TECHNOLOGY ADVANCES

Airbus works to develop smarter autopilots

Need based on TCAS problems when TA and RA occur coupled with desire for intelligent responses.


We flew the TCAS tests over the Pyrenees, southern France. At medium and lower altitudes in busy terminal areas, collision avoidance and evasion maneuvers are of utmost importance.

Botargues pressed the "Enter" button on her laptop, and both of us up front were alerted to conflicting traffic by the TA "Traffic! Traffic!" A yellow diamond appeared on the navigation display (ND) at the 10 o'clock position, climbing rapidly to our flight level.

Lelaie pointed out the first indication that AP TCAS was installed—below the green "ALT" indication on the flight mode annunciator of the PFD appeared a blue "TCAS" showing that the TCAS mode was armed. This is consistent with the Airbus philosophy of showing all armed AP/FD modes in blue.

We both habitually looked for traffic, but of course could not see anything in this reserved air­space except the skies over the beautiful French countryside. The simulated intruder did not go away—and, as expected, the RA came with a "Climb! Climb!" announcement from the speakers. We sat back and relaxed, merely observing as the blue armed TCAS mode indication became active (green) and moved up, replacing the previous green ALT indication.

Our A380 started to pitch up. The initial system target V/S was 200 fpm with a load factor increase of 0.3 g. As TCAS only provides vertical guidance due to unreliable azimuth information, the lateral mode of the AP/FD system remains unchanged. If the FD bars are switched off by the pilots for some reason, they automatically display similar to the go-around or windshear modes. The autothrust system engages or remains engaged in speed or Mach modes to ensure a safe speed for the maneuver.

The conflict was resolved smoothly. The intruding traffic had leveled off below us, the TCAS computer an­nounced "Clear of conflict," and the green and blue TCAS indications disappeared. Now the system reverted to V/S mode, as we were at FL256—600 ft above our cruise altitude. ALT was armed and, as we descended and returned to FL250, ALT was captured and changed to active (green) again.

We tried out several configurations and traffic situations as we descended (idle back) to TLS, first in OP DES (idle power) with speedbrakes extended. The "Climb! Climb!" RA was suddenly triggered, followed beautifully by the autoflight system—although we did have to retract the speedbrakes manually.

Then we were at 5000 ft with flaps 2, and finally we were at 5000 ft with flaps full and gear down. Each time, the AP TCAS system resolved the conflict as we were watching.

A380 prototype flight test engineer station. The laptop on the desk was used to feed simulated traffic signals into the TCAS system.

Airbus stresses the point that the crew retains the capability of disconnecting the AP and the FDs at all times, and can respond manually to a TCAS RA by flying according to the "conventional" TCAS procedure (ie, flying the vertical speed out of the red band). The final responsibility and authority of the captain are not diminished by this tool.

Availability of AP/FD TCAS mode makes it possible to define simple procedures for the flightdeck crew. When a TCAS TA is triggered, the crew procedure is to check that the TCAS mode is armed blue.

When a TCAS RA is triggered, the crew follows the AP/FD guidance or monitors that the aircraft V/S is outside the red area on the V/S scale. ATC has to be informed, just as always. When clear of conflict, autoflight system modes have to be monitored as they return to the previous flightpath.

I was impressed. A maneuver that at a minimum would have caused some spilled coffee and flightdeck excitement had been reduced to an automatic routine avoidance man­euver. Of course, you could ask why nobody had ever thought of this before.

Introduction pain

A "Climb!" RA results in AP TCAS vertical mode engagement. TCAS mode annunciation is still boxed white, showing that the mode was just activated. Aircraft climbs on autopilot out of FL100 to avoid traffic. Once clear of conflict, the autoflight system will revert to vertical speed (V/S).

Over the years, numerous pilots have asked their simulator instructors why they are taught to fly other critical maneuvers, such as a go-around or windshear recovery, using autoflight, but a TCAS avoidance maneuver always manually. The answer is actually certification and maybe history.

When TCAS was mandated by Congress as a result of a string of tragic midairs in the 1980s, it was imposed on aviation against the opinion of many experts who feared that it would interfere with the safe conduct of air traffic control.

It took many years of educating pilots and controllers to trust TCAS, but also many years of technical improvements. Only today do we have a situation where pilots do not question TCAS commands but follow them right away, and ATC controllers accept flightpath deviations caused by TCAS RAs as a helpful last line of defense.

Certifying such a system is not easy either, as the aircraft is doing something that the pilots did not ask it to do. To convince authorities that such a system should be certified and could actually improve aviation safety is an achievement in itself. And any certification for a new system always requires procedures for failures and warnings, creating new and unintended complexities and issues.

To support its case, Airbus also studied the impact of the new AP TCAS function on the entire air­space. If most aircraft were fitted with the new system, would that improve overall safety? Consulting firm Egisavia performed this analysis and looked at the safety impact, both systemwide and on individual fitted aircraft.

Significant safety improvements were found, both for the entire airspace system and for each aircraft outfitted with AP TCAS. This was largely due to the fact that delayed or wrong pilot response to TAs was eliminated.

Airbus is enthusiastic about AP TCAS and will certify the system on its entire fleet. However, a word of caution is always in order. New automatic systems are often introduced with the argument that they overcome shortcomings of the human pilot. But the safety benefits of new automated systems are sometimes offset by the new hazards they create.

FBW itself may be viewed as such a system. While it protects against dangerous or even reckless pilot inputs that most pilots would never attempt, the feel for the aircraft close to the ground in difficult landing situations seems to be less direct. And the fact that one pilot does not feel the other pilot's inputs may lead to unwanted flight trajectories due to dual inputs.

Recent years have seen reinforcement of the view that it is vital to maintain good basic pilot skills and training. Consider how many pilots may just sit back and monitor when there is a real threat of collision.

Peter Berendsen is a Boeing 747-400 captain for an international airline. He writes regularly on aviation-related subjects.


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