not know. $\endgroup$
    – randomhead
    Commented Apr 10, 2021 at 19:41

3 Answers 3


The TCAS algorithms are specified in RTCA/DO-185B (not freely available).

In case of an encounter at same altitude with a non TCAS equipped aircraft, assuming that neither climb nor descent are inhibited, the TCAS will produce a descent advisory (part 2, sec. - sense selection).

This is a consequence of the somewhat subtle signs used in the specification (<= for descend, > for climb).

A non mode-S intruder will broadcast altitude with a 100ft resolution, and a TCAS equipped aircraft will normally use barometric altitude with 25 ft resolution, so the two aircraft do not need to be at exactly the same altitude - in the worst case, a difference of up to 62 ft will be interpreted as "same altitude".

There are also provisions for climb and/or descent inhibits. In particular descend RAs are inhibited below 1000ft radio altitude.

Climb RAs are inhibited when the aircraft performance are not compatible with a 1500 fpm climb (for example, one engine out). The exact mechanism through which this information is provided to the TCAS computer is aircraft dependent.

In your case of same-altitude encounter, if descend RAs are inhibited, a climb RA will be generated.


As requested, more clarification.

A simplified description of RTCA-DO185B initial sense selection follows. This description assumes that the intruder is not TCAS equipped, and that the firmness (confidence in intruder altitude and position) is sufficiently high.

First, choose the direction (climb or descent) that will produce the highest separation at CPA (see below). If equal, select descent (see 3.52 Noncrossing_Biased_Climb and 3.53 Noncrossing_Biased_Descent). Denote this direction with D

  • If a RA in the D direction does not result in a crossing (see below), then issue an RA in the D direction.
  • Otherwise, if the opposite direction still provides at least ALIM separation, then issue an RA in the direction opposite to D.
  • Otherwise, issue an RA in the D direction.

Separation: The TCAS algorithm models a RA response as follows:

  1. 5 seconds of reaction time (pilot maintains current airspeed and rate-of-climb)
  2. A segment with a constant acceleration of 8 ft/s^2 in the direction of the RA
  3. A final segment at constant rate of climb (usually + or - 1500 fpm).

To select the sense, the TCAS algorithm considers:

  • Distance from intruder
  • Time derivative of distance from intruder (closure rate)
  • Time derivative of the closure rate
  • Ownship altitude and rate of climb
  • Intruder altitude and rate of climb

With these 5 parameters, the vertical separation at the CPA (Closest Point of Approach) can be computed. In this scenario, the relative altitude is zero, and the rate of climb of both intruder and ownship are 0. Due to the symmetry of the model, the separation will be the same for both climb and descent.


Crossing is defined as either:

  • A climb RA, with ownship at least 100 ft below the intruder
  • A descent RA, with ownship at least 100 ft above the intruder

In this scenario, ownship and intruder altitude are equal, so neither direction will result in a crossing RA.

  • $\begingroup$ Can you provide us more details on that equation? I think it's strange that TCAS II considers only the altitude as parameter (see my answer and the NASA document) $\endgroup$
    – ocirocir
    Commented Apr 11, 2021 at 12:33
  • $\begingroup$ And this FAA document: faa.gov/documentlibrary/media/advisory_circular/… $\endgroup$
    – ocirocir
    Commented Apr 11, 2021 at 12:36

It is not possible to give an answer, because the TCAS II RA algorithm is not so simple. It isn't like "if A is 50 feet above B then A climb and B descent", it consider many different parameters to take a decision. Under certain conditions, it is possible that even if our aircraft is below the intruder, the RA can instruct us to climb.

This paper from NASA describes the equations on which TA and RA TCAS II algorithms are implemented and states:

The sense of an RA is computed based on the direction for the ownship maneuver that provides a greater vertical separation, with a bias towards the non-crossing direction.

This depends on both aircraft TAS, vertical speeds, horizontal separation, and many other parameters. The final equation is the following one (I highlighted the return values):

enter image description here

The comparison highlighted in blue tells us that in the rare case that performing a climb or a descent will give exactly the same vertical separation values, then CLIMB is performed.

See also Page 29 in TCAS II FAA booklet.

  • $\begingroup$ I used that same paper in an answer I just posted and deleted :D I think the answer based on RTCA/DO-185B by sbabbi is more correct, as that algorithm you and I came across is for UAS. $\endgroup$
    – user14897
    Commented Apr 11, 2021 at 12:25
  • $\begingroup$ @ymb1 I don't have access to DO-185B, but I suspect that is for TCAS I and not TCAS II $\endgroup$
    – ocirocir
    Commented Apr 11, 2021 at 12:26
  • $\begingroup$ I think it's TCAS II, see e.g. here: standards.globalspec.com/std/1607299/RTCA%20DO-185 $\endgroup$
    – user14897
    Commented Apr 11, 2021 at 12:27
  • $\begingroup$ @ymb1 I feel it as strange that depends only on altitude. TCAS II should consider also other parameters to decide the direction, not only the altitude $\endgroup$
    – ocirocir
    Commented Apr 11, 2021 at 12:32
  • $\begingroup$ NASA document also agrees with this FAA document: faa.gov/documentlibrary/media/advisory_circular/… $\endgroup$
    – ocirocir
    Commented Apr 11, 2021 at 12:36

The original poster's question assumes that mode S is not available. The original poster did not stipulate that only one aircraft lacks mode S, therefore, neither aircraft has mode S.

Mode S is necessary for TCAS II, and therefore one cannot have a resolution advisory without a mode S transponder.

Under the conditions stipulated, no mode S and only one aircraft with TCAS, that aircraft can only have TCAS I, which will provide a traffic advisory, but not a resolution advisory. With no mode S and no RA, there will be no direction given for a climb or descend, only a traffic alert. It will be up to one or both pilots to see and avoid and to take necessary action to avoid a collision.

  • $\begingroup$ Only Mode C or above is required for RA generation in TCAS II . Please make sure you're current with the information before answering. $\endgroup$ Commented Apr 15, 2021 at 14:18
  • $\begingroup$ What are you talking about? Mode S is a basic requirement for TCAS II. TCAS II is necessary for a resolution advisory. If neither aircraft has a mode S transponder, there is no TCAS II possible, and there is no resolution advisory possible. Please educate yourself on TCAS II, mode S, and resolution advisories. faa.gov/documentLibrary/media/Advisory_Circular/20-151B.pdf $\endgroup$
    – Will
    Commented Apr 18, 2021 at 20:23
  • $\begingroup$ Yes. Thank you. $\endgroup$ Commented Apr 20, 2021 at 9:39

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