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?
.... A) Does TCAS know what you can and can't achieve ?
AC20-131A provides guidance for approval of TCAS II systems. One of the requirements, in para (6), is that
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.
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.
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.
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.
Greetings 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.
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.
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.