Turbine Pilot

See and avoid gets a big upgrade

TCAS, short for traffic alert and collision avoidance system, is a little bit like the fax machine, cellular telephone, or microwave oven. When those inventions first appeared, a lot of us scratched our heads and wondered why we possibly needed them. Now, of course, many of us just can't live without them. While TCAS is a long way from enjoying the equivalent popularity in aviation circles of, say, a microwave oven, it is changing the minds of some who early on doubted its usefulness. Although far from perfect, it has already proven itself to be a powerful tool in helping prevent midair collisions.

I'll admit it. I was one of those who doubted TCAS would ever work. Because it was designed mostly with airliners and corporate heavy metal in mind, it all seemed to me like expensive window dressing — nice to look at but far from a complete solution to the problem of midairs. Most general aviation aircraft wouldn't even be equipped with it. I visualized frequent chain reactions in busy airspace, where one aircraft's TCAS- generated avoidance maneuver would cause other aircraft to do the same, until the ATC system itself experienced a kind of meltdown. Even worse, TCAS reeked of a Big Brother mentality. It was somehow offensive to me that see and avoid was no longer good enough. Now we needed a machine to do what pilots had been doing all along. Never mind that nearly every midair reflected a breakdown in the see-and-avoid doctrine. Never mind that such breakdowns were statistically certain to continue occurring. A little voice kept repeating to me: TCAS will never work.

The day came when I found myself sitting in ground school, learning about the Bendix TCAS II system that my carrier had begun installing in the fleet. I was unhappy to learn it would share the same screen as my weather radar and that it would automatically switch from weather view to TCAS mode if it detected threatening traffic. What if we were deviating around a monster storm and suddenly the cell disappeared off the screen? Not a problem, the instructor assured us. We could quickly switch among a choice of weather, TCAS, or both modes. What if TCAS told us to climb at the same time it commanded head-on traffic to climb? That can't happen, he told us. The system will always command opposite vertical maneuvers between conflicting, TCAS II-equipped aircraft. Just do what it says, and you'll be fine. From the sound of muttering coming from some of my classmates, I knew I wasn't the only one not buying it. I left class feeling, if anything, even more unhappy about entrusting myself to this new technological wonder.

Well, as the saying goes, that was then and this is now. After flying more than 700 hours with TCAS, I would sooner give up my morning coffee than fly without it. To carry the microwave analogy one step further, I've gone from exploding hot dogs and baked potatoes to knowing how to cook with it. It's a terrific help in spotting traffic, far better in some situations than just another pair of eyes. It can find airplanes through cloud cover, haze, or darkness as easily as it can in CAVU conditions. It's reliable, never relaxing its vigilance, except for those times when its central processor is overloaded from receiving too many transponder returns at once. It has definitely increased the number of aircraft I acquire visually during a typical flight. For the most part, it has proven my early fears ungrounded.

Not that it doesn't have its little quirks. In my opinion, it still gives too many false alarms in the vicinity of busy airports, where aircraft often maneuver in close but safe proximity to each other. For example, aircraft on parallel approaches can trigger warnings, even though IFR separation is adequate. Likewise, helicopters flying helicopter-only traffic patterns and even taxiing aircraft can fool TCAS into wrongly believing a conflict exists with traffic operating near the airport. Other false alarms are generated at altitude during high-vertical-rate encounters, as when climbing and descending aircraft are converging. TCAS looks only at the vertical rate of closure and has no way of knowing that the pilots are planning to safely level off before their altitudes intersect. Some of these problems will be eliminated as the software continues an evolutionary process of refinement. But in any event, I'm willing to overlook them for now in return for the wider benefits TCAS offers.

TCAS works by computing the range, relative bearing, and altitude of intruder aircraft (if squawking altitude). It then predicts the time to and the vertical separation from the intruder at its closest point of arrival to one's own aircraft. Because TCAS works by interrogating transponders of other aircraft, those without transponders cannot be detected.

There are currently two kinds of TCAS in use: TCAS I and TCAS II. If either system predicts that safe separation boundaries will be violated, it issues a traffic advisory, alerting the pilot to acquire the traffic visually. TCAS II has the added capability of being able to issue a resolution advisory when the threat of collision is especially high. A resolution advisory is a TCAS-computed avoidance solution, telling the pilot to either maintain or change altitude so as to avoid the traffic. Neither TCAS I nor II installations are able to command course changes, a feature in store for us when TCAS III makes its debut at some future date.

So what's it like to fly with TCAS? For starters, it takes very little time to feel comfortable with it. The easy learning curve is due to a simple method of depicting information on the TCAS display. A small airplane symbol in the middle of the screen represents your aircraft. Any other transponder-equipped aircraft within the selected range (5, 10, 20, or 40 miles) appears as either a diamond-, round-, or square-shaped symbol. If an intruder aircraft is squawking altitude, its altitude relative to your own is displayed. An up or down arrow indicates whether the intruder aircraft is climbing or descending.

The collision threat level of other aircraft is indicated by the kind of symbol displayed by TCAS. A hollow white diamond shape represents a non-threatening aircraft within 2,700 feet vertically of your own aircraft and up to 40 miles distant. When the diamond turns solid white, the target is still not considered a threat, but it has closed within 6 miles laterally and 1,200 feet vertically.

