T-cas Descent
Thread Starter
Join Date: Aug 2004
Location: malaysia
Posts: 404
Likes: 0
Received 0 Likes
on
0 Posts
T-cas Descent
I just finished my night shift and I have a query and hope someone will explain what exactly happened.
A Singapore Airlines B777 was maintaining FL260. A MAS B747 was crossing right over the SIA777 descending to FL270. At the point of crossing, the B747 was passing FL277. At this moment the SIA777 reported T-CAS DESCENT. The B747 reported a RA.
How could this be when both acft are vertically separated?
This is the first time I have encountered such a scenario.
Need some explanation.
A Singapore Airlines B777 was maintaining FL260. A MAS B747 was crossing right over the SIA777 descending to FL270. At the point of crossing, the B747 was passing FL277. At this moment the SIA777 reported T-CAS DESCENT. The B747 reported a RA.
How could this be when both acft are vertically separated?
This is the first time I have encountered such a scenario.
Need some explanation.
Join Date: Jul 2001
Location: Fort Worth ARTCC ZFW
Posts: 1,155
Likes: 0
Received 0 Likes
on
0 Posts
howdy Veloo;
It's quite simple, you had an aircraft coming down and you and the pilot knew was going to stop 1000 feet above the other aircraft. The TCAS however doesn't know this and if a certain rate of descent is observed, the TCAS will cry foul and give an RA and then tell the aircraft to descend. We see it here more often than I care to think.
regards
Scott
It's quite simple, you had an aircraft coming down and you and the pilot knew was going to stop 1000 feet above the other aircraft. The TCAS however doesn't know this and if a certain rate of descent is observed, the TCAS will cry foul and give an RA and then tell the aircraft to descend. We see it here more often than I care to think.
regards
Scott
Join Date: Aug 2002
Location: Southampton
Posts: 97
Likes: 0
Received 0 Likes
on
0 Posts
Veloo
As Scott rightly points out this sort of occurence happens all the time. When first introduced these were called "nuisance" events - however, you never know when TCAS may save your bacon!
An interesting fact about your incident is that it appears an RA was issued prior to a loss of standard vertical separation. There is a lot of literature out there on what actually happens during a TCAS incident and it requires an amazing amount knowledge amongst all parties to be truly effective.
Suffice it to say that your MAS aircraft was probably descending at a fairly high rate - this led to the on board system on the SIA aircraft getting "nervous" and an RA being issued.
If TCAS was operating exactly as we would like then the following would have happened;
*As the MAS was approaching its cleared level the aircraft's descent rate was reduced.
*The TCAS system on the SIA would "spot" the approaching confliction and a TA would be provided to both crews.
*As vertical separation was still reducing, but at a shallow rate, (even still in excess of 1,000ft) the MAS would be given a "Monitor vertical speed" Resolution.
*This would require the crew to follow a vertical profile that the systems had decided upon to maintain separation.
*As the approaching angles were, in effect, shallowed out no drastic RA action would be needed.
It's not an ideal world though is it guys!
One thing you can do to possibly help in a situation like this in the future though would be to provide Traffic Information to both aircraft (time permitting) to help begin the above.
I've used things like;
"ABC123 maintain FLxxx on reaching there is traffic crossing left to right 1,000ft below in 90 seconds 10 o'clock range x miles, caution possible TCAS alert"
a pre-emptive strike (in this situation) never hurt anyone!
As Scott rightly points out this sort of occurence happens all the time. When first introduced these were called "nuisance" events - however, you never know when TCAS may save your bacon!
An interesting fact about your incident is that it appears an RA was issued prior to a loss of standard vertical separation. There is a lot of literature out there on what actually happens during a TCAS incident and it requires an amazing amount knowledge amongst all parties to be truly effective.
Suffice it to say that your MAS aircraft was probably descending at a fairly high rate - this led to the on board system on the SIA aircraft getting "nervous" and an RA being issued.
If TCAS was operating exactly as we would like then the following would have happened;
*As the MAS was approaching its cleared level the aircraft's descent rate was reduced.
*The TCAS system on the SIA would "spot" the approaching confliction and a TA would be provided to both crews.
