As I look up into the sky I see two jet airliners on what could be a collision course. As they pass each other harmlessly, their wakes streaming behind, I think about all the technology protecting them. Looking up at them from the ground, or even from a higher altitude, it might not be possible to be certain that the planes won’t collide. Protecting them is TCAS (Traffic Alert and Collision Avoidance System).
Before getting into what TCAS is all about I reflect on some flights I had piloted and had near misses. One that first comes to mind, was a flight from western Massachusetts to Burlington, VT in my B-55 Baron. Flying VFR at 7500 feet in the clear, suddenly a plane shot by from my right rear diagonally in front, at my altitude, missing me by what seemed only a few feet. The only thing I saw was that it was a low winged single, possibly a Bonanza. When I recovered enough to key the mike, I called Boston Center and reported the near miss. They weren’t in communication with that plane, although they did have it on radar. This was before most planes had some form of altitude reporting, so they really couldn’t have effectively warned me. Which one of my nine lives had I used on that one?
There were others, but none as close as this one was. I remember flying in and out of the murk and looking down at a large fighter bomber, B-1, about 500 feet below me on a diagonal course. I had received an alert from ATC (Air Traffic Control) on that one. Another was encountered somewhere over Massachusetts again. It was marginal VFR, and again an alert from ATC advising about an airliner at my 10 o’clock and ½ mile away. We barely saw that one. TCAS would have been nice to have there.
Now it is time to briefly explain what TCAS is and how it operates. TCAS is a system independent of the ground based ATC system. To work at the simplest level (TCAS-I), an airplane must have an operating transponder with an altitude reporting capability, (Mode C or S). It is designed for general aviation aircraft with a passenger capacity of up to 30. “Intruder” aircraft up to 40 miles away can be detected and an alert issued. The alert is depicted on a gage on the instrument panel. Approximate bearing and relative altitude data are shown. But it is up to the pilot to visually spot the “intruder”, and take whatever evasive action is needed. This seems ok in good visibility conditions, but not in IFR type weather. But in IFR conditions, one would normally be in radio contact with ATC, who should issue traffic alerts and offer vectoring to avoid collisions.
The next higher level of TCAS is TCAS-II. This is much more complex and required for larger transport type aircraft. Briefly, traffic alerts (TA’s) as well as resolution advisories (RA’s) are automatically issued by the system in the form of aural and visual data. The RA’s offer only climb or descend commands, that are to be followed immediately, regardless of ATC commands. Eventually, lateral type evasion commands may be given under the newer TCAS-III and TCAS-IIII systems as well as climbs and descents.
So the next time you look up and see planes that may be on a collision course, don’t worry, big brother TCAS is at work. Fly safely and do keep your eyes peeled for traffic, even if you have a TCAS system aboard, as no system is 100% perfect.
*See Wikipedia and Google for more on TCAS.