TCAS or ATC priority? Re. DHL 757 midair and TU-154
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TCAS or ATC priority? Re. DHL 757 midair and TU-154
Hi people - have just been watching the Air Crash Investigation's re-enactment of this particular tragedy (or at least the part of it I can find), and it raised one thought to mind - I have always assumed that a TCAS warning was to be followed, whether that meant going against an ATC command or not, as this is precisely why the system was first designed? Can anybody shed me some light on this particular issue - I apologise in advance if this has been covered elsewhere but I have tried a search, and I also apologise for any ignorance I may be displaying in asking.
The link to what I can find is - YouTube - Air Collision Mid-Air
All the best, Jack.
The link to what I can find is - YouTube - Air Collision Mid-Air
All the best, Jack.
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Bashkirian Airlines Flight 2937 - Wikipedia, the free encyclopedia
Quite some useful information in this article, more specifically under the header 'Conflicting Orders'
Quite some useful information in this article, more specifically under the header 'Conflicting Orders'
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Always listen to your TCAS!
Remember that we ATC's dont know what your TCAS is telling you. TCAS is only between you and your conflicting traffic. If for some reason you have gotten too close to other traffic, ATC might try to solve the problem by descending/climbing you. But if TCAS tells you otherwise, always adhere to the TCAS, unless you have confirmation from the conflicting traffic that you should both adhere ATC instructions. (and there is seldom enough time for this)
Classic example of this was the Swiss midair! The ATC instruictions were in conflict with the TCAS instructions. The DHL flight adhered to TCAS, while the russian crew adhered to ATC. They hit each other more than 1000feet below cruising FL's.
Under these circumstances, a good ATC should instruct you : (...TCAS is priority, descend FL...)
Remember that we ATC's dont know what your TCAS is telling you. TCAS is only between you and your conflicting traffic. If for some reason you have gotten too close to other traffic, ATC might try to solve the problem by descending/climbing you. But if TCAS tells you otherwise, always adhere to the TCAS, unless you have confirmation from the conflicting traffic that you should both adhere ATC instructions. (and there is seldom enough time for this)
Classic example of this was the Swiss midair! The ATC instruictions were in conflict with the TCAS instructions. The DHL flight adhered to TCAS, while the russian crew adhered to ATC. They hit each other more than 1000feet below cruising FL's.
Under these circumstances, a good ATC should instruct you : (...TCAS is priority, descend FL...)
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Remember that it took place some years ago, and the policy about TCAS was not as strict as today. Also SOPs for Tu-154 (follow TCAS) and for the airline (follow ATC) conflicted, hence the disagreement in cockpit of Tupolev.
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Under these circumstances, a good ATC should instruct you : (...TCAS is priority, descend FL...)
TCAS is the safety net. If it generates a resolution advisory, that indicates that the air traffic control system has failed, (or there has been a level bust) and therefore it is common sense, and SOP to promptly obey the RA regardless of what ATC has instructed. As soon as time permits, the crew should communicate that fact to ATC, using the phraseology "...TCAS climb/descent".
Incorrect. Under these circumstances the ATC should do nothing, except advise the location of any known essential traffic, and plan any recovery strategy that may be necessary. ATC should not interfere in any RA generated, even to pass a recommendation to follow it.
In the case of the TU154/B757 event, the controller was unaware that at the time he became aware of the impending loss of separation and took steps to resolve that, that the TCAS of both aircraft was about to command a different response. That the crew of the TU154 elected to follow the ATC instruction was, regrettably, the final link in the chain.
The controller had no way of knowing the TCAS had generated an RA.
There is still no automatic feature in the ATC system (that I know of) that provides this information to the controller. If any of us become aware of a potential conflict/loss of separation, we will take all steps to prevent a collision and restore the appropriate required spacing. It is not only in our nature to do this; it is a requirement. Right up to the time one of the crew informs ATC they are responding to a TCAS event.
