In Response to Thomas Turner’s “On a Swivel”

In Thomas P. Turner’s article in the June issue, he states that in order to be detected by TCAS or TCAD a transponder-equipped target (a Cessna 150 in his example) must be “in radar contact with ATC” and that “even if the aircraft had a transponder, the transponder would not be emitting a signal to be picked up by the Citation’s TCAS.” In fact, “active” TCAS and TCAD traffic systems (such as Skywatch) broadcast their own aircraft-to-aircraft interrogation signals, which all nearby transponders will reply to whether or not they are being interrogated by ATC. This is actually one of the chief advantages of an active TCAS or TCAD, in that they will detect nearby transponder-equipped traffic even if they are so low that ATC radar is not painting them. Here’s an excerpt from a typical Skywatch manual proving my point: 

Santa Barabara, CA 

Thomas Turner responds:

You are absolutely right that TCAD and TCAS are active systems that trigger a response from transponder-equipped airplanes which then (in most cases) will appear on the display in the airplane that sent the interrogation. That system works quite well – most of the time. The passage you cite, and item #3 that follows, explain why this probably would not have been in the case that prompted June’s article. 

Transponders are line-of-sight radios. The descending Citation’s active traffic system signal would probably have been blocked by the Cessna 150’s wings and fuselage from reaching the trainer’s transponder antenna on the belly of the airplane. Any signal that might have been sent from the C150’s transponder would likely have been blanked out from the Citation’s position above.  

I point this phenomenon out to my students in flight when the opportunity arises; airplanes approaching near head-on but slightly below our aircraft will commonly appear on the TCAD screen until they’re within a mile or so, and then disappear from the scope as the target airplane’s structure comes between us and its bottom-mounted transponder antenna. If I’m lucky on a training flight, I can show that the biggest threat, a nearby airplane at our altitude and turning toward us, will be indivisible on our display. I remind pilots that the way to use our cockpit traffic display is to identify airplanes as far from us as possible and then acquire them visually, so we can avoid them if they come close enough they may disappear from our scope.

Back to the article: my point in the entire discussion on traffic avoidance displays is that they are extremely helpful in visually acquiring other airplanes, but like most technology, to use it effectively we need to know its limitations as well as its strengths.

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