*As vertical separation was still reducing, but at a shallow rate, (even still in excess of 1,000ft) the MAS would be given a "Monitor vertical speed" Resolution.
*This would require the crew to follow a vertical profile that the systems had decided upon to maintain separation.
*As the approaching angles were, in effect, shallowed out no drastic RA action would be needed.
It's not an ideal world though is it guys!
One thing you can do to possibly help in a situation like this in the future though would be to provide Traffic Information to both aircraft (time permitting) to help begin the above.
I've used things like;
"ABC123 maintain FLxxx on reaching there is traffic crossing left to right 1,000ft below in 90 seconds 10 o'clock range x miles, caution possible TCAS alert"
a pre-emptive strike (in this situation) never hurt anyone!
Join Date: Mar 2001
Location: Karlsruhe, Germany
Posts: 12
Likes: 0
Received 0 Likes
on
0 Posts
The magic number is 1.500!
If the vertical speed of acft approaching each other is more than 1.500 fpm this will generate a TCAS alert. If both are in a vertical movement this number will count for the one and the other cumulative. i.e. if both make about 800 fpm it will trigger an alert as well as one doing 1.500 or more.
Companies like Lufthansa have an inbuilt FMS-programme that automatically reduces the rate during the last thousand feet of climb/descent.
In your case it seems the pilot is not very experienced referring to TCAS. We controllers are much more familiar with TCAS-alerts than pilots. They tend to overreact once they get a TA/RA.
What you can do is what Scott said.
Troublemakers are especially overpowered sportscars like B736. They give you 3 or 4000 fpm unless you´d need it!
If the vertical speed of acft approaching each other is more than 1.500 fpm this will generate a TCAS alert. If both are in a vertical movement this number will count for the one and the other cumulative. i.e. if both make about 800 fpm it will trigger an alert as well as one doing 1.500 or more.
Companies like Lufthansa have an inbuilt FMS-programme that automatically reduces the rate during the last thousand feet of climb/descent.
In your case it seems the pilot is not very experienced referring to TCAS. We controllers are much more familiar with TCAS-alerts than pilots. They tend to overreact once they get a TA/RA.
What you can do is what Scott said.
Troublemakers are especially overpowered sportscars like B736. They give you 3 or 4000 fpm unless you´d need it!
Join Date: Oct 2000
Location: Berkshire, UK
Age: 79
Posts: 8,268
Likes: 0
Received 0 Likes
on
0 Posts
You saw something, the like of which I saw enough times to make me wonder why pilots have so much trust in TCAS!! I had hoped that in the (nearly) two years since I retired things would have improved... apparently not.
Join Date: Sep 2001
Location: 24 27 45.66N 54 22 42.28E
Posts: 987
Likes: 0
Received 0 Likes
on
0 Posts
Veloo, I have to say I am surprised that you as a controller in this day and age, haven't been trained or advised on this occurence, as it is very common. In both Oz and here in the Middle East it is good practice (if not a regulatory requirement) to advise any traffic in this situation of the other traffic and vice versa. If you are climbing or descending an aircraft to one thousand feet within another, if you have time, it is good practice to give both pilots a heads up to the fact the other aircraft is levelling off with a thousand feet separation. In the case of a high rate of climb/descent ie. MD11, 767 or A320 I think it should be a requirement to advise both pilots of the siuation, so as to avoid an unnecessary RA maneouvre that could possiblly create a real RA situation.
TCAS is probably one of the best improvements in aviation safety in the last couple of decades. When you get used to flying with one you hate to go back. Your lot may not be keen on it, but its wildly popular with those on the other end of the microphone.
Ohcirrej
Join Date: Feb 2003
Location: This is the internet FFS.........
Posts: 2,921
Likes: 0
Received 0 Likes
on
0 Posts
Your lot may not be keen on it
Last edited by Jerricho; 25th Aug 2004 at 21:58.
Join Date: Nov 1999
Location: EIDW
Posts: 176
Likes: 0
Received 0 Likes
on
0 Posts
You saw something, the like of which I saw enough times to make me wonder why pilots have so much trust in TCAS!! I had hoped that in the (nearly) two years since I retired things would have improved... apparently not.
