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Old 30th Jul 2008, 12:52   #1 (permalink)
 
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Does TCAS 'know' the aeroplanes performance limitations ?

OK, let's say you're cruising along at FL400 and conditions are such that you're not going to get much out of the aeroplane, perhaps even you're not far from coffin corner. All of a sudden you get a TCAS RA commanding a 2000fpm climb.

A) Does TCAS know what you can and can't achieve ?
B) Given the above example, what would you do?

Thx
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Old 30th Jul 2008, 15:36   #2 (permalink)
 
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A) Generally not.
B) I wouldn't be there in the first place...cruise lower for better performance.
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Old 30th Jul 2008, 15:50   #3 (permalink)
 
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Quote:
B) I wouldn't be there in the first place...cruise lower for better performance.
Absolutely, why be there in the first place?
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Old 30th Jul 2008, 21:50   #4 (permalink)
 
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Quote:
Originally Posted by buzzc152 View Post
....
A) Does TCAS know what you can and can't achieve ?
Sometimes, yes.

AC20-131A provides guidance for approval of TCAS II systems. One of the requirements, in para (6), is that

Quote:
The collision avoidance maneuvers posted as RAs by TCAS II assume an aircraft's ability to safely achieve them. If it is likely they are beyond the capability of the aircraft, then TCAS II must know beforehand so it can change its strategy and issue an alternative RA. These performance limits shall be provided to TCAS II from the aircraft interface and discretes relative to altitude and/or configuration. However, the need to inhibit TCAS II CLIMB or INCREASE CLIMB RAs should
Testing/analysis is conducted to determine a flight envelope within which RAs can be complied with safely, and the inhibit is outside that enveleope.

Note that the enevelope so defined is not exact, nor necessarily correct for the specific aircraft on a specific day, since any deterioration or excess performance on a given aircraft cannot be known. Also, there is a desire to not overly restrict the RA envelope by making excessively conservative assumptions, as this would deny useful RAs in many cases.

Thus there will be "edge of the envelope" RAs which some aircraft can achieve, but other cannot. But this design approach at least ensures that RAs which that type cannot comply with under any reasonable assumptions are excluded.
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Old 31st Jul 2008, 00:07   #5 (permalink)
 
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Even when cruising 1000 feet above "optimum altitude," the airplane would always have enough speed and inertia to quickly climb another 500 to 1000 feet in order to avoid a collision. There will be airspeed decay and you'll have to lower the nose immediately afterwards and descent and re-accelerate to cruise Mach with MCT setting.
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Old 31st Jul 2008, 08:45   #6 (permalink)
 
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Thanks for the answers...... purely a theoretical question that popped into my head.
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Old 31st Jul 2008, 09:45   #7 (permalink)
 
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Usually not thats why we switch it to TA only after an engine out. Tcas comes into its own on clb or dsc into busy terminal areas . If you dont notice tfc. bearing down on you at fl400 you shouldnt be asleep. No offense.
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Old 31st Jul 2008, 15:20   #8 (permalink)
 
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I was told in a previous aircraft I flew that above a certain altitude, the TCAS was configured for that aircraft to not be able to give a climb RA.
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Old 31st Jul 2008, 22:07   #9 (permalink)
 
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ZFWT.
Even if you see the other traffic on the screen, what are you going to do?
TCAS manual says don't do anything until you get an RA.
Safe flying.
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Old 1st Aug 2008, 11:31   #10 (permalink)
 
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TCAS installations include inhibitions of the various upwards RA's based on performance. A number of parameters can be considered, most obviously Altitude and Configuration, but other things can be considered, e.g. is the ice protection on to an extent that implies the aircraft is in Icing?

So there may be multiple envelopes for the enabling / inhibition of Increase climb and climb RAs.

Glueball, respectfully I must disagree. There is a lot of type dependancy in there. Cruising at ceiling isn't the worst case anyway, but rather near top of climb is. When TCAS was Mandated I was flying ATRs at the time, and the idea that we would be able to always zoom climb 500 feet is pretty optomistic. (A typical climb in icing conditions would be at or near to minimum clean speed, with a ROC of maybe 300 fpm as MAUW and a climb power pretty much equal to MCT. Putting the Props to Max would get us a bit more, but 1500 fpm... no way)

Needless to say, in that aircraft a strengthened climb RA was inhibited pretty much always IIRC, and a normal climb RA most of the time, but even the performance envelopes specified were pretty optomistic.

Which is why everyone should know that protecting the flight envelope is more important than following the RA, and what they should do if they can't achieve the RA, which is to get as close as possible with a cardinal rule of not manouvering opposite.

pb
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Old 3rd Aug 2008, 01:16   #11 (permalink)
 
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Thanks dixi.
What would we do upon a T.A.?
1.Look for traffic using traffic display as a guide. call put any conflicting traffic.
2.If traffic is sighted, maneuver as required.

Understandably there are many scenerios but I hope we wouldnt sit there at fl400 fat dumb and happy waiting for an R.A. if you noticed an ac on a collision course.
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Old 3rd Aug 2008, 08:27   #12 (permalink)
kijangnim
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Greetings

I am surprised TCAS doesnot anything about aircraft performance, if it was the case then what would happen if both aircrafts were at max cruise level, will systems give descent to both?
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Old 3rd Aug 2008, 15:27   #13 (permalink)
 
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Quote:
Originally Posted by kijangnim View Post
Greetings

I am surprised TCAS doesnot anything about aircraft performance, if it was the case then what would happen if both aircrafts were at max cruise level, will systems give descent to both?
If both aircraft are at an altitude where climb RAs are inhibited, then TCAS will give one aircraft a descend RA and have the other remain level.
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Old 3rd Aug 2008, 15:51   #14 (permalink)
kijangnim
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Do you think that only one aircraft manoeuvring is enough to create safe separation
Furthermore, it is true hat I dont know everything but I never heard inflight RA inhibition (without warning the crew that they dont have RA available), since it is the last safety net we have.
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Old 3rd Aug 2008, 20:54   #15 (permalink)
 
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Quote:
Do you think that only one aircraft manoeuvring is enough to create safe separation
Yes. Do the sums. The alerting times, and changes in flight path, are sufficient to generate a miss if the threat aircraft does not manoeuvre. Hence it is adequate against Mode C (non TCAS) threats.

Which is why, in most coordinated encounters, both aircraft can expect the RA to weaken well before clear of conflict, because safe separation is met with time to spare and the aircraft can be directed to paralled their original flight paths, thus minimising their deviations and reducing the chances of a third aircraft above or below becoming involved.

Quote:
Furthermore, it is true hat I dont know everything but I never heard inflight RA inhibition (without warning the crew that they dont have RA available), since it is the last safety net we have.
They get an RA, but it'll be preventative.

Same thing happens in reverse in a low level encounter.

pb
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