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TCAS saves the day again - or not...

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TCAS saves the day again - or not...

Old 21st Dec 2003, 04:35
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TCAS saves the day again - or not...

First there was the JAL incident, then Uberlingen. Now one on our own patch. From the ATSB:

"FACTUAL INFORMATION

An infringement of separation standards occurred 70 NM east of Darwin, NT, between a descending Boeing 737-376 (737) and an Embraer EMB-120 (Brasilia) that was maintaining level flight. The event took place during the hours of darkness and in visual meteorological conditions. The crew of the 737 intentionally flew the aircraft through its assigned level in response to a traffic alert and collision avoidance system (TCAS) warning. The Brisbane sector controller also received a short-term conflict alert (STCA) between the two aircraft from the Australian Advanced Air Traffic System (TAAATS). The STCA alerted controllers when the radar trajectories of two aircraft indicated that separation standards might be infringed. The 737 and the Brasilia passed within 1.6 NM horizontally and 600 ft vertically. The required separation standard was either 3 NM or at least 1,000 ft.


...



The 737 crew were on their fourth consecutive day of duty and completing the last sector of a four-sector day when the incident occurred. It took place at the transfer of control point between Brisbane Centre and Darwin Approach. The 737 was en-route from Cairns to Darwin and was on descent to FL220. The Brasilia was enroute to Groote Island, under the control of Brisbane Centre, with instructions to maintain flight level (FL) 210. Those routes placed the two aircraft on almost reciprocal tracks. As the 737 was passing FL235, the crew were instructed by the Brisbane sector controller to contact Darwin approach control for further descent, but were not advised of the opposite direction traffic.

...

Shortly after acknowledging the instruction to change frequency, the 737 crew received an aural `traffic, traffic’ warning and a display indication of an aircraft 5 NM ahead. The pilot in command stated that the TA quickly changed to a RA with a `descend, descend, descend’ aural alert. As the aircraft was approaching its assigned level he disconnected the autopilot and pitched the aircraft nose down with the intention of following the RA commands. He stated that the required rate of descent shown on the IVSI was 1,200-1,500 ft/min. On passing FL220 the TCAS command abruptly reversed to a climb RA (aural `climb, climb now’) which was followed positively. The climb annunciation continued until the aircraft was at FL225. No more commands were issued and there was no TCAS `clear of conflict’, which is normally generated once a RA is removed.

Analysis of recorded data indicated that as the 737 descended through FL230, its rate of descent was approximately 2,900 ft/min. At FL227, the automatic flight system commenced a transition manoeuvre to achieve level flight at FL220. At approximately FL225 the autopilot was disengaged and the descent was continued manually at a rate of descent in excess of 3,200 ft/min to FL215. That was followed by a climb to FL225 at 2,900 ft/min as the pilot responded to the RA reversal (`climb now’ advisory).

Maintenance files from the TCAS computer were examined and no indication of TCAS failure was found. Technical expertise was requested from the manufacturer of the TCAS equipment. Their evaluation of the event presented two possible scenarios.

`Explanation 1. The reported `descend’ advisory was actually a `reduce descent’ advisory that was misunderstood by the crew. A `reduce descent’ would be consistent with the expected TCAS response per the reported geometry of the aircraft. Because the advisory was misinterpreted, the rate of descent was increased rather than decreased until the aircraft was below the TCAS required 700 feet vertical separation. Thus the TCAS was required to issue a `climb now’ advisory. The lack of a `clear of conflict’ annunciation is explained in the following paragraph.

`Explanation 2. There is a possibility that the intruder aircraft’s (equipped with mode C transponder) altitude report was not correctly received by the TCAS. There have been instances when a Mode C reply will not contain all the appropriate pulses in the message or it transmits pulses that are too narrow for the TCAS to detect. This could cause differing altitude reports and could result in multiple unstable tracks at different altitudes for the same intruder aircraft. This being the case, the TCAS could have issued a `descend’ advisory for the intruder because it appeared (due to erroneous altitude report) that it was actually above its own aircraft. If subsequent replies had the correct altitude, the erroneous track would be dropped by the TCAS and the TCAS would issue a `climb now’ advisory on the track with the correct altitude.

`The reason that `clear of conflict’ was not annunciated can be attributed to low track firmness of the intruder aircraft. Mode C equipped aircraft are typically only equipped with a single antenna mounted on the lower hull. Since the intruder aircraft was below the 737 aircraft, it is likely that the TCAS was not able to receive regular replies at close proximity. The TCAS computer unit will `coast’ the track of a previously established intruder file if it does not receive a valid or reasonable interrogation response. If the track of the intruder that generated the RA is coasted during the time of the associated RA, then the `clear of conflict’ is not announced.’