Under these circumstances, a good ATC should instruct you : (...TCAS is priority, descend FL...)
In the case of the TU154/B757 event, the controller was unaware that at the time he became aware of the impending loss of separation and took steps to resolve that, that the TCAS of both aircraft was about to command a different response. That the crew of the TU154 elected to follow the ATC instruction was, regrettably, the final link in the chain.
The controller had no way of knowing the TCAS had generated an RA.
There is still no automatic feature in the ATC system (that I know of) that provides this information to the controller. If any of us become aware of a potential conflict/loss of separation, we will take all steps to prevent a collision and restore the appropriate required spacing. It is not only in our nature to do this; it is a requirement. Right up to the time one of the crew informs ATC they are responding to a TCAS event.
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UK Instructions
I can't speak for the rest of the world but the UK the Manual Of Air Traffic Services Part 1 basically says that the Resolution Advisory will have priority until the conflict has passed.
Take care
Take care
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Its good to see that everyone seems to speak the same language. There are some "cultures" however that still seem to prefer ATC instructions above TCAS instructions. This has been a very extensive topic of discussion during the investigation of the swiss midair. Cockpit recordings have shown that the crew of the russian airliner were in dissagreement with each other re this topic.
The comment I made regarding that a "good" (and I hate to use the word good) ATC should/could add "TCAS has priority", is off course only in the case where uncertainty exists to the ATC, on whether a RA has been given by the TCAS.
You are quite correct in saying that if any of the pilots have reported a TCAS descend/climb, ATC should do nothing, and let the TCAS do its work.
I have been part of an investigation (after the swiss midair), where the ATC was involved in a reduction in seperation with a CL60 and a B732 head-on at FL350, and after noticing the conflict, the ATC instructed the CL60 to descend immidiately, after which the B732 reported that they were already on a TCAS descend. Luckily nothing happened, but in this case it would've been helpful to add "TCAS is priority, descend...) A Few seconds hesitation by any of the crew could be fatal in such a case.
The comment I made regarding that a "good" (and I hate to use the word good) ATC should/could add "TCAS has priority", is off course only in the case where uncertainty exists to the ATC, on whether a RA has been given by the TCAS.
You are quite correct in saying that if any of the pilots have reported a TCAS descend/climb, ATC should do nothing, and let the TCAS do its work.
I have been part of an investigation (after the swiss midair), where the ATC was involved in a reduction in seperation with a CL60 and a B732 head-on at FL350, and after noticing the conflict, the ATC instructed the CL60 to descend immidiately, after which the B732 reported that they were already on a TCAS descend. Luckily nothing happened, but in this case it would've been helpful to add "TCAS is priority, descend...) A Few seconds hesitation by any of the crew could be fatal in such a case.
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The controller had no way of knowing the TCAS had generated an RA.
There is still no automatic feature in the ATC system (that I know of) that provides this information to the controller.
There is still no automatic feature in the ATC system (that I know of) that provides this information to the controller.
Financially, there appears no willingness to invest in this life-saving feature to automatically inform ATC of a TCAS RA.
Safety is not always priority number 1 in aviation.
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A Few seconds hesitation by any of the crew could be fatal in such a case.
Pilots are instructed to always follow TCAS because there is no other choice. There's not enough time to sort it all out. You have to folllow TCAS -- even if it is offering the wrong solution.
I'm looking forward to the long term data -- mid air collisions pre-TCAS and post-TCAS. So far, between Uberlingen and Brazil -- I've haven't seem any proof that TCAS has made anything safer. It make people feel safer. But there's a big difference.
TCAS and Intercept by F-16
AIM 5-6-2. Interception Procedures
This is just one area with a serious disconnect in policies. Mr. Murphy has dozens of others awaiting us.