We all know the TCAS RA is unneccesary 99 out of a 100 cases. It's that one case out of a hundred where is saves us from a close encounter or worse, that causes us to follow all RAs. Wouldn't you do the same if you were in our place?
Join Date: Jul 2001
Location: Fort Worth ARTCC ZFW
Posts: 1,155
Likes: 0
Received 0 Likes
on
0 Posts
Exactly...
I too am getting tired of telling an aircraft to turn for traffic and I need it NOW! and then have the pilot come back and say yeah he is no factor we have him on TCAS. Well YEAH the one that you are looking at may not be a factor but someone else is and I need for you to turn NOW and not blow me off. That is happening more and more these days. Not to mention some back seat driving by thinking that they know what the traffic situation is by watching the "fish finder." Hate to tell all the tin drivers out there, but TCAS does NOT show you the big picture by any means, nor does it show you all the aircraft.
rant off
Scott
I too am getting tired of telling an aircraft to turn for traffic and I need it NOW! and then have the pilot come back and say yeah he is no factor we have him on TCAS. Well YEAH the one that you are looking at may not be a factor but someone else is and I need for you to turn NOW and not blow me off. That is happening more and more these days. Not to mention some back seat driving by thinking that they know what the traffic situation is by watching the "fish finder." Hate to tell all the tin drivers out there, but TCAS does NOT show you the big picture by any means, nor does it show you all the aircraft.
rant off
Scott
Moderator
Join Date: Jul 1997
Location: Europe
Posts: 3,228
Likes: 0
Received 0 Likes
on
0 Posts
TCAS is a great thing when used for what it was intended, namely 'Collision Avoidance'. No controller nor pilot should believe that it is infallible, but as a last gasp life saver the odds of it providing a resolution are pretty high and certainly better than leaving everything to fate. Indeed it can be argued that is even preferable to instructions from a controller whose radar picture is usually a little bit on the back side of the drag curve.
As Scott says, for use as an onboard radar display or to provide accurate situational awareness, it's useless in its current iteration.
As Scott says, for use as an onboard radar display or to provide accurate situational awareness, it's useless in its current iteration.
I fly the airways a few hundred days a year. The type of situations in which Jerricho and Scott mention are few and do not represent the vast majority of us who understand the importance of controllers. To bitch about the occasional pilot who bucks the system is no worse than us bitching about the odd controller out there that tries to fly the aircraft for us. They are out there, but seldom seen and of no real relevance to the discussion about the importance of TCAS. If however its therapeutic, have at it.
As Phoenix said it, its that 1 in a 100 theory. What if that guy you believe is going to level off 1000 ft above doesn't? What if he/she has some sort of vertical mode selected that doesn't allow alt capture? Dialed in the wrong altitude in to the alerter...what if....what if.
Many companies here in the US and I'm sure in the UK have safety programs that allow pilots to submit reports on screw ups. The product of this is a database that one can read through and try to garner some degree of insight in to the chain that caused the infraction to begin with. Suffice it to say the folder with altitude busts is the thickest.
As Phoenix said it, its that 1 in a 100 theory. What if that guy you believe is going to level off 1000 ft above doesn't? What if he/she has some sort of vertical mode selected that doesn't allow alt capture? Dialed in the wrong altitude in to the alerter...what if....what if.
Many companies here in the US and I'm sure in the UK have safety programs that allow pilots to submit reports on screw ups. The product of this is a database that one can read through and try to garner some degree of insight in to the chain that caused the infraction to begin with. Suffice it to say the folder with altitude busts is the thickest.
Ohcirrej
Join Date: Feb 2003
Location: This is the internet FFS.........
Posts: 2,921
Likes: 0
Received 0 Likes
on
0 Posts
Westy you have to understand that while it isn't the norm, these occurences are beginning to become more and more prevalent, especially in busy traffic situations in a Terminal area when you really don't need it. The mere fact that 3 controllers have mentioned it here must indicate it isn't exactly a "seldom" seen situation as you perceive it to be.
It wasn't a cheap shot, it wasn't a sweeping statement for scoring points, nor for therapy. Just a statement of something that SOME of us have experienced which is directly attributable to the system not being used for what it is intended for. So yes, it should be included in the discussion.
(How did I know you were going to react this way?)
