Controller or TCAS RA: which would you obey?

 - May 5, 2008, 9:59 AM

After the tragic collision between a Russian Tu-154 and a DHL cargo Boeing 757 at 35,400 ft over southern Germany on July 1, AIN questioned whether regulations, or any other body of accepted procedures, clearly state which command took precedence–that of the onboard traffic alert collision avoidance system (TCAS) or that of the human controller. Seeking input from our readers, we posted the following question on “You are flying at 35,000 ft at night. Suddenly, ATC tells you to descend immediately due to traffic, but at the same moment your TCAS tells you to climb. What do you do and why?”

The question evoked response in greater numbers than any in recent memory. A total of 81 answers were tallied. (Seventy-nine readers responded via e-mail, while a pair of ink-pen jockeys wrote in the old-fashioned way.) Sixty-two respondents (77 percent) said they would climb in response to their TCAS. What surprised us here is that 14 (17 percent)–including one with a Honeywell e-mail address–advised they would obey the controller. Finally, five rugged individualists (6 percent) said they would maintain their current altitude and turn–some said “either left or right.”

The AIN question-and-answer was far from a scientific analysis. It could be called a small survey, perhaps, and nothing more. But the level of pilots’ confusion over the issue surprised many of us here at AIN, and led to further research into a question that seemed, at first, to be cut and dried.

According to FAA Advisory circular 120-55B, which deals with the air carriers’ operational use of TCAS (ICAO and JAA have nearly identically worded advisory circulars):

“ATC may not know when TCAS issues RAs [resolution advisories]. It is possible for ATC to unknowingly issue instructions that are contrary to the TCAS RA instructions. Safe vertical separation may be lost during TCAS coordination when one aircraft maneuvers opposite the vertical direction indicated by TCAS and the other aircraft maneuvers as indicated by TCAS. As a result, both aircraft may experience excessive altitude excursions in ‘vertical chase’ scenarios due to the aircraft maneuvering in the same vertical direction. Accordingly, during an RA, do not maneuver contrary to the RA based solely upon ATC instructions.”

The interactive nature of TCAS II is designed, in part, to eliminate the airborne equivalent of two people trying to sidestep each other on a sidewalk only to find they both zigged to the same side thinking the other would zag to the other. The TCAS commands the converging aircraft to go in opposite directions, one up, the other down.

Again from AC 120-55B:

“For TCAS to work as designed, immediate and correct crew response to TCAS advisories is essential. Delayed crew response or reluctance of a flight crew to adjust the aircraft’s flight path as advised by the TCAS due to air traffic control clearance provisions, fear of later FAA scrutiny or other factors could significantly decrease or negate the protection afforded by TCAS.”

For some pilots, learning to defy a controller’s command in deference to a cockpit instrument may have been as big a leap as early pilots ignoring their inner ear’s balance cues in favor of following the gyro horizon. Indeed, many pilots who wrote in evoked the old mantra, “Trust your instruments.”

TCAS, with its two-per-second updates, is much closer to real-time information about a conflicting aircraft’s path and position than the controller’s radar with its 12-sec sweep. Perhaps more important, pilots should realize that by the time a traffic conflict generates an RA, the ATC system has already failed, somehow, to provide adequate separation.