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What has priority ATC / TCAS ??

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I was watching a program on "Wings" last night about ATC - made well before last weeks Swiss/German crash. In it they were interviewing a Dulles air traffic controller who had a very similar experience. She was attempting to "seperate" two aircraft by instructing one to climb and the other to descend, TCAS kicked in on both aircraft and instructed both pilots to do the opposite of what ATC was telling them to do, resulting in a very near miss.Couple of questions :::1) Which instruction has priority for pilots ??2) Whose fault is it that TCAS & ATC conflict ?? Does TCAS work to certain rules and if so does ATC know what they are ?? (and if so why do they give conflicting instructions ??)From a survival point of view I would have thought a pilot would follow TCAS (I am not a pilot myself), but if ATC has already given one / both of the aircraft conflicting instructions - it sounds like a recipe for disaster !!!

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I'm not an expert, but from what I've heard pilots always have to obey the TCAS instructions. If everybody does their job well, controllers as well as pilots, obeying the ATC instructions first will not allow TCAS to kick in (which it does only in the final 30 seconds or so).Looking forward to someone more knowledgable to give a more extensive answer :-)Vince

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As I understand what happened both crews followed the instructions from their TCAS.Both TCAS systems instructed a dive, resulting in the aircraft colliding anyway.ATC warned the Tu several times but it did not oblige orders to change altitude. What I find strange is that ATC did not order the Boeing to change altitude (or course) when the Tu did not respond to their communications.TCAS is an emergency response system that should never be needed unless ATC screws up (or is blatantly ignored by one of the crews involved like in this incident).

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I did wonder that when the news reports were on - why only order one aircaft to alter their altitude, surely if it is getting that critical they should have ordered both to change altitude ?? At least then the DHL plane would know what ATC was trying to do !?!

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I had read that the DHL 757 was equipped with TCAS, but the Russian Tupolov was not.I'm not sure how it works in other countries, but in the U.S. 14 CFR 91.3 says that the pilot in command is the "final authority" regarding his/her aircraft's operation and they may deviate from any ATC instruction or flight rule in an emergency.In the US, 14 CFR 91.123© says: "Each pilot in command who, in an emergency, or in response to a traffic alert and collision avoidance system resolution advisory, deviates from an ATC clearance or instruction shall notify ATC of that deviation as soon as possible."My understanding was that ATC told the non-TCAS equipped Tupolov to descend at the very last minute, but the DHL 757 pilot had also begun descending in response to a TCAS directive. The rate at which they were closing in on one another was incredibly fast and, in the absence of timely instructions from ATC, there was very little time to react.My experience is that air traffic controllers are very good at what they do, but they are still only human. I fly much slower moving aircraft (130 knots or less), but on more than one occasion ATC has given me instructions for avoiding other aircraft that would have taken right into the other aircaft's path. In those situations, I told ATC "unable" and followed the standard traffic right-of-way rules. With faster moving planes, there's much less time to see and avoid.John

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Eric,I just read the latest news report, and you're right that the Tupolev had TCAS (an earlier report I read said otherwise) and it apparently directed the pilot to climb. The DHL 757's TCAS had directed it to descend, so if the pilots had followed their respective TCAS directives instead of the controller they might have lived to fly another day.It appears the controller was way behind what was happening and should have resolved the conflict long before the TCAS systems came into play. Hindsight is 20-20 ...John

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Yep, they confirmed it today. And the controller in question has been suspended. Aside from the obvious mistake by the controller, there was the fact that he was alone, that 3 phone lines out of 4 were out of order (I think it was 3), that the 4th was engaged...etc.But on a broader scale, there's now the problem of solving who has priority. TCAS or the controller ? And should there be a link between aircraft TCAS systems and the controller equivalent (they're not linked at the moment, so they said it is now irrelevant that the ground system was shut down for maintenance) ? Of course, if the controller had been quicker to react, there may not have been any conflicting orders between controller and TCAS...__________________________________________________________EricList of all airlines, aircraft manufacturers and aircraft types recognised by ATC:http://www.geocities.com/eric_2203/orhttp://ftp.avsim.com/library/esearch.php?D...atID=fs2002misc

