When not to follow the controller

Air Safety Week,  Oct 10, 2005  

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From the summer 2005 issue of CHIRP FEEDBACK, the UK journal of the Confidential Human Factors Incident Reporting Program, is this tantalizing item on the Airborne Collision Avoidance System (ACAS) and its conflict with guidance given by the air traffic controller (ATC):

ACAS vs. ATC - A Reminder

Report Text: We were just innocent bystanders who heard the following [radio] exchange between a Northern European air traffic controller and an aircraft of a third nationality, which was cruising at FL380:

ATC: "[Callsign] contact AAA on channel XXX.XXX."

Aircraft: "AAA on XXX.XXX. [Callsign]"

ATC: "Negative! [Callsign] descend immediately FL370."

Aircraft: "Roger. Descending FL370. [Callsign]"

ATC: "The traffic is in your twelve o'clock at 12 miles at your level, converging. He will maintain FL380. If you get an ACAS alert, do not follow it."

Aircraft: Silence.

We were aware of no exchange with other aircraft involved, but there was a lot of chatter in the national language on the [radio]. I just hope that the pilots would have obeyed any [TCAS, or Traffic Alert & Collision Avoidance System, which is the same thing as ACAS] resolution advisories, RA, to avoid a collision] and ignored the controller's incorrect instruction. I also hope that the controller went home and re-read his manuals!

CHIRP Comment: Following the berlingen mid-air collision (see ASW, May 31, 2004), ICAO [International Civil Aviation Organization] issued revised guidance regarding TCAS RAs ... The revised guidance is unequivocal and states that in the event of an RA pilots shall:

* Respond immediately by following an RA as indicated, unless doing so would jeopardize the safety of the airplane;

* Follow the RA even if there is a conflict between the RA and an ATC instruction to maneuver;

* Not maneuver in the opposite sense to an RA; and

* As soon as possible, as permitted by flight crew workload, notify the appropriate ATC unit of the RA including the direction of deviation.

Since the revised guidance was issued, at least one further serious loss of separation incident has occurred in Europe, in which a crew continued to follow an ATC instruction and did not respond immediately to the RA that they received.

In the UK, the importance of the flight crew responding immediately to an RA has been emphasized to ATC providers in Air Traffic Services Information Notice (ATSIN) No. 15.

(Note: An excellent discussion of ACAS and the berlingen mid-air collision may be found at www.rvs.uni-bielefeld.de/publications/Reports/ACAS- Midair-www.html)

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