TECHNOLOGY ADVANCES

Airbus works to develop smarter autopilots

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

By Peter Berendsen
ATP/CFII. Boeing 747, MD11


Author Berendsen (L) with Airbus Senior VP & Test Pilot Claude Lelaie over southern France aboard F-WWOW, the original A380 prototype used for flight testing.

It was while the Airbus A340 was cruising over the southwestern US that a TCAS traffic advisory (TA) suddenly popped up, followed immediately by a resolution advisory (RA). The command "Climb! Climb!" from the TCAS computer was displayed on the primary flight display (PFD) and called out via loudspeaker.

The 2 pilots looked for traffic, and the captain as pilot flying disconnected the autopilot but not the autothrust. He could not see the traffic, but followed the commands of the TCAS computer anyway by trying to fly in the green area of the vertical speed band, as per recommended procedure. The first officer did the same thing, pulling on the sidestick as well, instinctively following the "Climb!" command.

What happened next can only be understood with the help of some knowledge of the Airbus fly-by-wire flight control system. The 2 sidesticks are not connected mechanically, and 1 pilot cannot feel the other's control inputs.

There is a visual warning when dual inputs occur, but this is easy to overlook in a stressful situation. If both pilots move the sidestick-control, the inputs are added. For example, if the inputs are in opposite directions, they cancel each other out.

In this case, both pilots pulled in the same direction and the inputs were added. Since the TCAS evasion maneuver calls for a rapid pilot response, the inputs were crisp. Added together, they became dramatic.

Passengers and crew walking about in the rear section of this long aircraft suddenly lost the floor underneath their feet and were thrown violently against the ceiling, some sustaining injuries. For the TCAS computer, the conflict was resolved with a "Clear of conflict" message—but the flight had to divert to attend to the passengers and crew.

Problem and solution

Crew procedures call for clear task separation between pilot flying and pilot not flying. But, as this incident showed, in a stressful situation—seconds away from a possible collision—a less-than-optimum crew response to TCAS commands cannot be ruled out.

While it's true that the problem of adding sidestick control inputs is very specific to Airbus aircraft, it can be said that, generally, manual flying skills—especially at high altitude—are not practiced very often by today's civil jet flight crews.

But currently TCAS is only certified for manual avoidance maneuvers, and autothrust may be left on, as most operators recommend. At high altitude and high speed, the pitch changes needed to comply with the evasion command are actually quite small.

But jet crews usually do their manual flying during the low-altitude portions of flights and they are used to the aircraft's response at lower and medium altitudes and speeds.

On all aircraft, the switch from a relaxed cruise mode, possible with a drink or a meal tray on the lap, to a manually flown high-performance evasion maneuver can sometimes be too sudden. This may result in overreactions, excessive load factors, excessive vertical speeds and possible secondary RAs with a 3rd aircraft.

Airbus would not be Airbus if the company did not think of some automated solution to this problem. The solution is called Autopilot TCAS (AP TCAS) and I was invited to actually fly this new system and see how it works on the Airbus A380 at TLS (Blagnac, Toulouse, France).

Flying AP TCAS

The PFD shows the autothrust (SPEED) and lateral mode (NAV) active as in normal operations. The vertical mode is TCAS, showing that an evasive maneuver is being performed by autopilot or flight director.

As we cruised over the vineyards of the Bordeaux region at a block level between FL200 and FL250, Airbus Senior VP & Test Pilot Claude Lelaie was in the right seat while I had the privilege to fly in the left seat of this giant airliner.

But there was not a lot to do—the autopilot was engaged and following flight management system (FMS) commands along a preprogrammed route. In other words, it was an ordinary cruise situation.

But way in the back, on the main deck of F-WWOW—the A380 prototype still retained by Airbus for flight test use—was AP TCAS Project Leader Paule Botargues. Her laptop computer was plugged into the flight test station under the careful watch of Flight Test Engineer Gérard Desbois. Botargues is a young engineer for automatic flight systems at Airbus in Toulouse.

Her laptop did not just read data from the test aircraft, but actually fed data into the aircraft avionics systems—something you can only do in a flight test situation. The data she fed would make the giant aircraft move, since her data went straight to the TCAS computer. She simulated actual conflicting traffic, making the black box think that real aircraft were dangerously close.

The basic concept of AP TCAS is simple—make the TCAS evasion just an additional mode of the AP/FD system, providing guidance based on the TCAS target. This allows crews to remain in a familiar operating mode while in a critical situation. Further design goals were to minimize the deviation from the initial trajectory, maintain the current AP engagement status and avoid any need for pilot activation.

The AP/FD TCAS mode is a vertical guidance mode built into the autoflight computer. It controls the vertical speed (V/S) of the aircraft on a vertical speed target adapted to each RA, which is acquired from TCAS.

With the autopilot engaged, it allows the pilot to fly the TCAS RA maneuver automatically. With the autopilot disengaged, the pilot can fly the TCAS RA maneuver manually, by following the TCAS flight director pitch bar guidance. It has to be considered as an add-on to existing TCAS features (traffic on navigation display, aural alerts, vertical speed green/red zones on the vertical speed indicator).

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