...

ANALYSIS

As the 737 descended towards FL220, the crew was faced with the apparently conflicting demands of an ATC clearance and a TCAS resolution advisory. Given that the 737 was above the Brasilia, it would be normal for the initial TCAS advisory to have been a `reduce descent’ or a climb advisory. Although no evidence of a TCAS or transponder malfunction was found, the investigation could not exclude the possibility of an equipment failure contributing to this incident.

It is possible that the crew may have misidentified the TCAS aural warning. Prompt action was required to resolve the apparent ambiguity and the crew may have been guided more by the aural warning than by the IVSI display. That may have been, at least in part, due to the limitations of the IVSI display, where a pilot may initially rely more on the aural alert. Compared with a TCAS IVSI display, traffic information that is displayed on an EFIS screen increases the crew’s situational awareness. However, pilots are trained to use all the information at their disposal and an aural alert would be the trigger to look at the IVSI display immediately. Therefore if the green band of the IVSI was indicating a required rate of descent of 1200-1500 ft/min, then the correct procedure would be to disengage the autopilot and smoothly adjust the pitch to attain that rate of descent.

The probability that the crew of the 737 would receive a TCAS RA on the Brasilia could have been reduced had the Brisbane sector controller provided some indication to the crew of the 737 that there was another aircraft restricting further descent. That would have enabled the crew of the 737 to adjust their rate of descent in lieu of possibly maintaining a level, and would have provided additional information that the crew could have then used to improve their situational awareness and optimise their decision making."


So, despite plenty of genuine saves it is slowly beginning to sink in that TCAS is not infallible either. It can make a bad situation rotten or, as in this instance, make a fully controlled situation go pear-shaped.

Good thing we aren't relying on it for collision avoidance in the NAS airspace!
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Old 21st Dec 2003, 05:53
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Correction - it was NOT during the hours of darkness and also - the aircraft were on different frequencies.

Similar to the two metro's on day 2 of NAS 2B, in the same area, on different freqs, but no TCAS - however WTF - they were just Metros!!!!!

Common thread? trend? I say again, if we change the reporting requirements and the specification of what constitutes a safety incident or dangerous act - how can we possibly gain any statistics and safety analysis (to objectively determine if this new system really is safe enough) and subsequently trend analyse and provide corrective action where required?
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Old 21st Dec 2003, 12:43
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Important points (imho);
-TCAS is good, but has it's own set of problems
-TCAS is not infallible
-pilots can over-react to alerts, and cause more problems (see point 1).
-controllers need to be aware that they are not fully in control any more, and the pilot's must have every available piece of information in order to keep up SA. ie pass traffic lots, in order to prevent "control" being wrested from you.
-the idea that TCAS can be used as a primary seperation tool, as under NAS class E, is dangerous and irresponsible.
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Old 21st Dec 2003, 15:23
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The Reverend
 
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the idea that TCAS can be used as a primary seperation tool, as under NAS class E, is dangerous and irresponsible
Ferris, I hope I'm not sitting behind you if you ever get a TCAS RA in conflict with an ATC instruction.
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Old 21st Dec 2003, 16:22
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Hotdog.
I have been in that situation. What are you getting at? I think you have misunderstood my point.
I don't believe TCAS can be used as a primary seperation tool. If you think otherwise...........
TCAS RA in conflict with an ATC instruction.
That situation still hasn't been sorted out. There has been no change in Russian thinking, and threads on PPRuNe have indicated many, many pilots are operating under conflicting instructions as to what to do TCAS vs ATC (latest was the Japanese one? pilots charged with not following ATC?). So, whilst you may follow the RA, how do you know what the other guy is doing? A totally unacceptable situation. The TCAS version about to be released has enhanced resolution logic which will not ask the aircraft to cross levels unless absolutely necessary, something the current versions pay no heed to. I have long thought this a serious flaw. As I said, TCAS has a raft of issues, so using it as a primary seperation tool in classE is dangerous and irresponsible. NASites will tell you that see and avoid is the primary seperation device, but in practice that isn't the case between jets and lighties (ie. "the 421 pilot reported at no time did he sight the B737").
What I was really trying to say, in relation to the opening post,
So, despite plenty of genuine saves it is slowly beginning to sink in that TCAS is not infallible either. It can make a bad situation rotten or, as in this instance, make a fully controlled situation go pear-shaped.
Good thing we aren't relying on it for collision avoidance in the NAS airspace!
TCAS has produced a subtle change, an extra empowerment if you will, to the cockpit. A pilot is no longer blindly following instructions, and has a partial situation display in TCAS. This isn't going to change, and will only grow as technology grows. ATCs need to help that "partial" SA with as much info as they can. Otherwise you will keep getting situations , as described in the topic, where the controller "loses control" (the pilot follows an RA) where there was no danger. Further, the implied reliance that is going on in E is a disgrace. The rules have leapt ahead of technology. Eventually ADSB might provide what is being sought for NAS, but TCAS doesn't.
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Old 21st Dec 2003, 16:39
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Hey Ferris, it also means that by having TCAS the pilots can complain about their slot in the sequence and why it is that with 80NM to run AUH and 8 miles behind with closing why he isnt No.1. Ah you gotta love SpeedBird pilots! cheers

