I do not want to interfere too much in this fascinating debate but there are a couple of misconceptions and assumptions ( most coming from Fullwings ), that I’d like to correct :
Fullwings :
Yes but those instructions came too late to stop the aircraft coming within 7NM of each other at the same level, on a collision course. That, by definition, is a "failure of separation". "Late ATC intervention" is a "failure of separation", involving much form filling, suspension of the individuals involved, official reports, etc. I'm coming to think your definition of "separation" is "not actually hitting each other", which is not an industry standard!
Not quite. In real life, late ATC intervention is rather common for various reasons, not all due to ATC errors, and 99,9999 % of the cases it solves the problem. There is no consequence. If a controller had to be replaced every time he use the word “expedite” there will be little staff left in my center !
ATC works on establishing separation ( typically 1000 ft and 5 NM )
TCAS was not designed to re-establish separation ..It was designed to prevent metal from touching each other.
(In most cases TCAS missed distance is unlikely to be more that 300 feet.)
Mixing the 2 leads to misconceptions on how both work in real life.
The misconception that once you get below “separation “ a controller should not issue any instruction is totally wrong and dangerous. ATC is responsible to provide separation , and anti collision avoidance
at all times , unless the PIC in command has informed him that the crew is following an ACAS RA. From this moment on , pilot takes over responsibility of anti collision from ATC., but before that moment, the controller is responsible, above or below 5NM .
The problem we encountered in Ueberlingen ( and in many other less dramatic incidents ) is that ACAS intervenes BEFORE separation is lost , and pilots do not inform ATC that they are following an RA, enforcing the controller’s mind that everything is still under HIS control, and might even continue to issue instructions well after TCAS has interfered.
Note : As a controller you do not know if an aircraft has a functioning TCAS or not (e.g. : one can fly 10 days with a U/S unit) and the same service is given regardless of having TCAS or not.
Fullwings :
From a pilot's POV I have to assume that there has been a failure of ATC when in receipt of an RA and act appropriately.
From my ATC experience and recorded data, until very recently, most RAs were not due to ATC but caused by excessive vertical rates by crew. Therefore the ICAO note to reduce vertical rate before leveling off.
There are many non-ATC other cases , such as ghosts targets, or level busts for instance , that are causing RAs.
The notion that an RA only occur when ATC has failed is wrong.
Lastly :
Fullwings again :
Or it's a cruise missile, a weather balloon, a flying saucer, a flock of birds, etc. Very easy to find edge cases where the system may not work but how realistic are they in a positive radar environment? If you stray into the path of a UFO during an RA it's just not your day is it?
Not a UFO, but a military a/c ( quite a few where I come from ) or an out of tolerance SSR that will not be accepted by TCAS., but visible on radar.
In my ATC system, an SSR failure will be compensated and tracked on primary , that one too will be invisible on TCAS . Remember that I have legally to separate aircraft from what I know.
If ATC instructions differ from what TCAS is telling you , there you have a dilemna. Do I have , as a controller, the right picture, with every player, moving as my radar tells me, or does TCAS has the correct solution ? . You will never know until you try , but either way, it is not a 100% guarantee that what you chose is the correct action.
Finaly :
Bsieker :
The problem is when you get conflicting advice from ATC and TCAS, particularly if in the order:
- ATC advice
- starting following ATC
- TCAS RA
This is indeed one of the real issues .Seeing the data I see , all post 2002, leads me to believe that we have not solved the problem yet and that another accident is probable.