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ATC and TCAS ????

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ATC and TCAS ????

Old 2nd Jul 2004, 04:14
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ATC and TCAS ????

I find it hard to enjoy Canada day (July 1) like I used to.

Two years ago today, a good friend of mine and an excellent pilot passed on. He was the co-pilot aboard the DHL 757 that was lost when they were hit by a russian airliner over Germany.

During the course of the investigation (and from comments on this board) it came out that different parts of the world had different procedures when it came to TCAS and ATC instructions. I stopped trying to follow this shortly afterwards. All I want to know is: Did my friend die in vein? Or has some worldwide accepted procedure come about from this tragedy?
Old 2nd Jul 2004, 05:19
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A TCAS last resort instruction was always accepted as an overriding factor in collision avoidance. Unfortunately some indidviduals chose to ignore this discipline. However the tragic death of your friend has reinforced the rules so he did not die in vain.
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Old 2nd Jul 2004, 06:48
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No, your friend did not die in vain. The world wide aviation community was made very aware of TCAS and following its commands. Training was reinforced at most carriers…simulator scenarios modified to incorporate TCAS scenarios. And numerous articles in industry and in house publications on TCAS.

Following is a piece I prepared for our in-house publication shortly after the event.

The German Midair – Lessons to be Learned

By now most of us have seen the press and industry accounts of the tragic midair collision between a DHL 757F and a Bashkirian Airlines Tupolev 154 over southern Germany, killing all 71 aboard both aircraft. Like many accidents, it resulted after a chain of events, any link of which, if broken, would have prevented the accident. And the scenario leading up to the collision was eerily similar to an event in the skies over Japan in January of 2001.

As of this writing, this much is known:

- Both aircraft were operating at the same Flight Level (FL360) in European RSVM airspace (1000’ vertical separation). Both aircraft were equipped appropriately for participation in Europe’s RSVM airspace ( CADC’s, servoed altimeters, etc., and TCAS (version 7.0 – see sidebar)).

- Both aircraft had been handed off to “Skyguide”, the Swiss ATC agency controlling southern Germany.

- Just prior to entering Skyguides airspace, the “conflict resolution” software function of the enroute radar was turned off for routine maintenance. Existing Skyguide regulations call for two controllers to be monitoring airspace under these circumstances, and increased lateral separation. One of the two controllers on duty was on break during the accident sequence.

- Parts of the main telephone system at Skyguide were also down for maintenance at the time of the accident sequence.

- The one controller on duty was working an enroute sector and an approach sector at the time of the accident. There were problems related to the phone system in contacting the tower of the approach sector.

- An adjoining sector (Karlsruhe ATCC) was warned of the impending loss of separation by its conflict alert system. When it observed no action being taken, they attempted, with no avail, to contact Skyguide (phone problems).

- Both aircraft received RA’s (TCAS Resolution Advisory) as designed – The Russian aircraft was instructed by TCAS to Climb, the DHL to descend. The DHL aircraft started its descent and transmitted so to ATC. Almost simultaneous with the initial TCAS issued RA to climb, the Russian aircraft was instructed by ATC to descend. There was a repeated ATC instruction, and the Russian aircraft descended to comply with the ATC instruction, overriding the TCAS RA command to climb (however, the latest information showed the Russian aircraft climbing 100 feet prior to the descent – possibly indicting that the aircraft initially responded to the RA, and then chose to descend on ATC command)..

- Unfortunately, the lateral geometry was perfect for the collision (more on this later).

Break any of the events in that chain, and the accident would have not occurred. Or, had the lessons of the Japanese extremely near miss the previous year been widely disseminated and trained for, this event may not have occurred.

