Thursday, December 11, 2008

Merging Targets



No matter what you read in the coming days, the only failure that mattered in the midair in Brazil, back in 2006, was a failure of Air Traffic Control. It was not the failure of the pilots, the transponder, TCAS (Traffic Collision Avoidance System) or anything else. Just air traffic control. On a fundamental level, it was a failure of the air traffic controllers. Having been one, I realize that they may have been victims of their technology and/or their work environment. Be that as it may, the controllers are part of the ATC system.

Much is being made of the transponder failure and the subsequent failure of TCAS. For the general public, this is nothing more than an unfortunate distraction. Airplanes fly everyday without transponders or TCAS and the air traffic control system is still responsible for separating them safely.

Some of procedures we use in air traffic control have -- seemingly -- lost their relevance over time. They seem out of date. They seem quaint. We don’t have time for them. Take this paragraph from the FAA’s Order 7110.65 -- Air Traffic Control:

5-1-8. MERGING TARGET PROCEDURES

a. Except while they are established in a holding pattern, apply merging target procedures to all radar identified:

1. Aircraft at 10,000 feet and above.

2. Turbojet aircraft regardless of altitude.

REFERENCE- P/CG Term- Turbojet Aircraft.

3. Presidential aircraft regardless of altitude.

b. Issue traffic information to those aircraft listed in subpara a whose targets appear likely to merge unless the aircraft are separated by more than the appropriate vertical separation minima.

EXAMPLE- "Traffic twelve o'clock, seven miles, eastbound, MD-80, at one seven thousand."


Right now, I’m being cursed by various air traffic controllers because they know what is coming. They will tell you that they don’t have time to apply Merging Target Procedures. I will say that they have a point. Due to the increased accuracy of GPS, targets “merge” constantly now with the “appropriate vertical separation” as they navigate precisely on the center line of the airways they are assigned. If a certain hard-headed safety rep. reminded them that they needed to apply the “merging target procedure” they would rationalize, “Why ? They can see each other on TCAS.”

Going back to the FAA 7110.65 5-1-8, there is another subparagraph:

d. If the pilot requests, vector his/her aircraft to avoid merging with the target of previously issued traffic.

NOTE- Aircraft closure rates are so rapid that when applying merging target procedures, controller issuance of traffic must be commenced in ample time for the pilot to decide if a vector is necessary.


I don’t know if there is a similar section in the Brazilian air traffic control manual. But after reading through the accident report (a 6+ meg .pdf file), I can tell you that ATC was the only entity that was aware of -- and charged with -- keeping those two airplanes apart. Without a traffic call to alert the pilots of the other's presence -- without a transponder to alert the TCAS system of the other aircraft’s presence -- the only entity aware of the two aircrafts’ proximity to each other was the ATC system. And it failed.

Don Brown
December 11, 2008

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