It wasn't a cheap shot, it wasn't a sweeping statement for scoring points, nor for therapy. Just a statement of something that SOME of us have experienced which is directly attributable to the system not being used for what it is intended for. So yes, it should be included in the discussion.
(How did I know you were going to react this way?)
(how did I know you would have to finish with a less than endearing statement)
Back to the point. TCAS is a good tool and in the situations controllers see as a nuisance I will respond to the RA not only because I'm required but because its the best course of action.
As to using the TCAS to second guess ATC, a very unwise choice.
It does however allow my SA to shoot way up in relation to traffic in my immediate vicinity. As an example before requesting higher or lower I range out the TCAS to see if there are any obvious confliction's that would preclude. Does that mean I have the big picture? No, I think that's the answer you want to hear from me.
If questioning ATC by relatively small number of pilots is a by product of TCAS (and I'm sure it is) then it will simply have to be accepted as is more than offset the positives TCAS offers. It is an invaluable tool with uses far beyond simple collision avoidance.
Back to the point. TCAS is a good tool and in the situations controllers see as a nuisance I will respond to the RA not only because I'm required but because its the best course of action.
As to using the TCAS to second guess ATC, a very unwise choice.
It does however allow my SA to shoot way up in relation to traffic in my immediate vicinity. As an example before requesting higher or lower I range out the TCAS to see if there are any obvious confliction's that would preclude. Does that mean I have the big picture? No, I think that's the answer you want to hear from me.
If questioning ATC by relatively small number of pilots is a by product of TCAS (and I'm sure it is) then it will simply have to be accepted as is more than offset the positives TCAS offers. It is an invaluable tool with uses far beyond simple collision avoidance.
Join Date: Apr 2002
Location: Belgium
Posts: 138
Likes: 0
Received 0 Likes
on
0 Posts
West Coast
I can see where you're coming from, but your point of view is (quite naturally) from a pilot's perspective. Let me expand
Ok, so you see some traffic that you think could have an effect on your request....would that stop you making the request? If so it really shouldn't, for all you know what you percieve on TCAS to be a problem is traffic that we are planning to descend for whatever reason through your level, and your request for higher would help us out. Also, the request would give us time to plan for you, even if the traffic you see is indeed going to prevent you from getting your clearance immediately.
This is a point that controllers and pilots are always and forever going to disagree on. In my relatively short career in ATC I have had more losses of separation caused by TCAS RA's than anything else. Sure I will never have a midair caused by TCAS, and thats the bottom line (quite understandably) for you guys flying the big jets. But to watch a perfectly safe situation, with traffic info issued, turn into a loss of separation caused by a 'nuisance' RA is something thats becoming frustratingly more commonplace. My point is that following an RA is a requirement but not always the best course of action, unless we are happy to accept losses of separation as being "ok". Separation standards are paramount to us, whereas to pilots it seems to be the school of thought that as long as you don't bump into anybody its all fine. I would imagine I would feel the same in your shoes, but we have standards to maintain, and to have those standards taken away beyond our control makes our job very uncomfortable at times.
I know all the arguments, I think at the end of the day TCAS is a necessary tool, I just can't accept a RA induced incident as being a good thing.
I can see where you're coming from, but your point of view is (quite naturally) from a pilot's perspective. Let me expand
As an example before requesting higher or lower I range out the TCAS to see if there are any obvious confliction's that would preclude.
I will respond to the RA not only because I'm required but because its the best course of action.
I know all the arguments, I think at the end of the day TCAS is a necessary tool, I just can't accept a RA induced incident as being a good thing.
Join Date: Mar 2004
Location: Europe
Posts: 9
Likes: 0
Received 0 Likes
on
0 Posts
We all know the TCAS RA is unneccesary 99 out of a 100 cases.
And how did you determine this 99% rate of failure?
If you have a solid statistical (not anecdotal) data to support your statments, please share them with us.
Ohcirrej
Join Date: Feb 2003
Location: This is the internet FFS.........
Posts: 2,921
Likes: 0
Received 0 Likes
on
0 Posts
It is an invaluable tool with uses far beyond simple collision avoidance.
