TCAS query

[crackerjack]

Here's a question.....

How would TCAS resolve the following scenario?

Me in a Boeing on long(ish) final say at 6000'. The base of controlled airspace is 5000'.

My mate in a high performance aerobatic aircraft with mode C transponder - he pulls a vertical manoeuvre ahead and below me with an instantaneous climb rate in excess of 5000 fpm.

What now?

I think it might be an issue and have with reservations advised him to squawk 7004 without mode C ('cos I know he's a good bloke and won't bust airspace)

Would be grateful if anyone has any thoughts on this.......

[End_of_Descent]

I don't fly myself but I've done quite a lot of reading on TCAS theory (formulas, thresholds. operations and stuff) recently, so I'll try to answer that one based on pure theory.

For a TCAS RA to occur, both TCAS (slant) range test and TCAS altitude test must both be fullfilled.

D_mod for the range test at such low an altitude is about 0.35 NM which is equal to 2000' (slant range might also be pure vertical). The altitude test should be triggered when your aerobatics mate climbs through (roughly) 4300', that's 20 seconds away from climbing through 6000' (asuming still 5000 fpm at 4300'!). With such high a climb rate, TCAS II is unable to get a sufficiently good estimate for the vertical seperation and assumes 0' of vertical seperation, to be on the safe, conservative side.

I'd say this would quite likely trigger an RA in your Boeing, as both Range and Altitude Test are triggered. However, this is a pretty unfortunate conflict situtation where TCAS - to my best knowledge - is known to perform less than optimum (but still safe) against aircraft at slow closure but high vertical rates with no Mode-S transponder.
TCAS formulas choosing RA strength and direction are a bit difficult, but I'd guess that TCAS II chooses the non-crossing RA, e.g. a 2000 fpm climb, which might achieve a 600' vertical seperation.

In short, TCAS might work not too well in this encounter and it might be indeed be suitable for your mate to switch of Mode-C as long as he is doing aerobatics below class B/C airspace, and ensures staying clear of this airspace. This might still trigger TAs in your Boeing, but not RAs.
However, other guys from the front office might want to comment whether this is an acceptable advice from the flight deck's point of view.

Hope this helps,

EoD

[Duke of Burgundy]

crackerjack - I sense your moral dilemma and have to say that I think you are profoundly wrong to advise your friend to not select Mode "C" when carrying out his manouevres.

As I am sure you know TCAS is a last gasp safety net and despite your assurances that your friend is "a good bloke who won`t bust airspace" the road to hell is paved with good intentions as they say.

On the day when it transpires that your mate has feet of clay like the rest of us and makes a mistake during one of his zoom climbs or whatever, the fact that no Mode "C" is available will rob the other pilot`s TCAS of the ability to salvage the situation.

DoB

[crackerjack]

Thanks Duke. A thoughtful post and appreciated.

[INLAK]

Should your mate really be doing aerbatic manoeuvres on an extended runway centre-line??!!!!!!!

[Tcas climb]

hehe

[autoflight]

Let us not think that these things are impossible or unlikely. I had just become visual on ILS final at 400 ft AGL to find a jet trainer doing a barrel roll around my A321. More likely in South Korea, but possible elsewhere. TCAS totally useless due KAF trainers had no mode C.