The Japanese Near Miss

On January 31, 2001, JAL Flight 907, a Boeing 747, had departed Tokyo-Haneda with a destination of Naha. JAL Flight 958, a DC-10, was en route from Pusan to Tokyo-Narita. A trainee controller cleared flight 907 to climb to FL390. Two minutes later, JAL958 reported at FL370. Both flights were on an intersecting course near the Yaizu NDB (near Suruga Bay). The controller noticed the imminent conflict, but instead of ordering Flight 958 (DC-10) to descend, he mistakenly ordered Flight 907 (747) to descend. Immediately after this instruction, the crew of the flight 907 (747) were given a aural TCAS Resolution Advisory to climb in order to avoid a collision. At the same time, the crew of Flight 958 (DC-10) were given an aural TCAS advisory to descend. The captain of flight 907 (747) followed the instruction of the air traffic controller to descend instead of his TCAS RA to climb. A collision was averted when the pilot of the 747 put his aircraft into a steep descent (fortunately there was visual contact with other aircraft). The 747 missed the DC-10 by 105 to 165 meters in lateral distance, and 20 to 60 meters in altitude difference. About 100 crew and passengers aboard the 747 sustained injuries due the emergency maneuver.

PROBABLE CAUSE: The Aircraft and Railway Accident Investigation Commission concluded that the air traffic controllers error and the pilots decision to follow air traffic control instructions instead of the TCAS Resolution Advisory (RA) were the two main causes.

Lessons to be learned

The major lesson to be learned from these events is:

If your TCAS issues a RA, Air Traffic Control has already failed. At this point, the TCAS system is infinitely more valuable than ATC’s instructions. Most enroute radars have update rates between 6 and 10 seconds – the information your TCAS is providing is much more timely and, by design, training, and regulation, takes precedence over ATC commands.

Both the Russians and the Japanese, in their respective accident/incident, believed that ATC commands had priority over TCAS commands and were apparently trained accordingly. My guess it that this will be a favorite training scenario with many airlines around the world in the very near future.


According to Honeywell, for TCAS to do its job properly and insure separation, an RA must be complied with within 5.0 seconds and a G loading (+/-) .25 G’s. If an RA command is reversed (version 7.0), compliance requires a response within 2.5 seconds and a G loading of (+/-) .35 G’s . The autopilot should be turned off to accomplish an RA maneuver.

Some other considerations when using TCAS:

- Today’s avionics suites provide such precise lateral navigation, that in some ways it may contribute to the possibility of a near miss/midair with lack of vertical separation. Some routes/procedures may allow for offsets.

- Always communicate as soon as possible to ATC that you are climbing/descending in response to a Resolution Advisory. Future plans are for Mode S/ADSB transmission to ATC of an RA and it’s command, but for now, ATC has no idea why you are climbing/descending in response to a TCAS command.

- Do not try to use TCAS for lateral separation maneuvers. TCAS was designed
for vertical separation maneuvers only. Limits on current generation TCAS
antennae’s and processors allow for approximate azimuth information (may be
up to 30 degree error in azimuth at close ranges).

- While TCAS version 7.0 eliminates many of the false warnings associated with earlier versions, it is still recommended, particularly in 1000’ separation RSVM airspace, to have a reduced rate of climb/descent for the last 1000 feet of one’s climb/descent to assigned altitude, to prevent a possible overshoot and possible issuance of a RA (version 7.0 adds a slight time delay to avoid false warnings).

- Unfortunately, our current TCAS installations (747,757/767, most 727’s) have limited
range display capabilities. While the software is looking out to the maximum range of TCAS design, the crews cannot see a conflict developing as they could on a longer range display. The 757/767 fleet is due to be upgraded shortly with a longer range display.

- The first response to a “traffic, traffic” TCAS TA (Traffic Advisory) warning should be an attempt to visually identify the other aircraft. Keep in mind that azimuth information displayed on your TCAS display may be up to 30 degrees off.

- As reported in a previous article on runway incursions, there is a documented TCAS “save” by using TCAS information in a way not intended by the original designers. After the runway collision in Linate, Italy between a Cessna Citation and an SAS MD-87 in low visibility, the tower controller, unaware of the collision on the runway, issued a takeoff clearance to a Lufthansa jet. The Lufthansa crew did not observe the just departed SAS aircraft on its TCAS, and queried the controller as to its whereabouts. It subsequently refused takeoff clearance, almost certainly saving another collision (wreckage was on the runway).