I believe (and I'm relying on my poor memory here) there was a trial a couple of years ago over the Atlantic where TCAS was being considered for climb through "separation" or something like that, but I didn't hear any more about it.
And to pick up a point that 4-3-3 has spoken about reference a request for level change and "obvious conflictions", the azimuth of TCAS is no where as accurate as the radar we use (hence the reason RAs are only given in the vertical), and what may be perceived as a conflict on the flight deck may not be at all (adhering to our separation standards). Or perhaps a cheeky little vector could be used. On the other side, I have had pilots request further climb/descent whe they are about to get a face full of opposite direction traffic.
If questioning ATC by relatively small number of pilots is a by product of TCAS (and I'm sure it is) then it will simply have to be accepted as is more than offset the positives TCAS offers
Last edited by Jerricho; 26th Aug 2004 at 12:56.
Join Date: Nov 1999
Location: EIDW
Posts: 176
Likes: 0
Received 0 Likes
on
0 Posts
Oh really? Who is "we"?
And how did you determine this 99% rate of failure?
If you have a solid statistical (not anecdotal) data to support your statments, please share them with us.
And how did you determine this 99% rate of failure?
If you have a solid statistical (not anecdotal) data to support your statments, please share them with us.
By the way, I never mentioned the word 'failure.' This situation is about a limitation, not a failure. The system does exactly what it's designed to do, exactly to avoid the nasty results of the other a/c not leveling off, for reasons already explained by Westy, where it's supposed to.
433
It comes down to definition. What you see as a nuisance I see as a potential threat. A simple failure to input a proper mode on the AP, an altitude entered in error and what you see as a nuisance now becomes a potential midair. Just because you properly called traffic and issued clearances that would separate us doesn't mean I or the other guy got it right. We are humans capable of error despite your best efforts. I am willing to accept the risk of flying surrounded by fallible humans because not every mistake, blown clearance means disaster. If however I am bore sighted from above or below and one of us screws the pooch it likely does make CNN and my wife gets a lot of money.
Jerricho
I'll speak to your last point and then leave it alone as it detracts from a potentially good thread. I feel safe saying every controller has had a run in with someone over time. I remember myself the controllers that have been a pain in the ass trying to fly the plane for me more than the ones that have provided excellent service. Just the way it is. If you believe that some greater degree of pilots are questioning you based on TCAS, then we will agree to disagree. You may find it unacceptable but you are going to go away before TCAS does so learn to accept it. Enough said. Now on to the valid questions you raise.
As to me using TCAS for level changes, etc.
If I am sitting at 33000 wanting 37000 and I see someone 12 o'clock and 20 miles at 35000 opposite direction I understand that to be a confliction. The azimuth is not as accurate as your radar but I can still discern somewhat accurately direction of flight. Especially so as the range narrows. If I am willing to accept a vector off course, I'll ask now, if not I'll simply wait or put a different level on request.
As to other uses.
I watch the TCAS as I approach lines of thunderstorms. As the range closes I watch the routes other aircraft are taking through the stuff. I can overlay the wx radar on top of the TCAS to get a fairly accurate picture of the route the test ship is taking. Depending on exact location, altitude, gains used and tilt a cell make look different to others than what I'm looking at. What I see as a marginal route may be seen as a preferred one by others. I use the TCAS to help determine this.
ATC assigned separation aside we still have wake turbulence encounters. I use the TCAS to determine altitude of the heavy ahead. Is he joining from above, is he already descending on the GS, etc. I have had a number of wake encounter in to LAX. Heavies coming from the North downwind often join from above. This requires monitoring by the succeeding crew to avoid his wake if they are on a straight in approach on GS. 5/6 miles may be your appropriate Sep (or whatever it is) that however does little if the guy is a thousand feet higher. If nothing else I can sit the flight attendants down to avoid injury if I suspect an encounter.
Going in to airports in high numbers of VFR, such as Santa Ana (SNA) Burbank (BUR) it is an invaluable tool. Not sure of your exact duties as written but down here controllers separate IFR from IFR, I guess it goes down from there to additional duties as able. BUR as an example has a routing that has us down in the weeds in GA territory for quite awhile. The controller is balls to the walls working IFR aircraft and often we don't receive traffic on VFR aircraft. I suppose this is a form of separation but not one thought of much by controllers. Hitting a VFR Cessna is going to hurt just as much as a much larger aircraft.
