Aviation In Canada

Sunday, June 05, 2005

Use of TCAS as a Radar

I remember when TCAS started proliferating in aircraft around the Moncton FIR several years ago. Some of us were quick to criticize it, while others realized the value in pointing out traffic to pilots. Then there were some pilots who used it, in my view, in inappropriate ways.

One pilot called me to criticize my judgment regarding an aircraft in front of him, which I may have even written about recently. My memory isn't all that great. Turns out, he knew we used to need 10NM between aircraft in that airspace, claimed his TCAS showed only 9 between him and his traffic (there was 14 on my radar which has been flight-checked for accuracy), and we had recently switched to 5NM separation there. Don't get me wrong, by the way, I'm all for cross-checks and I never take this sort of criticism too seriously. If someone sees something that is dangerous and I don't, I'd much rather tell me about it than let things degrade, possibly to the point of no return.

Anyway, the real point of this is the use of TCAS in ways which may not be good. I've long wondered how useful TCAS would be as a radar tool for a pilot, since there is no "trail history" to show where the aircraft was coming from, no groundspeed or "predicted track line" to show where he's going. Just a target, drawn with azimuth and a range (and the altitude difference, of course). With the lack of any indication of what the other aircraft is actually doing, it doesn't show the much-needed information to the pilot when the thought of using it to make his own judgment about avoidance manoeuvers. The good part of this is that you no longer have to take my word for it. IFALPA did up a really nice presentation in Adobe Acrobat's PDF format that demonstrates real-world pilots making avoidance manoeuvers based on TCAS observations that caused problems where separation was being looked after. I've included a link at the bottom of this page, with no guarantee how long this PDF will sit at this location. Please have a read of this, and let me know what you think. It tells the story I've long been thinking about (and even arguing about from time to time), but lacked communication skills to describe, and also lacked real-world examples of the dangers.