As always, the Aircraft Operating Manual is the primary source for operating procedures for the TCAS unit and procedures installed on your aircraft. Currently, the procedures outlined in the 757/767 A.O.M. are:

TA’s – PF observes TCAS display and begins visual search. PNF calls out range, bearing and altitude of the intruder and joins visual search.

RA’s – PF clears airspace when conducting avoidance maneuver and alters vertical speed to move VSI needle out of the red and into the green “fly to” zone. PNF continues visual search, monitors intruder position on TCAS and verifies that PF is correctly following the RA. PF expeditiously returns to assigned flight path when clear of conflict (note: to prevent additional TA/RA from not being at assigned altitude).

Be careful out there.
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Old 2nd Jul 2004, 07:03
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A TCAS last resort instruction was always accepted as an overriding factor in collision avoidance. Unfortunately some indidviduals chose to ignore this discipline
Hotdog - I wonder if you have read the whole accident report, particularly with reference to the conflicting SOPs for the Tupolev crew regarding TCAS warnings - as Skyhawk comments? One side of those SOPs dictated that ATC instructions were over-riding. The statement, I feel, (mistakenly, I'm sure) puts a possible slur on the crew of the Tupolev which would not be entirely right?

Commiserations to Skyhawk1, and if I may add to 'Shore Guy's' post, I feel that the answer might be more accurately phrased along the lines of:

"It was apparent that some parts of the world had confusing instructions regarding TCAS. As a result of this tragic accident, many lessons have been learned both on the ground and in aviation SOPs. I understand that these areas of confusion have now been eliminated"

For Skyhawk1, although it always sounds so trite - no, he did not die in vain and things have changed, if that is any consolation for you and the others who lost.
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Old 2nd Jul 2004, 07:17
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<<If your TCAS issues a RA, Air Traffic Control has already failed.>>

As a retired Heathrow radar controller I take exception to this statement and I am sure that many of my ATC colleagues world-wide will agree that it is flawed.

I have on a number of occasions seen serious incidents caused by TCAS when aircraft started off properly separated. Example: DC-10 was cleared to 4000 ft and pilot announced "TCAS, descending". The only traffic within several miles of that aeroplane was one climbing to 3000 ft underneath. TCAS, not knowing that the outbound was going to stop at 3000 ft and the inbound at 4000 ft had decided that the best action was to get the DC10 down fast - the wrong decision because what was a 100% safe situation developed into a loss of separation.

I have also experienced TCAS RAs causing loss of separation in holding patterns - where an inbound a/c joining a hold is descending rapidly... and also with outbound a/c climbing up rapidly under a holding pattern. Until there is data exchange between the ground and the TCAS such incidents will continue.

TCAS is a most useful tool which has undoubtedly saved lives, but please don't ever in your wildest dreams believe that every RA is caused by an ATC (or aircrew) foul up.

Last edited by HEATHROW DIRECTOR; 2nd Jul 2004 at 07:35.
Old 2nd Jul 2004, 07:29
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In our company we`ve had a few occasions, involving older Russian aircraft, where following an RA would have caused loss of separation.
Apparently, some older Russian type of altimeters (Gilham-encoded altimeters) give erroneous altitude information to the transponders and therefore the intruder appears to be at an altitude different from the real one.
The cases mentioned occured over China and Mongolia.

Thank god it was VMC at the time.
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Old 2nd Jul 2004, 07:48
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I do agree with most of your post. I still will follow RA guidance in the situation you give, even if previously told of the impending traffic. What if in the crews haste to set up the hold they neglect to fly the plane and blow through their assigned altitude? I was inbound to KSLC last year and was told of a Lear that was to level a thousand feet above me. That was the controllers plan. however the Lear blew through his altitude causing a RA on our part. ATC cannot fly the plane, only fallible humans can despite whatever clearance they are given. That said don't be mad if you see a RA response to what you think is a resolved situation.

