Aviation In Canada

Monday, May 09, 2005


As mentioned yesterday, my faith in TCAS isn't what some people would carry. I was "lucky" enough to work before the days of TCAS, and have since seen the development of this system through from its introduction to the present state. I've watched aircraft get instructions from TCAS, tell me they're acting on a TCAS resolution, only to find out that the TCAS actually directed the plane back into the traffic. One example was a Dash 8 on departure out of CYYG seeing a target on TCAS that was already below him, and telling us that TCAS advised a descent. Early days of a new piece of equipment.

One of my favorite stories that I experienced personally happened when two regional airlines, always in fierce competition in our skies, were flying into a non-radar environment. At the time, the pilots were used to the fact that we used 10NM between aircraft due to the nature of our equipment in that area. AircraftA was flying at FL240, and on radar was measured at 14NM ahead of AircraftB at FL220, who was 10 knots faster. Given the distance involved, I would have less than 10NM between them by the time they wanted descent, so AircraftB would end up being too close to AircraftA to do anything but work hard to get AircraftA in. AircraftB was fully aware of this, as I found out later. I decided I'd push AircraftA down early to FL210, then let him descend at his discretion beyond this, using radar to establish the vertical separation before I lost the targets. AircraftB, upon hearing my clearance to A, started the following exchange:

"Moncton, AircraftB"
"AircraftB, Moncton?"
"We've been following AircraftA since we left PointA, watching him on TCAS all the way, and we're only 9 miles behind him."
"Sir, my radar shows him 14 miles ahead of you at this point."
"Well, sir, our TCAS only shows 12 miles ahead, and he's only three quarters of the way up the screen, so that's only 9 miles."
"Well, my radar has been flight checked for accuracy, he's showing 14 miles ahead, and we've had a change in equipment in this area and I only need 5 miles between you now."

Anyway, that exchange prompted an experiment that I carried on over the next 5 years or so. I would take opportunities to evaluate when traffic permitted. For example, I would have to aircraft crossing at shallow angles, or even opposing each other on nearly the same track. If my workload permitted in setting it up, and both pilots agreed, I would ask them if they could keep an eye on their TCAS and watch the traffic go by, and give an estimate of their closest point of approach, all the while watching intently with a distance measuring tool displayed on my radar which would give a readout down to 0.1 NM. For the first two years of this unofficial experiment, I got ranges that varied greatly from aircraft to aircraft, and over what I observed on radar. Many of the reported values differed by as much as 50% over what was displayed on my screen, and even more wildly between each other. After the first two years, the discrepancies began to diminish. I assume this was because better equipment became available on some of these aircraft. I stopped the experiment after finding the reported values hardly varied at all over what I was seeing over the next three years.

So TCAS has gained some trust with me, but as much as I will never board an aircraft without a pilot on board, I still wouldn't want to fly in an airliner in skies without a controller watching over my flight.