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When TCAS commands, don't look outside

A resolution advisory can conflict with your past training and experience

By J. Mac McClellan

Nearly all of us are now flying with the extra safety of a traffic advisory system (TAS).

Illustration by Neil Webb.
Zoomed image
Illustration by Neil Webb.
These systems track nearby traffic and announce a potential threat if an airplane is near our altitude and within a few miles laterally. TAS is, as its name implies, advisory. TAS is telling us a potential collision threat is developing, but the system leaves it up to us to locate the other aircraft causing the possible conflict and maneuver to avoid if necessary.

TCAS II (Traffic Collision Avoidance System) also provides the conflict advisory, but if a likely collision threat develops the system issues a resolution advisory (RA) that commands pilots to perform a vertical maneuver to escape the conflict.

TCAS was developed and then required in all airline jets after a series of tragic collisions in the 1970s and 1980s. The public and Congress lost all tolerance for collisions involving airliners and GA airplanes after a McDonnell Douglas DC–9 and a Piper Archer collided near Cerritos, California, in 1986. The result was mandated TCAS installation in all airliners.

TCAS was selected from among competing airborne collision avoidance systems because it tracks all Mode C transponder-equipped aircraft. If you have TCAS you have “protection” from any other airplane with an altitude reporting transponder. TCAS uses ADF techniques to determine the bearing to nearby aircraft, as do TAS systems. Both types of traffic warning systems actively interrogate other transponders and time the reply to accurately determine range to the possible threat.

Because it can take so long for a turn rate to develop after banking, particularly at jet speeds, changing heading to resolve a TCAS identified threat is not practical. And, it’s possible that two jets could be closing at ground speeds near 1,000 knots, so time to escape a threat may be very limited. That’s why a TCAS RA command is only a vertical maneuver.

When TCAS determines a collision threat is real, the system RA commands the pilot to maintain present altitude or initiate a climb or descent. If already changing altitude the RA commands to either maintain the present vertical speed or increase or decrease the rate.

When two TCAS-equipped airplanes come into conflict the systems automatically use the Mode S transponder datalink to exchange escape plans. For example, the TCAS in a descending airplane may transmit to a threat airplane in level flight that will RA command the descending pilot to slow or halt the descent. The TCAS in the level airplane will command nothing, or perhaps an RA “don’t climb.” Without this coordination between TCAS systems it’s easy to imagine how each box in a threat situation could command the same escape maneuver, such as an RA to each to climb. That would “harden” the conflict, as the regulators say, not resolve it.

TCAS RAs are issued by computer generated voice commands which are also shown on the vertical speed indicator. In new flat glass avionics systems, TCAS RAs also display a pitch target that will generate the necessary vertical rate to escape. If the autopilot is engaged when the RA is issued it disconnects so the pilot can quickly follow the RA command.

When you get an RA the FARs require the pilot follow the command even though it requires deviation from an assigned altitude. We’re also required to immediately tell the controller that we’re deviating to satisfy an RA.

If you get to fly a TCAS-equipped airplane be ready to ignore your previous experience and training. If an RA comes, don’t look out, follow the commands on the avionics system.For many years TCAS II equipment was too large and too costly to be installed in all but the largest business turbine airplanes. But advances in electronics now make TCAS II available—and much more common—in even light business jets and some turboprops. That means more and more pilots moving up from lighter GA airplanes will possibly encounter an RA and may need to overcome years and many hours of experience to react properly.

I know this to be true because I lived through a botched RA reaction by a pilot who responded to his decades worth and thousands of hours of flying experience in light airplanes instead of following the RA.

I was in the right seat for departure from a busy nontowered airport. We were on an IFR clearance, in good visual conditions, and cleared to climb runway heading to 3,000 feet. At about 1,500 feet above the runway a yellow diamond appeared on the PFD and the computer voice called a traffic alert. The yellow traffic symbol was directly ahead.

A few seconds later came the RA with the red flashing pitch target on the PFD and the voice calling “maintain rate, maintain rate.” TCAS had determined it would be close, but if we just kept climbing at our current vertical speed we would miss the threat.

But the pilot flying did what he had always done, and been taught, when a collision threat is perceived. He dumped the nose so he could look ahead through the windshield for the traffic.

Suddenly the TCAS solution to keep climbing was trashed. TCAS then switched to an urgent descent RA command to try to go under the threat. At the same time the collision avoidance software in the controller’s radar system was going off and he called with an urgent traffic alert.

It gets worse. Because we were at a fairly low altitude the ground proximity warning system (GPWS) took priority over TCAS and started shouting “terrain, terrain, pull up, pull up.” The same calculation was made by the controller’s radar system, and he stopped calling the traffic and changed to an urgent low altitude alert warning. The only fortunate aspect of this event—if there even was one—is that we had the cockpit speakers muted so all the warning and commands were coming through our headsets, not over the speaker to terrify the passengers.

We never did see the airplane that generated the collision threat. And we wouldn’t have flown into the ground because the visibility was good. But we sure could have hit that other airplane.

So, if you get to fly a TCAS-equipped airplane be ready to ignore your previous experience and training. If an RA comes, don’t look out, follow the commands on the avionics system. If you do that you will miss the threat. But if your experience and non-TCAS training take over and you maneuver to look for the traffic, you will almost certainly turn a “resolution” into a more serious threat of collision.

J. Mac McClellan is a corporate pilot with more than 12,000 hours, and a retired aviation magazine editor living in Grand Haven, Michigan.

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