I guess now that your retired you don't have to worry about that anymore. Only what beer to drink and what time to get up
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Old 2nd Jul 2004, 08:15
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Before the days of TCAS, we were cleared to descend to 10,000ft , Onjuku VOR inbound to Haneda in a L1011. Unbeknown to us, a JAL 747 was also cleared to Onjuku 10,000ft outbound. We were IFR in cloud when we reported 10,000 Onjuku VOR when the JAL 747 cut across our bows so close that we saw the white underbelly and main gear doors of the aircraft.
Heathrow Director, I appreciate your professional pride and you may say this could not have happened to you but it did happen to us, albeit in Japan. ( I am not insinuating that Japanese ATC is unprofessional by the way).

West Coast , right on buddy!
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Old 2nd Jul 2004, 11:07
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Two years after mid air collision there is no ICAO stipulated procedure for "automatic" notifications of RAs. Leaving those notifications at the expense of many circumstances, as happened in this tragedy.
It will be advisable to establish a procedure that unerringly notifies to ATCOs the fact that tCAS is issuing RA command to the crew.The quickest way would be to send a predetermined sqwack on the transponder.
Any delay on RA notification could have fatal consecuences.
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Old 2nd Jul 2004, 11:19
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A lesson has undoubtedly been learned, but no way did it need all those deaths to be applied. Should never have happened if the relative priority of ATC and TCAS RAs had been clearly stated by ICAO, not buried in "training guidelines". And what makes it worse is that the JAL incident occurred in Jan 01, with the Japanese authorities drawing the attention of ICAO to the incident and asking for guidance. ICAO responded guess when, August 02, one month after Ueberlingen.

The BFU accident report (available in English on their web-site) doesn't pull any punches in this regard. It identifies several systemic causes of the disaster, and top of their list was:

"The integration of ACAS/TCAS II into the system aviation was insufficient and did not correspond in all points with the system philosophy.
The regulations concerning ACAS/TCAS published by ICAO and as a result the regulations of national aviation authorities, operations and procedural instructions of the TCAS manufacturer and the operators were not standardised, incomplete and partially contradictory."
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Old 2nd Jul 2004, 11:37
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Thumbs down

Hot Dog, I'm unsure of the relevance of your last paragraph as a response to HD's post. Yes, in that instance you would have got a TCAS RA if you'd had a TCAS fitted, and yes, ATC had fouled up. These mistakes do happen, and all ATCOs are acutely aware that it could happen to them as easily as it could happen to anyone else. But I think what HD is trying to say is that many RAs (probably much more than 50% of them) are "nuisance advisories" and occur when ATC have provided the correct separation. In an en-route environment, this may merely be an minor annoyance, because it's quite unlikely that there'll be anything else in your way. In a terminal environment, a nuisance RA can quickly put an aircraft in much more danger than it was in before TCAS intervened.

As ATCOs we do our best to issue traffic information when situations occur where separation has been provided, but the profile of the traffic is such that your TCAS may become "interested", and issue a TA or RA. We DO expect you to respond to ALL RAs, but with any luck, early traffic information means that pilots will moderate their climb/descent profile accordingly, thus reducing the likelihood of a TCAS event.

The bald statement, <<If your TCAS issues a RA, Air Traffic Control has already failed.>>, isn't just flawed, it's untrue. Try opening a topic in ATC issues with that statement, and check the response you get.

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Old 2nd Jul 2004, 12:11
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I still will follow RA guidance in the situation you give, even if previously told of the impending traffic.
ANd that's exactly the right thing to do because as you said there could be a hundred reasons for this "resolved situation" in the controller's mind to turn into a loss of separation.

As an example, the latest RA I had was this situation where normally an aircraft was descending to level 110 and the other one (not on my freq) cleared to climb to FL100. The trouble was that the pilot ( american) was climbing to 10 000 feet on the QNH, which was pretty high that day:1028 or something. And as this plane was on another frequency I had absolutely no way to know or even have a clue of what was happening, so when I heard this "TCas Climb" I was surprised, maybe shocked ...but happy afterwards that the pilot followed his Tcas.

And however we have to remind that Tcas is not meant to provide separation (the way controllers see it, i mean like 3NM or 1000 ft) but to prevent collision. And the Uberlingen accident should have taugh that: whether you're pilot or a controller don't ever argue with Tcas !

