TCAS RA's

[jumbojohn]

My company policy is to switch TCAS to TA only, (RA off) after the loss of an engine, I assume that this is because of reduced performance available.

I have just read a safety flash from Eurocontrol titled "ACAS 2, Follow the RA" and it appears to me that we should try to comply with a TA, even with reduced performance as we don't turn it off on a heavy weight take off when performance is also limited. The only reason that it might be worth going to TA is if that info is passed to any possible target and "extra" RA information to avoid us is given by by the other aircfraft's equipment, but I don't think that is the case, does anyony know?

Also, SOP's of sighting the other aircraft and ignoring an RA appears to be a flawed policy. Can we really identify for sure a target in a busy terminal area and decide to do our own thing? I think that I would be inclined to just follow the RA as the equipment appears to knows best.

Other thought please?

[tone-uncage-fire]

Im not sure if I am reading your post correctly?

You should ALWAYS follow the RA..... no exceptions, even if you can see the traffic. DHL vs Aeroflot springs to mind.

It is my understanding that your assumptions on swtching to TA after an engine fail are correct (ie: performance related)

[Phoenix_X]

Especially on the a/c with the RA indications on the EADI, the brackets are only correct for 2 engines. Also, as said, with 1 engine inop, the perf limitations are such, that it's better to switch RA off, which will command a more agressive maneuvre from the other A/C. That A?C will base it's RA on your current trajectory, which is unlikely to chance on one engine.

[Capt Pit Bull]

Jumbojohn,

When you are at TA only mode the other aircrafts TCAS doesn't coordinate its escape manoeuvre with your TCAS. This gives it a greater range of responses.

Bear in mind that there are other system inhibitions. So in some scenarios there may not be that many options for avoidance. It is even possible to imagine a scenario (at low altitude - say whilst flying a circling approach) like this:

Aircraft 1 (engine out)
Can't descend (inhibited due Rad Alt)
Can't Climb (or not enough) due engine out, but TCAS doesn't know.

Aircraft 2
Can't descend (inhibited due Rad ALt)
Can Climb.

Both in RA mode:
One aircraft gets preventative RA (coloured band / arc says not to climb)
The other aircraft gets a climb.

Two possible solutions:

1:
Aircraft 1 gets preventative.
Aircraft 2 gets Climb.
All is well.

2:
Aircraft 2 gets preventative.
Aircraft 1 gets climb. Fails to do it.
Collision.

So, 50/50 chance of disaster.

However, if aircraft 1 had been at TA only mode, then aircraft 2 would have definitely been issued a Climb.

So what it boils down to is that when your aircraft is malfunctioning to the point where being able to comply with an RA is going to be difficult, its better to be non TCAS.


Regarding the freedom to disregard an RA on visual spotting:

There are some great axample around of why this is a bad idea.

My favourite is one from the FAA TCAS transition program, very abbreviated here:

Aircraft 1 has two RA's in quick succession, versus 2 different aircraft (aircraft 2 and 3)

Gets RA versus aircraft 2. Unable to see any aircraft. Follows RA. Gets another RA, this time versus aircraft 3. Coincidentally, crew happen to spot aircraft 2, passing clear down their side. They say 'This RA not needed'. Fortunately, they followed it anyway, and then spot aircraft 3 as it flashes 300 feet overhead!

The morale of this incident is 'So you think the RA is not necessary? Follow it anyway!"

Even if you can see the other aircraft, its virtually impossible to second guess TCAS's RA anyway.

See this thread (http://www.pprune.org/forums/showthread.php?s=&threadid=58559)



Tone-uncage-fire

Having said that, there are times when it may not be appropriate to follow an RA, and this is why most Ops manuals (and guidance from regulatory authorities and manufacturers) usually give final authority over RA compliance to the crew.

Remember, TCAS only know about transponding targets. It doesn't know about non-transponding aircraft, or terrain (other than straight down - a ridge will spoil your day) or antenna, so it is always possible to come up with a scenario where RA compliance is more dangerous than doing nothing. At the end of the day a certain terrain impact is worse than a possible air impact, so that is why TCAS is subserviant to GPWS and Windshear.

So I take issue with your statement that there are no exceptions, and that an RA must always be followed.

At the risk of repeating myself, the recent collision was not caused by non-compliance with RA's. It was caused by manoeuvres opposite to the RA's. Such manoeuvres are universally prohibited by manufacturers and regulators alike (and hopefully by all training departments!).

Ultimately, if the tupolev had stayed level, all would have been well (that would have been non compliance). It was the descent when told to climb, followed by increasing desccent when told to increase climb. Two manoeuvres opposite in sense to the issued RA.

CPB

[tone-uncage-fire]

CPB

I agree with you entirely, my answer was too simplistic..... I wouldnt, for example, advocate following an RA that could cause ground impact or advocate doing nothing to avoid a visual target just because there is no RA.

Your post very informative. Thanks.

TUF

[jumbojohn]

CPT

Thanx for your posting, it all makes lots of sense now.