Originally Posted by
Max Angle
As for TCAS, if you have the presence of mind (and I not sure I would) you should set it to TA only (as for an engine failure) so that it does not give you an RA and any unit in a conflicting aircraft will know not to try and co-ordinate with yours.
The co-ordination is in fact only "I say UP - you say DOWN" so a RA reversal would probably happen very quickly. I have the same idea but it is nowhere to be seen on paper. I wonder if keeping RA on is perhaps to prevent the double descent formation situation.
Imagine this: Aircraft starting ED @ FL300 v/s 4000 fpm increasing to 7000 fpm. (RA threshold is 35s to closest point of impact). Same track/direction traffic @ FL 280, one mile behind. (target RA miss distance is 400 ft vertical, lateral RA threshold is about 1,2 NM).
Case1) the ED A/C is TA only - no changes to its flight profile. The 2nd A/C TCAS has 3 options
a) climb RA: this in fact may do little to no service as far as separation goes, also it is movement against the intruder whose trajectory is parabolic(? - you get my point) as the A/C accelerates into the dive. May not be an option.
b) maintain v/s RA: Not an option, this is why the RA was issued in first place.
c) descent RA: creates a double descent situation, the lower aircraft may be pushed to such rate its forward speed will start to increase (closing in on the ED A/C), or, even worse, the lower aircraft may not be able to keep the rate great enough thus gliding towards the ED A/C. Maybe not an option.
Case2) The ED A/C is TA/RA.
Options for the FL280 A/C are pretty much the same. But the ED A/C may get REDUCE V/S RA, and the other DESCENT (other combinations also available). While this is far from final resolution to the conflict, the initial manoeuvre will INCREASE the separation unlike all three options in Case1 above.
In vertical plane it is quite hard to find critical situations and very complicated to solve them. What is for a long time obvious to any pilot, that you ultimately need to change course to solve the equations. In fact, any lateral input greatly reduces the time/space exposure to the 400 ft-1,2 NM target miss bubble.
In vertical plane, maneuvering capability is limited let's say by (sustained) +4000 / -7000 fpm. Forward speed however, is about 450 TAS i.e. 45500 fpm! To get 1/10th reduction (4500 fpm), change track by 25 deg (arccos(0,9)=25[deg]). If you change track by 45deg, you get 30% reduction in forward speed (cos(45[deg])=0,7), that is 13000 fpm!
From this point of view, turn away, start descent, keep TA/RA on sounds like a good advice. But, what have I missed? (apart from the obvious turn exactly into the trajecotry of below crossing traffic).
FD.
(the un-real)