by
Darryl Phillips
Another midair. They keep happening. Collisions often occur in the pattern, almost always in good weather. With all the avionics technology, why do we keep running into each other? Why doesn't somebody do something?
Traffic Collision Avoidance System is the system the air carriers are using. TCAS on the airliner talks back and forth with transponders on nearby aircraft. TCAS interrogates on 1030 MHZ, transponders within 10 or 20 miles (and sometimes 100 miles or more) automatically reply on 1090 MHZ. Using a directional antenna, TCAS can determine the relative bearing of the traffic. By timing how long the reply takes to come back, distance can be determined. And if the traffic is equipped with Mode C capability, TCAS can read altitude. By taking successive measurements of range, bearing, and altitude, TCAS can determine if the traffic will pass nearby. Targets are shown on a color display, and warnings are seen and heard when the target becomes a threat.
A recent article in the WALL STREET JOURNAL tells of the struggle between controllers and airline pilots over TCAS. Should the pilot deviate when TCAS tells him to? Or should he wait for a break in a constant stream of radio communication to tell ATC first? If the pilot is not supposed to believe this instrument, then what's it for? And how should the controller react when the pilot responds to a command the controller never saw or heard?
TCAS worked swell in the laboratory. And perhaps ATC will learn to live with it, as they have with other congressionally-mandated ideas. In any event, TCAS isn't coming to general aviation. It costs more than most airplanes.
A sort of little brother to TCAS is TCAD. While TCAS is an industry standard produced by several companies, TCAD is proprietary and manufactured only by Ryan International. TCAD also uses the transponder signals, but it only listens. The basic idea is that two aircraft can't collide unless both are at the same altitude. Makes sense. Using signal strength, TCAD can make a rough guess (not always accurate) of the distance to the other plane. If the target has Mode C, TCAD determines altitude and issues advisories if there is a conflict. Of course if the target doesn't have altitude reporting capability, or Mode C is not turned on, then TCAD can only report a target at unknown altitude. And TCAD doesn't give the pilot a clue regarding which way to look.
There are several TCAD models, and they're much less expensive than TCAS. Figure a few kilobucks. TCAD is a passive system (receive only, no transmitting) so it doesn't contribute to the clutter on the transponder frequency. That's good. The downside is that it only works with transponder-equipped traffic, and those sponders must be on and working. Pattern altitude at uncontrolled fields is where midairs often happen, and frequently the pattern is below radar coverage. Transponders don't work unless there is an interrogation, so even with a transponder turned on and functional there will be no replies for TCAD to pick up just when they are needed most.
The newest concept on the market is the Collision Warning System (CWS) by B.F. Goodrich. It combines some of the best parts of TCAS and TCAD, and falls between them in cost. If you don't fly at least a King Air or Citation you probably won't be interested in the price of CWS. But the techniques CWS uses to search for traffic are unique and worth noting.
CWS doesn't just listen to the transponder replies, it examines the interrogations from ATC radar sites first. Different dishes rotate at different speeds, and have differing interrogation rates. The computer in CWS keeps track of the various radars, and matches up each transponder reply with the ATC installation that requested it. After a little electronic voodoo and some complex math CWS can tell where the traffic is. Again, it's worthless against aircraft that don't have functioning transponders. But planes that can afford CWS spend most of their time in airspace where transponders are required. CWS displays the targets in color just like TCAS, and does so without all the interference to ATC radar that TCAS causes.
In airspace below radar coverage, or someplace like the north pole or the middle of the ocean where there isn't any ATC radar, CWS changes over from strictly listening to a mode where it interrogates nearby sponders like TCAS does. In this mode CWS loses some of the ability to determine target location, but at least with CWS you know the presence and altitude of nearby aircraft. And it works at pattern altitude where the danger is greatest.
For years pilots have asked if ATC radar pictures could be relayed to the aircraft, so traffic could be seen first hand instead of relying on word of mouth. The idea was long resisted by FAA, but recently they've been touting Traffic Information System, or TIS. The idea is to take the ATC radar data, pretty it up in full color, and then use radar again, via Mode S datalink, to relay the picture to the aircraft. Each aircraft would have to carry both Mode S and the TIS equipment itself. Mode S datalink, if it ever works at all, relays data from the ground to one aircraft at a time. Although the traffic picture is the same, a separate copy would be sent to each TIS recipient.
The common thread, and common flaw, that runs through all these systems is the reliance on transponders. Why is this?
In the early 1940s radar was in it's infancy. With the war raging, there was a need to sort out which blips were the good guys. The answer was called Identification, Friend or Foe. IFF for short. This was the earliest transponder. There were 64 secret codes that could be set, and when IFF picked up radar pulses it replied with the proper code. IFF saved a lot of our boys who otherwise might have been hit by friendly fire.
Of course the transport planes were IFF equipped along with the fighters and bombers, and it was natural after the war to use IFF for air traffic control. In the war, a single code was perhaps used on a given day or in a given operation. But ATC wanted to give each plane a separate code, and soon 64 weren't enough. By splitting each pulse into two, the 6 pulses that made up 64 combinations became 12 pulses with 4096 possibilities. And that's why our transponders have 4096 codes today.
Decades passed. From the years of the DC3 to the Concorde and beyond. Transistors and integrated circuits and computers were invented. We even went to the moon. But the only thing that changed about transponders was how many of them there were. And how often they replied. As the number of ATC radar sites grew, so did the number of replies. When general aviation transponders were required the jump in replies was tremendous. Then came Mode C, which effectively doubled the number of replies again. TCAS added even more. According to reports issued by the General Accounting Office, over-saturation has reached the point where ATC is sometimes losing aircraft from the radar screens, and occasionally seeing the whole screen go blank for minutes at a time.
Air traffic management is one task, collision avoidance is something else altogether. Traffic management involves TCA and ARSA and tower and approach control and clearance delivery and a hundred other creatures of the government. Collision avoidance involves airplanes.
The IFF idea had only to distinguish the good guys from the bad guys. It worked well for that, and is marginally capable in the traffic management role. But to avoid running into another plane, each aircraft needs data on where the other guys are. Position in three dimensions. Transponder signals have only the altitude dimension. And that limited data isn't available when flying below radar coverage, exactly where many midairs happen. Of course no data is available from the thousands of aircraft not transponder equipped. Collision avoidance based on sponders just won't hack it.
Which brings us back to the beginning. Why do we keep running into each other? Why doesn't somebody do something?