TCAS RA @ Low Level
 

If you receive a Descend TCAS RA above the inhibit height of 1000' ft (+/- 100 ft) what aural alert does the system generate as (or if) you descend below the inhibit height? If someone could point me towards some supporting documentation that would be much appreciated. Cheers.

According to the FAA TCAS II v7.1 into booklet, if a "descend" RA is in progress as you decend through 1100' it will change to a "do not climb" RA.

Thanks for the reply. As far as I can research 'do not climb' is not an aural callout in the menu of TCAS 7.1, so what do you think the aural callout would be to satisfy a 'do not climb' RA?

Perhaps "Level off".

Hi Fly3 - yes - 'Level Off, Level Off' is what I think it is, but I'm struggling to find it written in a document somewhere.

Sounds :) correct, it's a weakening one.

See Table 4. TCAS Aural Annunciations (#2 by rudestuff, his ref. is pg. 30). N.B. below 500' the audio is inhibited altogether.

I believe they are inhibited in that if you did have an active RA as you descend through the inhibit height it would stop giving commands. (Good question though, I haven’t thought of that.)


“Increase Descent Resolution Advisories are inhibited below 1450 feet AGL.

• All Resolution Advisories are inhibited below 900 feet descend-ing or 1100 feet ascending.

• All TCAS II audio warnings are inhibited below 900 feet Radio Altitude on approach and up to 1100 feet Radio Altitude on departure.”

Honeywell Collision Avoidance System Pilot's Guide

TCAS II CAS 67A, 2004

pompey777

i don’t think that is a command. The closest would be: “MAINTAIN VERTICAL SPEED, MAINTAIN,”

CM, your document is out-dated, perhaps even installation-specific. Ver. 7.1 has been around for a decade already. The level-off aural alert is a new feature to avoid some very freaky pilot reactions observed through incident investigations.

Check the document mentioned in post no. 2, the table is on page 36.

Ah, thank you, I learn something new everyday. I am biased as to the TCAS currently installed on our fleet.

For descent there are the following inhibitions

• Below 1450 there will be no ‘INCREASE DESCEND x2’ RA.
• Below 1000 there will be no “DESCEND x2’ RA.
• Below 900 there will be no more RA: TA only.
• Below 400, TA aural alerts are inhibited.

Exactly as CaptainMongo stated. :ok:

When one of the Inhibitions is active, it is what it is. That means that below 900, the RA in progress is suppressed and converted into a TA.

Dear Open Des,
Thank you for the reply. Do you have the source document for that please?

Actually CM is not "wrong" as "Maintain Vertical Speed, Maintain" is a TCAS Aural Annunciations (ref. table 4 of the FAA document discussed) BUT in the case we are considering of a weakening RA the proper instruction would definitely be "LEVEL OFF, LEVEL OFF".
Sometimes the confusion arises between "adjust" vertical speed (7.0) and "maintain" vertical speed (7.1) but the latter has a different meaning.

“Level Off” (v7.1j is the new “Adjust Vertical Speed” (v7.0) which in turn was the new “Reduce Climb / Descent” (v6.04).

These are all weakening of initial corrective RAs.There is some interesting history there, relating to what the call meant versus the original call versus what the aircraft was actually doing, particularly in the case of non compliance with the original RA.

There were also cases of pilots pitching in response to the words of the callout without referring to the (type dependent) vertical speed or pitch targets on their PFD. This can lead to excessive changes of ROC and larger deviations from clearance, increasing the probability of “knock on” encounters with other aircraft.

Note that the 7.0 wording forces you to refer to your PFD. “Adjust my vertical speed? How Much? Better look in now, not out”. This was a major step forward. However, since the intent with all weakening RAs is to level off, at 7.1 it was decided it was clearer to just explicitly say so.

in practice, a corrective RA will generally weaken anyway as either RA is big enough to achieve the target separation. Especially at low level, where that is only 300’. So there is only a very narrow altitude band (150’) where you get inhibited before you would have weakened anyway. In this case the other aircraft just weakens slightly later than it would have.

Incidentally, “Maintain Vertical Speed” is a preventative RA and thus wouldn’t apply in this case. It is used when the flight path (a) needs to stay the same and (b) when the aircraft is already climbing or descending. It can be modified with the word “crossing” if altitude crossing is unavoidable, to try and mitigate the natural human desire not to carry on climbing (or descending) when the intruder is above (or below) you.

When a preventative RA is required in level flight (or gradual climb/descent) this is when you get “Monitor Vertical Speed”.

Ultimately folks, all TCAS RA callouts mean the same thing: look at your PFD and fly just out of the red.

It needs to be done promptly, so in the air don’t overanalyse the words just crack on with it.

Hope that helps.

sonicbum

He could not find level-off in his paper, only maintain V/S. Because it is outdated (2004, post #7) so it still shows both adjust V/S and maintain V/S. Instead of the correct level-off and maintain V/S combo. The rest as per your explanation.

EDIT: all wrong on my part, apologies sonicbum for not listening what you were actually saying. :ok:

OPEN DES

Care to disagree. Already mentioned and reference provided, the low level inhibition does not affect an RA that is already in progress. Opposite to your conclusion.

You seem to have used Table 3 on pg. 32 but missed the text on page 30: "If a Descend RA is being displayed as own aircraft descends through 1100 feet AGL, the RA will be modified to a Do Not Climb RA." (*)

Ref. Introduction to TCAS II Version 7.1, FAA 28 February 2011.

(*) = for which the aural alert is "level-off, level-off" on v 7.1 (#)
(#) = unlike on the 7.0 that calls out "adjust V/S, adjust"

Thanks. I stand corrected!

I asked the same question of our Training Standards guys some time ago and got a very similar answer.

Practically, it does mean that if someone is still descending above you when you are <1,000’, you will have to invent your own way of deconflicting, especially if the offending traffic is not coordinated, as TCAS is now playing a much more passive role...

FlightDetent

Hang on, a 'do not climb RA' is preventative and therefore wouldn't the callout be 'MONITOR VERTICAL SPEED'? (Tables 3 and 4 refer)

Plot thickens. Capt Pit Bull, point taken.

"Do Not Descend RA" is also described as "weakening" and "corrective" on pg. 33, albeit in a different circumstances.

• During an RA, if [...] response to a Positive RA has provided ALIM feet of vertical separation prior to CPA [...] the initial RA will be weakened to [...] Do Not Climb RA (after an initial Descend RA). This is done to minimize the displacement from the TCAS aircraft’s original altitude.
• after ALIM feet of separation has been achieved, the resulting Do Not Descend [...] RA is designated as corrective. [...] In Version 7.1, the RA is annunciated as “Level Off, Level Off.

Interestingly, both of are entirely appropriate to the edge-case discussed.
+ There was a hard corrective RA so it is a weakening change.
+ To minimize altitude loss is exactly why the downgrade of RA happens
+ Preceding manoeuvre was corrective, demanding action.

Unlike the "preventive" RAs where passive monitoring is the plan - "preventive RAs do not require a change in vertical speed." (pg 30)
- Not modifying the V/S instilled by the previous RA is would clearly the wrong choice here.

IMHO technical possibility exists that passing 1100 ft some internal element nulls the ALIM value which would cascade the described desirable behaviour.

As well, the call "level-off" has the appropriate good advice once 40 seconds from ground contact. Unlike "monitor V/S" which might fixate the pilot on keeping the needle steady and/or observing where the red arc is - instead of watching the altimeter!

Admittedly, the document we have does not provide any guidance in this direction.