I think that the problem of this Tcas awareness for controllers is linked to the very short term action of Tcas, which doesn't really leave time to the pilot to advise ATC of the RA. In fact when a pilot hears a resolution is merely 30 seconds away from collision, so definitely his priority is to take action and sorry to say that but not to advise ATC. That's where, as someone has already said, a special and automatic squawk could be useful but then, still you would depend on the speed of your radar...

One last thing, to those who have read the report from Uberlingen, one thing I noticed is the that the Tupolev crew is five . Can you imagine the troubles of CRM between five ? and it might have played a minor role in this... This said, I don't want to blame the crew for the tragedy or to disculp the controller, they've already paid a hard price for what happened.
May they all rest in peace.
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Old 2nd Jul 2004, 16:39
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Shore Guy :

Very good paper you wrote for your crews, small factual errors but the message getting out is the good one.
One error however I cannot let pass is your reference to the internet rumour about the Lufthansa refusing the clearance to take off on account of his TCAS … total bull..
There was no Lufhansa on the sequence in Linate at that time, the aircraft that was lined up and wait after the SAS took off was I-LUBI and after the TWR called the SAS 5 times (unsuccessfully ) , they ordered the I-BI to vacate the runway via R1 back to the apron. ( If you want to be sure, check Appendix P of the report , the transcript of the TWR freq. )

My dear Skyhawk

Sorry about your friend. Really.
Did he die in vein ?
No because , much more people are now aware of the interface ATC-ACAS problem . ICAO wrote additional text in November 2002 that has found its way this time in training manuals.
Will it never happen again then ? That is far from certain. There are still may traps in the TCAS logic and grey zones in the human interface.
The line is still, ATC stop to be responsible as soon as pilot report a n RA. If a pilot follows an RA but does not report it, controller may ( and will ) interfere. So Ueberlingen could happen again .

In our case here the TU154 crew did not report they were receiving an RA and the B757 crew did so 23 seconds after starting to manoeuvre ( a very, very long delay for ATC and especially in that case ).

The controller issued the clearance to the TU154 to descend prior the RA were started, and did not issue any different instructions after the RAs started.

Standard ATC – TCAS interface would you say.

So why a collision then ?

First , to all of you out there with an ATPL :
Consider the human factors aspects of the following :

The airline you fly for has all manuals ( translated ) in Russian . always had, (an ICAO language even ).
Your national AIP sates that a pilot must always follow an ATC instruction unless in emergency . ( like in many other AIPs as this is an ICAO recommendation )
ACAS RAs are not defined as emergency and the word RA ( Resolution Advisory ) is translated in verbatim : an advisory, not a command.
Then you fly at night, your are under supervision and your instructor is sitting besides you on the right seat. You FO is in the jump seat behind you. You set your TCAS display on max range and spot, well before the TA , an aircraft at same altitude .( you say : “ he is showing us zero “)
When the TA starts, it confirm what you saw all the time and you are not surprised.
When you later receive a clearance by ATC to descend , you are not surprised and comply
Your instructor even tells you : “ descend ! “ .
When an RA starts, you are surprised , your FO tells you : “it says “ climb “ but you are already descending. Your instructor is telling you : “ He is guiding us down “ but before you are finished thinking the controller repeat his clearance to descend, since you forgot to acknowledge the first clearance.
You continue to descend.
You still see the traffic ( despite the wrong indication by the controller of 2 o,clock instead of 10 ) and say : “here on the left ….” Then things go wrong , the 757 grow bigger and bigger of course you try to avoid visually, you dive further and turn, and deviate from both TCAS RA and ATC clearance , and you hit with your left wing the vertical stab of the 757. , missing it by about 2 meters…..

Now my friends, tell me how you think you will have reacted at that time if you were the TU154 captain flying that night ?
No benefit of hindsight, no new procedures, with what you knew about TCAS in July 2002 ?