Don Brown
Don
How your assertion can be quantified, I have no idea. What I can tell you is TCAS has saved my ass a few times. Would it have been a hit, not sure but close enough that TCAS and not a controller separated me from the other player. Not a gig against ATC, separating me from a VFR guy isn't their highest priority in high workload situations. (am I correct with a shade of gray for the type of airspace?) It's my priority however and TCAS likely saved the day.
You can't simply look at TCAS and say that the numbers justify it or not simply based off some empirical data that reflects a singular parameter. The things I use it for day in and day out make it worth its weight in gold. Am I catching the guy ahead on the visual? What's that VFR guy doing over there when landing at an uncontrolled airport or after the tower closes? Is the heavy I'm following a bit high over that fix that I'll cross in a minutes time on profile? Perhaps I should seat the flight attendants as such or adjust my profile.
I'll overlay the TCAS on the WX radar and find gaps in what otherwise looks like a solid line that I didn't see due to radar limitations or display set up. The list goes on and on. Each of those and other uses beyond simply producing RA's make it one of the best tools along with (E)GPWS mandated in the past couple of decades. Remember as you mention Uberlingen that the error of not complying with the TCAS was what allowed the accident to happen. As far as I'm concerned, the pilots have more shoulder more blame than controller as they had the tool in hand to avoid the crash and they are the ones ultimately responsible for the safe operation of the aircraft, not ATC.
How your assertion can be quantified, I have no idea. What I can tell you is TCAS has saved my ass a few times. Would it have been a hit, not sure but close enough that TCAS and not a controller separated me from the other player. Not a gig against ATC, separating me from a VFR guy isn't their highest priority in high workload situations. (am I correct with a shade of gray for the type of airspace?) It's my priority however and TCAS likely saved the day.
You can't simply look at TCAS and say that the numbers justify it or not simply based off some empirical data that reflects a singular parameter. The things I use it for day in and day out make it worth its weight in gold. Am I catching the guy ahead on the visual? What's that VFR guy doing over there when landing at an uncontrolled airport or after the tower closes? Is the heavy I'm following a bit high over that fix that I'll cross in a minutes time on profile? Perhaps I should seat the flight attendants as such or adjust my profile.
I'll overlay the TCAS on the WX radar and find gaps in what otherwise looks like a solid line that I didn't see due to radar limitations or display set up. The list goes on and on. Each of those and other uses beyond simply producing RA's make it one of the best tools along with (E)GPWS mandated in the past couple of decades. Remember as you mention Uberlingen that the error of not complying with the TCAS was what allowed the accident to happen. As far as I'm concerned, the pilots have more shoulder more blame than controller as they had the tool in hand to avoid the crash and they are the ones ultimately responsible for the safe operation of the aircraft, not ATC.
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I may be mistaken but seem to remember that since the option of TCAS, in case of such a conflict, avoidance instructions are to be issued IN THE HORIZOTAL PLANE ONLY precisely to avoid such confusion.
Seems like common sense to me anyway.
Seems like common sense to me anyway.
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ATC rule for air conflic
I think the TCAS's RA is based on the aircraft position / flight direction.
For the aircraft flying West to East or v.v, the TCAS will give Ascending command. And for the aircraft flying North to South or v.v, the TCAS will give Descending command.
To avoid a conflict between the TCAS'RA and the ATC command, I think the ATC-man should has a Rule , which should be the same as the TCAS's rule.
It is, when a conflict seen on the Radar display, the ATC man should order the East-West flying aircraft to climb, the North-South flying aircraft to descend.
It would be good also to tell the pilots, as a Pilot's rule, that for pilot flying East-West should fly ascending, and for the pilot flying North-South to fly descending, in case of a flight conflick.
By applying the same Rule , I hope there will be no bad news again in the future.
For the aircraft flying West to East or v.v, the TCAS will give Ascending command. And for the aircraft flying North to South or v.v, the TCAS will give Descending command.
To avoid a conflict between the TCAS'RA and the ATC command, I think the ATC-man should has a Rule , which should be the same as the TCAS's rule.