P.S Dubais great
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Old 21st Dec 2003, 18:00
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The Reverend
 
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Ferris, I agree that the present situation of differing SOPs with regard to TCAS RAs with different operators is fraught with danger but the sooner a unanimous agrement can be hammered out, the safer we will be by following TCAS RAs. Ueberlingen being the latest example and a personal experience, many years ago, of being cleared to 10,000ft Onjuku VOR on approach to Haneda, only to have our windscreen filled with the view of the under belly of a JAL 747 departure cleared to Onjuku at the same level on a different frequency. That was before the days of TCAS.

To quote from TCAS Pilot's Handbook:

Pilots are authorized to deviate from their current ATC clearance to the extent necessary to comply with the TCAS Resolution Advisory (RA)
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Old 21st Dec 2003, 20:19
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Given the scenario reported above, I rather think that an Explanation 3 is possible and seems to fit all the facts:

Due to the high rate of descent and distance between the a/c when the TCAS Resolution Advisory was issued, the TCAS had calculated that the best manouevre was for the 737 was to continue descent to avoid conflict. Coincidentally, the altitude capture mode was entered at this time (F227), which required a reduced rate of descent to level at F220. This was dutifully followed by the autopilot. Immediately following the autopilots reduction in rate of descent, the 737 pilot disengaged the autopilot (F225) in accordance with TCAS procedures and attempted to follow the “descend, descend, descend” order. However, because the rate of descent had been reduced by the autopilot following the alt capture mode the TCAS now recalculated that the new best manouevre to avoid conflict would be a reversal, “climb, climb now”.

I surmise that:

1. What the pilots reported was accurate.
2. TCAS did initially issued a descend RA order.
3. The rate of descent was reduced by the autopilot during level off, before the pilot had a chance to respond to the RA.
4. The pilot disconnected the autopilot and attempted to follow the descend RA.
5. Because of the reduced rate of descent, the TCAS recalculated and issued a climb reversal RA, Climb, Climb Now”.
6. There was no failure of the TCAS equipment.

The initial RA given by TCAS is a known problem when climbing or descending at high rates close to level off with traffic occupying an adjacent level. We usually accommodate this by reducing the climb/descent rate approaching level off if there is known traffic. Known traffic may be seen on the TCAS display (if the signal is not blocked) or by advice from ATC. Just my two bob’s worth.
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Old 21st Dec 2003, 22:04
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Wonder why the CVR didn't provide the factual answer to whether the RA was a "descend descend" or "reduce rate of descent" aural?

If they were approaching FL220, they'd have been about 70 miles to run, and 20 mins or so from block-in. For a 30 min CVR record, I guess it may have just been overwritten by the time they shutdown.

My company recommends (mandates?) the CB manually be tripped to preserve the CVR record in the event of an incident.

Any other companies recommend this?
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Old 22nd Dec 2003, 02:05
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FlexRes

You hit the nail right on the head.

There should always be a pilot induced response to an RA.

The auto pilot is in a greater percentage of times engaged at the time of the RA warning and the initial response by the pilot should be to disengage the AP and smoothly and quickly respond to the RA.

ATC instructions must be disregarded untill the RA warning is extinguished and ATC must be informed of the RA.

Some airline operators do have in their SOPs a clause which allows pilots to ignore RA warnings if they have visual sighting of the intruder target.

A wise pilot would still continue to follow the RA instructions of the TCAS in an area of high traffic density just to ensure that they have the correct target visually.

Remember, SOPs are written by human beings. NO HUMAN BEING IS INFALLIABLE!!!

The TCAS is a fantastic tool only as long as all aircraft have transponders operating. There are still areas of NZ airspace (those which do not have SSR coverage) which are Non-Transponder-Mandatory.

This, in my opinion, is a flaw in the NZ airspace management system, which needs to be addressed urgently, especially as most Air Transport aircraft are required to have TCAS by 2005.

Pilots still need to use the Mark 1 eyeball for midair collision avoidance.

