Query regarding TCAS.
 

During a sim session yesterday (ATC) we were discussing transponder failures. The scenario was then posed if aircraft A has a transponder failure (lets say mode A and C) and aircraft B with a fully serviceable transponder comes within range of a TCAS RA is it reasonable to say

1/ Aircraft B will receive no TA/RA BUT Aircraft A will
2/ Aircraft B will receive no TA/RA NOR will Aircraft A

The question(I guess) is whether or not an aircraft requires a serviceable transponder to RECIEVE TCAS instructions.

Thanks in advance

EW

Depends on partial or total failure of the transponder. If it fails in just transmit/receive, then 1. might be correct. Air to air coordination would still be impossible, probably causing own TCAS Fail annunciation, however. Actual performance may depend on the TCAS/transponder manufacturer.

A total transponder failure would shut off own aircraft altitude info to the TCAS, causing it to be inoperative, making 2. correct for sure.

GB

On the Honeywell device used by my company, the aircraft altitude input to the TCAS is direct from the ADC. Therefore aircraft A would receive normally TCAS warnings, but aircraft B would be blind to the presence of aircraft A: answer 1.

I think the way you wrote down the choices is a bit... discombobulating :ugh:

The one w/o TXPR will happily fly away ignorant of what might have been, while the other w/ a functioning unit will try to maintain separation. :ok:

What category airplane, Pub User, and what level TCAS?

Arinc spec for ACAS/TCAS II has the transponder relay altitude data to the TCAS, and there is no direct connection between the ADC and the TCAS interrogator.

TCAS doesn't interrogate mode A from the other aircraft's transponder, only mode C and mode S (if equipped). Aircraft B cannot see a plane without operative mode C.

Aircraft A TCAS will annunciate Fail if it is not receiving own aircraft altitude - and aircraft id number - from its transponder, which isn't clear in the question. Without a working transponder, moreover, there can be no aircraft to aircraft maneuver coordination.

GB

This is basically the same scenario as if the transponder of aircraft A is not powered. There is no communication from this particular unit to any other aircraft or ground unit. Data to the acas unit are non-existent or void.
The XPDR and acas unit of aircraft A, however, are not switched off, they MAY be able to receive a communication through . Due to the unavailability of comparing these data with own data it will, however, not calculate any position or threat at all.

As Greybeard said, there is a FAIL annunciation and no further data are displayed.

And from where will the one with the functioning unit get its data? The other one's transponder has failed. There is no data.

Wasn't this almost identical to the circumstance of the midair in Brazil?

good post graybeard.Your avio knowledge is very good.

A 1 mode s - B 1 mode s dual RA
A 1 mode s - B 1 mode c single RA/single TA
A 1 mode c - B 1 mode c dual TA only
A 1 mode s or c - B total failure - nothing nada or zip maybe splat

Are there not general piloting rules to make decision to climb or desend based on heading with the lack of a RA (if you have the time)?

Mode a is completely useless to TCAS it does not report altitude as I understand.

Also to add that I am referring to TCAS II (most do not make the distinction anymore).

The next form of collision avoidance is supposed to operate on a horizontal platform as well, there is much debate in justifying it's need.

Not so. TCAS will still plot the target in azimuth and issue a T/A. You certainly should not take avoiding action based soley on this, but it is a tool to try and aquire the aircraft visually.

Lots of Mode A only aircraft in the UK, and you quite often get T/As from aircraft that are seperated vertically from you.

Mode A is used and if received could result in TA but not RA.

In the original scenario there isn't enough info to make a call. If the transponder has failed completely then aircraft A doesn't exist to aircraft B. If the transponder can still receive then it could provide TA for aircraft A.

RA is a contract and both sides need to be able to negotiate it. That means listen and talk. TA is one sided and that side only needs to be aware of the other.

TCAS does not interrogate mode A. It doesn't care what the intruders 4096 code is so interrogating it is a waste of bandwidth. It only sends Mode C interrogations (plus Mode S calls to the aircraft on its roll call).

What people call "Mode A" transponders are in fact mode A and C transponders, but without an altitude encoder (or if it's turned off). i.e. they still reply to mode C all call, but they just send an empty pulse frame. But this allows TCAS to perform a time versus speed of light calculation for range.

So, if you have an A+C aircraft whose mode C has actually failed, i.e. it is not responding to mode C all call, then TCAS will not see it.

pb

No, see my previous.

That is only true for coordinated RAs.

CB,

Not aware of that,(re mode "A") thanks for the info.

That being said, are actual Mode A only TXPs even still being manufactured?

Quite right that a TCAS equipped aircraft will issue an RA to avoid a mode C but NON TCAS one.

As far as I can tell, no. You notice that all recent transponders say things like "On" and "On + Alt" rather than "A" and "A+C". I asked the CAA about this and they said that all transponders have to be at least A and C.

(Which I guess also implies you wouldn't get a radio licence for an older non compliant model).

Thing is there is a vast ammount of literature that is wrong about TCAS and Mode A, but usually its "two wrongs make a right" on the end result so its almost an academic distinction.

pb

Capt Pit Bull
 

My quote was directly from a Honeywell certification paper so you may need to give them a call and put them right.

Sure thing. Give me the title of your reference and I'll point them at the appropriate ARINC spec for data formats.

Capt Pit Bull,

PM'd.

Which is all a lengthy way of saying "you still need to look out of the window to stay alive....."

Folks

Thank you all for your input(even if I seemed discombobulating !) I have copied the replies to our guys and they are enjoying the debate.

Best


EW