Once TCAS calculates that an intruder aircraft represents a collision danger, it changes its traffic symbol to a solid yellow circle. TCAS issues a traffic advisory when the intruder's closest point of arrival is calculated to be between 35 and 45 seconds away. This advisory is a loud "traffic, traffic" voice annunciation, which calls attention to the threat. At this point, the pilot should attempt to acquire the traffic visually, but no evasive maneuver is called for.

The highest level collision threats are displayed as solid red squares on the TCAS screen. When a target turns from a yellow circle to a red square, it means TCAS has computed that a collision is likely to occur within 20 to 30 seconds, based on current trends. At this point, TCAS II will issue a resolution advisory. This is either a command to maintain altitude, climb, or descend to avoid the traffic.

Resolution advisories are communicated to the pilot in two ways besides the depicted red square symbol. The first is a loud voice command such as "climb, climb," "descend, descend," "monitor vertical speed," or "stop descent." The second is a band of red and green lights incorporated in the aircraft's vertical speed indicators. Green lights indicate an acceptable vertical speed range, and red lights show vertical speeds to be avoided. If TCAS commands a 1,000-fpm or better climb rate for instance, the band of green lights on the VSI will illuminate from the plus-1,000- feet-rate point to the upper climb limit of the instrument. Maintaining a climb rate somewhere in the green range will be sufficient to avoid a collision, based on the existing conditions. Because a descent or shallow climb would be inappropriate in this example, the entire descent range of the instrument, as well as the climb range less than 1,000 fpm, would be outlined in red.

Incidentally, TCAS usually supplies enough warning of a collision threat that any called-for evasive maneuver is fairly gentle. The resolution advisory is predicated upon the pilot reacting within five seconds of the warning, incurring G forces no greater than would be called for if air traffic control asked for an expedited climb or descent. Altitude deviations resulting from a resolution advisory typically are less than 600 feet. Climbs or descents greater than what TCAS calls for are not desirable, because such a maneuver could create a conflict with other traffic. Pilots need not advise ATC prior to making TCAS-commanded altitude changes although an advisory call is appreciated. They are, however, expected to immediately return to their last assigned altitude once the conflict is resolved.

TCAS makes use of the Mode S transponder. Mode S has all the capabilities of Mode C and also includes a data link allowing a TCAS unit to communicate with other TCAS units in range. This ensures that resolution advisories between two conflicting TCAS-equipped aircraft will always be coordinated in opposite fashion. If one is commanded to climb, for example, the other will be commanded to descend. For obvious reasons, pilots are warned not to maneuver in a direction opposite to that specified by the resolution advisory. Resolution advisories can be coordinated among as many as three TCAS aircraft at once.

Using TCAS is as easy as turning it on. I like to select the 5- mile range when departing an airport, because this scale allows for the clearest depiction of aircraft operating in or near the traffic patterns. During climb-out, I switch to progressively longer ranges, usually leaving it set at the 40-mile range for cruise flight at high altitude. If TCAS issues a traffic advisory, I can always switch back to the 5- or 10-mile range to help me more precisely locate the traffic visually. During descent, I reverse this process, selecting closer ranges as our flight nears the terminal area. Although the Bendix units can display as many as 30 aircraft simultaneously, I find that more than 10 or so at one time makes the screen difficult to interpret. Selecting closer ranges eliminates the more distant targets, reducing screen clutter in very busy airspace.

One benefit of flying with TCAS is that pilots acquire more aircraft visually than when operating without it. With a little practice, it becomes second nature to quickly spot a distant target out the window, based on the altitude and azimuth information displayed on the screen. I conservatively estimate that I spot twice as many targets visually when using TCAS than when flying without it. Often, these aircraft are not pointed out by ATC through traffic advisories, either. In fact, it's a bit humbling to realize just how many airplanes we don't see during a typical flight without TCAS.

Being able to spot non-threatening targets is nice, but being alerted to the possible collision threats is nicer. It is, after all, the reason why TCAS exists. In the course of using TCAS, I have experienced numerous resolution advisories. Although in most cases we were able to spot the intruder aircraft visually, I recall several occasions when we did not. In two of these encounters, ATC issued no traffic advisories, and we would have been oblivious to the danger were it not for TCAS.

TCAS has another less obvious use as well. I find I sometimes depend on it to supply spacing information when flying approaches. Often it gives the first clue that the aircraft we are following has begun to slow down or that, conversely, there is plenty of spacing and we can keep our speed up a bit longer. At high altitude, it serves a similar purpose, allowing us at times to plan our requests for altitude or speed changes more precisely with ATC. Although pilots need to resist the urge to play air traffic controller, TCAS can help boost our situational awareness, just like paying close attention to all radio transmissions can.

In the short time since TCAS has been in widespread use, there are already plenty of anecdotal tales of close encounters of the worst kind that had happy endings because of it. Warts and all, it's doing a respectable job of helping keep airplanes apart in the wild blue yonder. With a little more polish, it won't be long before we'll all be wondering how we made it this far without it.

Vincent Czaplyski, AOPA 690264, holds ATP and CFI certificates. He flies as a Boeing 737 captain for a major U.S. airline.