From an ATC point of view, the situation in which the controller found himself that night is a total management failure. Alone with degraded radar system, main telephone out of service, wrongly programmed back up telephone, unscheduled inbound to a normally closed airfield at night forcing him to open a separate position with a different frequency , leaving him 2 frequencies and 2 radar pictures to monitor on 2 positions several meters apart. Short tem conflict alert disabled, etc…
He spotted the conflict late, issued an expedite descend clearance to the TU154 ( as it was anyway coordinated lower with adjacent centre) and when he saw the Mode C of the TU154 winding down , he considered the problem solved. What he did not know is that the 757 had already started his descent as the radar return rate on that degraded mode was very slow. The 757 FO was PNF ( doing the R/T) and had left the cockpit leaving the Capt alone at the time of the RA, so he did not warn ATC immediately. When F/O came back 23 seconds later, he informed ATC using a wrong call sign, but irrelevant since the transmission was blocked by the A320 inbound to FH on the other frequency. ( the controller had admitted he did not hear the 757 call ).
There you go.
Without knowing that any of the aircraft was following an RA , the ATC clearance issued was a solution that should have solved the conflict in the mind of the controller.
But even If both pilots had reported they were following an RA, the controller would have said nothing as ATC is supposed to stop issuing instructions ( and be responsible for anti collision )after that point .
Without ATC , no collision. without TCAS no collision, if they would have been IMC , probably no collision.
Small things , errors, or small deviations from procedures, insignificant if taken in isolation ,are causing a tragedy when put in a certain sequence. Mixing automated systems with humans decisions is not and never will be a good choice.

Are we so sure this will never happen again ? I doubt it.

TCAS is far from being perfect and will contribute mathematically to more collisions . ( I remember hearing from the MITRE corp in the US , the owners of the sofware, something like for every 30 collisions it saves , TCAS will cause one ) But each collision will highlight more problem areas and more people will learn from them and try to avoid them in the future.

For this your friend probably did not die for nothing .

A last wish :
If I had some power I would mandate the following :

ALWAYS FOLLOW THE RA and never ever manoeuvre against it ! is the sticker that should be printed and put above every TCAS display .

Last edited by ATC Watcher; 3rd Jul 2004 at 06:06.
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Old 2nd Jul 2004, 17:25
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"I think what HD is trying to say is that many RAs (probably much more than 50% of them) are "nuisance advisories" and occur when ATC have provided the correct separation"

I understand what your saying, but its only a nuisance RA if he does level where he was supposed to. If you issue appropriate clearances to separate and traffic calls I would hope the accident board completely clears you. The one I worry about is the other pilots most of the time, not the controllers doing their usual stellar job of separating aircraft. We as pilots are keenly aware of the envelopes that will set off a RA and tend to fly outside those parameters. That is, to avoid setting off his or mine as we come within the 25 second CPA. That said, all it takes in a bit of inattention in one aircraft, a screw up on the automatics (using a mode on the A/P that may not capture altitude) heads down reviewing an approach plate, etc., and your legal clearance in voided and I'm in a world of hurt.

We simply have to respond, whether you think we are separated of not. Its not a knock against you, You did your job. I simply must fly as if I have your family in the back because I do have someone's.
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Old 2nd Jul 2004, 18:23
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ATC Watcher,

Thanks for the complement and correction on the Linate/Lufthansa situation. I wrote the piece shortly after the accident, and at the time, there was no denial of the situation (in fact, I believe I picked up the story off PPRUNE).

Don’t believe everything on the net is the lesson.
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Old 2nd Jul 2004, 18:38
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It had been mentioned previously: If you get a T/A it would enhance situational awareness if you were to reduce the TCAS range immediately to 10 miles, and in the case of an R/A to have the non flying pilot further reduce the range to 5 miles or 3 miles. This would help you to also steer away (laterally) from the approaching target while climbing or decending as directed.
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Old 2nd Jul 2004, 22:45
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Glueball, I wouldn't trust the accuracy of the TCAS readout to use it to provide your own lateral separation.

From my side, the tragic events 2 years ago only re-inforced that what we are taught is 100% correct, and that following the RA as apposed to my instructions is what pilots should do. I will issue the vectors to avoid even further, but no reference to height will be mentioned.

