View Full Version : TCAS- PRIMACY
Question for the Pros please-
If in recept of a TCAS alert, a pilot were to receIve a contemporaneous instruction from ATC to turn in an opposite direction or height to that commanded by TCAS and assuming no time to discuss ( does TCAS give closing speed/time?) ,what is the procedure in terms of primacy? Is it laid down in law, company policy or is it always PIC discretion? I am assuming that ATC would have the same proximity data. Is TCAS less likely than the human to be wrong?
Many thanks
Tarq57
26th Oct 2011, 10:13
The TCAS system overides any ATC instructions to the contrary.
TCAS provides commands in altitude only; there are no turn instructions.
Once the alert has been received in the flight deck, and the crew have a spare second, they should inform the relevant ATC unit that they are responding to a TCAS RA (resolution advisory.) Once that communication has been received by ATC, no instructions are to be issued in respect to that flight; information only (if relevant) may be passed.
In regard to your second question, it is not a matter of whether TCAS or ATC are more likely to be accurate; rather the TCAS system is a safety net. The last line of defence. As such, it is to be followed, the rationale being that the ATS system has already failed to provide the separation.
It does produce false alarms, particularly when aircraft are in descent or climb, and have not yet commenced a level-off at an altitude separated below (or above) other closing traffic. The TCAS system is not aware that the aircraft is about to level off - it predicts only closure rate and if that rate is above a certain threshold, will generate firstly a TA, and if the closure continues, and RA, which must be followed by the crew.
Many thanks for taking the time to give a most informative response
Tarq57
26th Oct 2011, 10:45
No trouble at all.
I'm curious about why you asked, though. Anything specific prompt this?
I seem to remember ( vaguely) the collision over Switzerland ? involving a cargo B757 an Russian aircraft where the pilpt allegedly followed an ATC command rather thab TCAS. Have I got this right?
Tarq57
26th Oct 2011, 11:15
I had a sneaky feeling the question may have been concerning this. It's been widely discussed, here and other forums.
In a nutshell, the controller became aware of the confliction at a late stage, and issued instructions to the Tupolev pilot to descend. The crew commenced descent, and almost immediately received a TCAS resolution advisory commanding a climb. Simultaneously the DHL crew received an RA commanding a descent. (TCAS systems of flights thus affected "talk" to each other in order to coordinate the appropriate response.)
For various reasons, the crew of the Tupolev disregarded the TCAS and followed the ATC instruction. The collision occurred quite shortly after that.
That is a gross simplification of the event, which had many contributing factors, as all accidents do, at the ATC, ANS management, technology, and airline training level. I hesitate to say such an event could not happen again, but the promulgation of the information/recommendations that came out of the investigation has done much to prevent a re-occurrence.
Tarq57
26th Oct 2011, 11:21
More detailed info on the accident, including a link to download the report if you're interested in the detail, here ASN Aircraft accident Boeing 757-23APF A9C-DHL berlingen (http://aviation-safety.net/database/record.php?id=20020701-1)
I add that I believe the 'rules' in force at the time in the Tupolev's company were to obey ATC at all times.
To also note that TCAS technology is 'on the way' to 'knowing' the level-off altitude selected by the crew to mitigate para 4 of Tarq's post #2.
BOAC-
Many thanks , as ever.
Just one other thing ,if I may -
So affording TCAS primacy is esssentially mandatory; if notwithstanding, the PIC ( yourself) is convinced that ( for whatever reason- visual awareness? - I don't know) the TCAS command is wrong and dangerous, is the commander authorised to override?
Many thanks
Hmm! Difficult question - well done:) It is, as always, 'never say never' but the first 'natural' instinct has to be to follow the TCAS command.
Since you asked - hypothetically - IF a TCAS climb would cause me to collide with a non-TCAS equipped a/c I could see just above me, or - if. for some reason - a TCAS 'Descend' would cause me to definitely collide with an obstacle.................................I cannot think of any others - and I would need to be 100% certain.
It is good. It has proved very reliable. It must be followed. It must be instinctive.
BOAC ( and TARQ)
I am grateful to you Pros for takiing the time to rely to the pax- very much appreciated. I love this site :)