Maintaining spacing on visual approaches is another. Every aircraft and pilot is a bit different and minus a speed adjustment is going to fly the plane a bit different. I may be happy at 190 to the marker while the guy ahead throws the anchor out at 10 miles and slows to 150. I use a five mile display with range marks at 5 and 2 1/2 miles and I can determine if I am closing. Conversely I also have a limited ability to look behind me to determine if someone is crawling up my ass and can adjust my speed.
These are the obvious ones that come to mind, I'm sure there are other secondary and tertiary uses of the metal detector, fish finder of whatever you like to call it
It comes down to definition. What you see as a nuisance I see as a potential threat. A simple failure to input a proper mode on the AP, an altitude entered in error and what you see as a nuisance now becomes a potential midair. Just because you properly called traffic and issued clearances that would separate us doesn't mean I or the other guy got it right. We are humans capable of error despite your best efforts. I am willing to accept the risk of flying surrounded by fallible humans because not every mistake, blown clearance means disaster. If however I am bore sighted from above or below and one of us screws the pooch it likely does make CNN and my wife gets a lot of money.
Jerricho
I'll speak to your last point and then leave it alone as it detracts from a potentially good thread. I feel safe saying every controller has had a run in with someone over time. I remember myself the controllers that have been a pain in the ass trying to fly the plane for me more than the ones that have provided excellent service. Just the way it is. If you believe that some greater degree of pilots are questioning you based on TCAS, then we will agree to disagree. You may find it unacceptable but you are going to go away before TCAS does so learn to accept it. Enough said. Now on to the valid questions you raise.
As to me using TCAS for level changes, etc.
If I am sitting at 33000 wanting 37000 and I see someone 12 o'clock and 20 miles at 35000 opposite direction I understand that to be a confliction. The azimuth is not as accurate as your radar but I can still discern somewhat accurately direction of flight. Especially so as the range narrows. If I am willing to accept a vector off course, I'll ask now, if not I'll simply wait or put a different level on request.
As to other uses.
I watch the TCAS as I approach lines of thunderstorms. As the range closes I watch the routes other aircraft are taking through the stuff. I can overlay the wx radar on top of the TCAS to get a fairly accurate picture of the route the test ship is taking. Depending on exact location, altitude, gains used and tilt a cell make look different to others than what I'm looking at. What I see as a marginal route may be seen as a preferred one by others. I use the TCAS to help determine this.
ATC assigned separation aside we still have wake turbulence encounters. I use the TCAS to determine altitude of the heavy ahead. Is he joining from above, is he already descending on the GS, etc. I have had a number of wake encounter in to LAX. Heavies coming from the North downwind often join from above. This requires monitoring by the succeeding crew to avoid his wake if they are on a straight in approach on GS. 5/6 miles may be your appropriate Sep (or whatever it is) that however does little if the guy is a thousand feet higher. If nothing else I can sit the flight attendants down to avoid injury if I suspect an encounter.
Going in to airports in high numbers of VFR, such as Santa Ana (SNA) Burbank (BUR) it is an invaluable tool. Not sure of your exact duties as written but down here controllers separate IFR from IFR, I guess it goes down from there to additional duties as able. BUR as an example has a routing that has us down in the weeds in GA territory for quite awhile. The controller is balls to the walls working IFR aircraft and often we don't receive traffic on VFR aircraft. I suppose this is a form of separation but not one thought of much by controllers. Hitting a VFR Cessna is going to hurt just as much as a much larger aircraft.
Maintaining spacing on visual approaches is another. Every aircraft and pilot is a bit different and minus a speed adjustment is going to fly the plane a bit different. I may be happy at 190 to the marker while the guy ahead throws the anchor out at 10 miles and slows to 150. I use a five mile display with range marks at 5 and 2 1/2 miles and I can determine if I am closing. Conversely I also have a limited ability to look behind me to determine if someone is crawling up my ass and can adjust my speed.
These are the obvious ones that come to mind, I'm sure there are other secondary and tertiary uses of the metal detector, fish finder of whatever you like to call it