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>As I understand what happened both crews followed the instructions from their TCAS.Both TCAS systems instructed a dive, resulting in the aircraft colliding anyway.AS I heard today after blackbox interrogation, this isn't quite accurate. ATC commanded a descent, but TCAS called for climb. The pilot decided to conform to ATC with the obvious disastrous and very heartbreaking results. A spokesman from Flight Safety Int. said today that in the event of conflicting instructions, the pilot must always follow his cockpit equipment.(edit: also read today, the TCAS systems of each aircraft "confer" with each other before offering instruction to the pilot. That way, the instructions cannot be in conflict, ie. both TCAS command a descent/climb).Paul

http://homepage.ntlworld.com/paul.haworth/Fortress.gifVoted Best Virtual Airline of 2002 and Best CEO of 2002 by participants in the BIG VA Vote organized by FSPILOT.comVANF "Best" New Virtual Airline Awardhttp://homepage.ntlworld.com/paul.haworth/saint_georgex1.gif

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In the US the pilot in command has priority over the controller anytime an emergency exists or the pilot believes a controller's instruction would compromise safety. I suspect that similar rules exist in other countries. The pilot can base his/her decision to say "unable" to a controller on TCAS or any other source of information.To read about another incident that had a better outcome, check out , scroll down and read the article "Unable to Comply" ... John

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Hi Paul,Thanks for the extra info on TCAS. Where did you read about the TCAS procedures? I'm always curious to learn more ...John

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Hi John, The info I had relating to the accident was obviously third-party (BBC World News), the TCAS info came from my Dads manuals (772/744 Command Cpt.)Cheers,Paul

http://homepage.ntlworld.com/paul.haworth/Fortress.gifVoted Best Virtual Airline of 2002 and Best CEO of 2002 by participants in the BIG VA Vote organized by FSPILOT.comVANF "Best" New Virtual Airline Awardhttp://homepage.ntlworld.com/paul.haworth/saint_georgex1.gif

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Not really an "issue" between the pilot's vs ATC "authority" in the true sense, but the authority of the TCAS CR (conflict resolution) command vs ATC.And in TCAS training the instructors use the term "Command" vs "advice", "suggestion", "guess" to instill that the TCAS is to be followed. As soon as a traffic alert is issued by TCAS my Mk 1 eyeballs would be on the display to see what the TCAS was "talking" about and setting up in my mind what might have to be done.There is little time for debate, and the Captain needs to immediatly balance CRM/ATC/TCAS and make the final choice.Situational awareness as well plays a part. Even in cruise by watching TCAS and listening to ATC a pilot can get a pretty good picture of what is happening around him.A plane cleared direct to a fix I am heading to, or any plane reporting descending out of an altitude above me, climbing below me or being vectored for the same approach I am on gets a little attention in my thought-process.Timothy

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A great rule of aviation: "Your insturments never lie." A very sad case here..I imagine a sky full of stars (at that alt), easy routine flight, and the impossible all comes to one single point in time.

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You are correct, the two TCAS boxes in each plane actually "Talk" to each other and work out a resolution. In this case it sounds like TCAS did it's job. When the ATC controller told the one plane to do the opposite of what TCAS said the pilot should have continued following the TCAS instructions and told the controller he was following a TCAS conflict resolution. I have on many occasions had TCAS go off and say "Climb" or "Descend" by the time it goes off the situation is usually critical, we are taught to follow the TCAS (unless it is going to put you into terrain) and then tell ATC what is going on as soon as you can. Maybe the training these poor souls had did not prepare them to react properly to the situation but ATC put them there to begin with so it look like once again a chain of errors led to this unfortunate accident.Ken

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>A great rule of aviation: "Your insturments never lie." ...except in that case of the GA pilot trying to find a small island in ther middle of the Pacific using the only navigational instrument he had - ADF. Of course, by the time he figured out that it was completely bust he was hopelessly lost...Cheers,Paul

http://homepage.ntlworld.com/paul.haworth/Fortress.gifVoted Best Virtual Airline of 2002 and Best CEO of 2002 by participants in the BIG VA Vote organized by FSPILOT.comVANF "Best" New Virtual Airline Awardhttp://homepage.ntlworld.com/paul.haworth/saint_georgex1.gif

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Pilot's gotta know the limitations of their equipment as well as trusting them.Timothy

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