It is, when a conflict seen on the Radar display, the ATC man should order the East-West flying aircraft to climb, the North-South flying aircraft to descend.
It would be good also to tell the pilots, as a Pilot's rule, that for pilot flying East-West should fly ascending, and for the pilot flying North-South to fly descending, in case of a flight conflick.
By applying the same Rule , I hope there will be no bad news again in the future.
"To avoid a conflict between the TCAS'RA and the ATC command, I think the ATC-man should has a Rule , which should be the same as the TCAS's rule.
It is, when a conflict seen on the Radar display, the ATC man should order the East-West flying aircraft to climb, the North-South flying aircraft to descend."
No - what everyone should do is exactly what the rules say they should do, & the rules should be clear & unambiguous. Uberlingen defined that as a problem; sadly people died before it was properly understood that there were different interpretations around the planet. What does not need to happen now is people making it up as they go along - read the book, follow the rules.
It is, when a conflict seen on the Radar display, the ATC man should order the East-West flying aircraft to climb, the North-South flying aircraft to descend."
No - what everyone should do is exactly what the rules say they should do, & the rules should be clear & unambiguous. Uberlingen defined that as a problem; sadly people died before it was properly understood that there were different interpretations around the planet. What does not need to happen now is people making it up as they go along - read the book, follow the rules.
M.Jansen Utter tosh.
What alfaman said is correct.
The TCAS on the aircraft cares not one iota about flight direction; it will compute the best avoidance action based on projected flight paths.
When a controller becomes aware there is about to be a loss of separation (prior to TCAS becoming involved), the controller will take whatever steps required to put it right, and direction of flight according to the quadrantal level rules will have little to do with it.
What alfaman said is correct.
The TCAS on the aircraft cares not one iota about flight direction; it will compute the best avoidance action based on projected flight paths.
When a controller becomes aware there is about to be a loss of separation (prior to TCAS becoming involved), the controller will take whatever steps required to put it right, and direction of flight according to the quadrantal level rules will have little to do with it.
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I believe that originally it was thought that the controller should give horizontal avoiding action and then let TCAS deal with the vertical. But recently that has been changed (at least in the UK as far as I understand) so that the controller passes traffic but gives no instruction and lets the pilots sort out whatever they think is best. Presumably the thinking on this is two fold.. firstly giving a turn whilst climbing is going to reduce your climb rate and secondly having two instructions at once to follow could increase the chance of a mistake and worse happening? You end up with the Uberlingen problem of two voices screaming imperative instructions at you. You might only hear one of them.
From the NZ MATS:
I believe this is inline with the applicable ICAO standard/recommendation.
Basically, once an RA is reported, it's "hands off".
Bears a little thinking about, for in such a situation it is quite possible that the controller may have just become aware of a pending loss of separation, and be focused on preventing same (or worse).
Put yourself in the situation. You are rapidly communicating an avoiding action to one or both pilots, and extremely involved in the situation, and one of them says to you "TCAS climb" (or whatever the word is). At that point, watching the blips closing, you have to deliberately and immediately stop giving instructions, until it's reported as resolved.
The tendency to keep attempting your own resolution must be strong, for at least a second or three; the ability to instantly relinquish control is probably not well practiced, yet it is of high importance.
Having a notion in advance of such an event is, in my experience, more likely to make it easy to do the correct thing, when doing the correct thing runs counter to instinct.
(And just in case there are people reading new to this thread or the concepts within, controllers are aware of what TCAS is for, and absolutely do not consider it a personal safety net or backup plan.)
38.2 Traffic Advisory (TA)
If a pilot reports a TA the controoller shall advise the pilot of any measures that are already in place to ensure separation, if required, or reiterate the traffic information. If separation or traffic information has not been provided, the controller shall take immediate action to provide such.