To refer to Heathrow Directors post, I have also seen the consequences of when TCAS gets it wrong, that when vertical separation existed TCAS took over and eroded it. Even in these circumstances I would expect pilots to follow the TCAS call, and would not intervene with vertical instructions.

Skyhawk1, I can assure you that many lessons have been learnt following this accident. In the world of ATC its one of those events that is now rarely mentioned but is often in the mind, our thoughts always go out to those that were in the air as well as those that were on the ground and hope it will never happen again.
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Old 3rd Jul 2004, 04:44
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Glueball and all,

I would absolutely NOT try to use a lateral separation maneuver based on TCAS information only to avoid a collision. Laterally, TCAS is inherently sloppy – that is why there is no envisioned lateral escape maneuver based on TCAS only information (ADS-B, well that is another story for another day….you could do lateral escape on an ADS-B based system due to its inherent accuracy and update rate).

If you have the capability in your aircraft, bring the range down close and watch the TCAS targets “hop” laterally – that is why (but it is very accurate in range).

And, I’m wondering if some of the bad/erroneous TCAS RA’s referred to were events prior to TCAS 7.0 being implemented.
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Old 3rd Jul 2004, 06:36
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Absolutely correct Shore Guy. TCAS is very bad in azimuth, and I would never attempt to maneuvre lateraly on account of TCAS alone.
For instance , if intruder is dead ahead , slightly ( say 5 deg) on the right of your centreline on TCAS display, the aircraft can be indeed on the left by 5 or 6 degr... Azimuth tolerance on TCAS is 11 degrees. TCAS was never made for this.

I do not believe the number of false / erroneous RAs have not significantly changed with Version 7 even if the reports do get less. This is probably due to the fact , that since nothing hapens after reports are sent ( and nothing can currently can be done as the TCAS team has been disbanded by the FAA) peolple tend not to file reports anymore if it is not involving a safety issue .

The events that involved false mode C encodings and a/c picking up their own transponder have been solved has they were transponder problems not involving TCAS logic.

To echo HD, no an RA is definitively not only when ATC failed. In fact virtual level busts ( you intend to level off , but your vertical rate is that high that ACAS beleives you will not level off and trigger an RA ) are still , and by far, the most number of TCAS RAs.
Version 7 has reduced those numbers ( by delaying the trigger of the RA) but they are still there.
From a philosophical point of view, you would like to keep those RAs as , even if the intruder pilot intends to level off, you never can be 100% sure he will. And if you both follow the RA as intended, both a/c will only move 3 or 400 feet .
The only way to eliminate those is to reduce vertical rate before level off to 1000 to 1200 ft/min maxi.
Some Airlines ( BA is a good example ) already recommend this , but some others ( in the US especially ) refuse to apply this, arguing that TCAS is a tool, and therefore it should be modified, not having pilots adapting to a tool.

Both arguements are valid. TCAS is definitively not something easy , becauses it mixes crude blind automation with human (re)actions.
TCAS is not HAL ( from 2001 a space Odyssey for the youngsters ) but is the closest thing we have build yet.
ATC Watcher is offline  
Old 3rd Jul 2004, 07:18
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ATC Watcher and all,

I agree with everything you have said. TCAS is (excuse the expression) a bastardized system piggybacked on components not originally designed for use as collision avoidance systems. Assuming that, it works remarkably well.

A “clean sheet of paper” design, incorporating ADS-B or its equivalent and FMS interface will not occur during my career…..and right now, with airline economics being what they are would not be installed unless mandated.

One aspect that is seems would be relatively easy to design into existing FMS aircraft would be a reduced climb/descent rate in the last 1000 feet to prevent (false) RA’s. (Perhaps this is built into recently designed aircraft….I’m flying an early FMS design).

Another apology…this time to HD…..I agree – my statement that if you get an RA, ATC has failed was painted with a broad brush. Pilot error (altitude bust or lateral deviation) that triggers an RA is not the fault of ATC.
Shore Guy is offline  

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