38.3 Resolution Advisory (RA)
After being informed that an aircraft is responding to an RA, the controller shall pass essential traffic information or traffic information (as applicable) to the aircraft reporting the RA and to all other aircraft that may be affected by the manoeuvre. Control instructions shall not be issued to the aircraft responding to an RA or to any other aircraft involved with the RA event as this may interfere with the resolution action planned by the ACAS.
Once an aircraft has begun a manoeuvre in response to an RA, the controller is not responsible for providing separation between the aircraft that is responding to an RA and any other aircraft, airspace, terrain or obstructions.
If a pilot reports a TA the controoller shall advise the pilot of any measures that are already in place to ensure separation, if required, or reiterate the traffic information. If separation or traffic information has not been provided, the controller shall take immediate action to provide such.
38.3 Resolution Advisory (RA)
After being informed that an aircraft is responding to an RA, the controller shall pass essential traffic information or traffic information (as applicable) to the aircraft reporting the RA and to all other aircraft that may be affected by the manoeuvre. Control instructions shall not be issued to the aircraft responding to an RA or to any other aircraft involved with the RA event as this may interfere with the resolution action planned by the ACAS.
Once an aircraft has begun a manoeuvre in response to an RA, the controller is not responsible for providing separation between the aircraft that is responding to an RA and any other aircraft, airspace, terrain or obstructions.
Basically, once an RA is reported, it's "hands off".
Bears a little thinking about, for in such a situation it is quite possible that the controller may have just become aware of a pending loss of separation, and be focused on preventing same (or worse).
Put yourself in the situation. You are rapidly communicating an avoiding action to one or both pilots, and extremely involved in the situation, and one of them says to you "TCAS climb" (or whatever the word is). At that point, watching the blips closing, you have to deliberately and immediately stop giving instructions, until it's reported as resolved.
The tendency to keep attempting your own resolution must be strong, for at least a second or three; the ability to instantly relinquish control is probably not well practiced, yet it is of high importance.
Having a notion in advance of such an event is, in my experience, more likely to make it easy to do the correct thing, when doing the correct thing runs counter to instinct.
(And just in case there are people reading new to this thread or the concepts within, controllers are aware of what TCAS is for, and absolutely do not consider it a personal safety net or backup plan.)
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Are we still required to provide traffic information? ICAO 4444 has removed the requirement to pass traffic. I found this from Eurocontrol.
Eurocontrol on TCAS RAs
Eurocontrol on TCAS RAs
PROVISION OF TRAFFIC INFORMATION
Until now, controllers were required to provide traffic information to the aircraft responding to the RA. This requirement has been removed as it is believed that at that point ATC traffic information provides little added value to the flight crew and might be distracting. Moreover, traffic information may be inadvertently inaccurate as the position and altitude information are delayed in surveillance processing. In some case controllers found issuing traffic information difficult due to the proximity or overlap of the aircraft labels and symbols on the screen. Also, traffic information and the visual acquisition of the intruder could prompt the pilots to stop responding to the RA. The amended paragraph 15.7.3.2 reads as follows:
“When a pilot reports an ACAS resolution advisory (RA), the controller shall not attempt to modify the aircraft flight path until the pilot reports “Clear of Conflict”.
Until now, controllers were required to provide traffic information to the aircraft responding to the RA. This requirement has been removed as it is believed that at that point ATC traffic information provides little added value to the flight crew and might be distracting. Moreover, traffic information may be inadvertently inaccurate as the position and altitude information are delayed in surveillance processing. In some case controllers found issuing traffic information difficult due to the proximity or overlap of the aircraft labels and symbols on the screen. Also, traffic information and the visual acquisition of the intruder could prompt the pilots to stop responding to the RA. The amended paragraph 15.7.3.2 reads as follows:
“When a pilot reports an ACAS resolution advisory (RA), the controller shall not attempt to modify the aircraft flight path until the pilot reports “Clear of Conflict”.