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Old 22nd Dec 2003, 05:23
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Exclamation TCAS Infallible?

Regarding the Uberlingen incident, Bert Ruitenberg (human factors expert) recently presented a report on that incident at a conference. An interesting comment on TCAS (not verbatim):

There is a known software "glitch" in TCAS II Version 7 (the current iteration of the software) where a "mode reversal" (climb, then descend) can ultimately confuse TCAS to the extent where it no longer helps to resolve the conflict, but only makes it worse. This software glitch has been known to the developers of TCAS for many years, but they (probably the bean conters) decided some time ago that additional development of TCAS to resolve the problem was not warranted given the remote chances of its occurrence.
Remember, SOPs are written by human beings. NO HUMAN BEING IS INFALLIABLE!!!
So is TCAS.

I don't know whether this is related at all to the Darwin incident, but it was the last line of defence at Uberlingen ... and it failed.
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Old 22nd Dec 2003, 07:21
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The Reverend
 
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It didn't fail at Ueberlingen, DHL followed the TCAS RA but the Russian plane did not but followed ATC instruction instead. Had the Russians followed the TCAS RA, this collision would have been avoided.
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Old 22nd Dec 2003, 09:12
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HotDog

You beat me to the punch. If all had done as the TCAS had instructed and not followed ATC instructions, both aircraft and crew would be here today.
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Old 22nd Dec 2003, 09:16
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Uberlingen

I was going to leave that alone in the last post, but since you brought it up...
It didn't fail at Ueberlingen, DHL followed the TCAS RA but the Russian plane did not but followed ATC instruction instead. Had the Russians followed the TCAS RA, this collision would have been avoided.
Yes, the Russians followed ATC rather than their RA. However, TCAS re-calculates the appropriate manouevre during the RA sequence. In this case, it had given the DHL an instruction to descend. The crew subsequently followed their RA (as per SOP).

However, TCAS would have detected that the other aircraft was also descending, and this provides precisely the scenario above where the known software fault provided the "hole" (so to speak) for it to become confused when a mode reversal (i.e. a climb) should have been initiated during the initial descent RA.

Of course in hindsight a blanket "if everyone had followed their respective RAs then all would be well" sounds well and good but TCAS is supposed to re-calculate during the manouevre and a situation was encountered where that fault presented itself.
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Old 22nd Dec 2003, 17:57
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There has been a small amount of unhelpful and misleading comment on this thread regarding the reliability of TCAS as a one and only last chance of avoiding mid-air collision. For the benefit of those who have not yet been introduced to the system, to avoid you being mislead, may I say a few words?

We train using TCAS in the simulator. We can generate single and multiple targets to try and confuse the TCAS. We can even make it command manoeuvres to “thread the needle” for traffic both above and below. TCAS works; it has already prevented many close and possible midairs in our airline, and I speak from personal experience.

The principle of TCAS is very simple. If we were out flying in the same airspace, our transponders chat to each other if we are not too far apart vertically and laterally.

If you approach my safety-bubble, my TCAS will compare our Altitudes and Separation Distance and Closing Speeds with your TCAS. In you come inside my safety-bubble our TCAS’s will agree on a coordinated manouevre for each of us pilots to perform. That will be something like, for example, “If you tell your pilot to climb, I will tell my pilot to descend”. The TCAS commanded manouevre would only use the vertical plane due to limitations of antenna lateral resolution.

TCAS Resolution Advisory relies only and absolutely on the pilots to follow the manouevres commanded.

For the benefit of all professional pilots reading this, if you come into my safety bubble, please do what your TCAS commands and I hereby pledge that I will also do you the honour of precisely obeying what my TCAS commands.

If you don’t have a TCAS, please squawk Mode C and that will allow me to see you. And my TCAS and I will guarantee that we won’t hit you.
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Old 22nd Dec 2003, 20:09
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The Reverend
 
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Blastoid, there is obviously no point in arguing with you on this subject as you will not concede that your opinion is faulty. However rest assured, that the majority of operators will rely on and follow TCAS RAs to avoid potential midair collisions regardless of ATC errors like Ueberlingen or pilot errors. You don't have to stray too far from home, just ask any Qantas pilot who regularly operates in Calcutta FIR for instance.
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Old 22nd Dec 2003, 20:20
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Blastoid

You just answered your own question. If both had followed their RA, there wouldn’t have been a collision. The TCAS in the DHL aircraft recalculated the command because the Russian aircraft didn’t follow the command that its TCAS had given it and been agreed to by the two TCAS systems. If everyone followed his or her TCAS RA’s to the letter this type of scenario wouldn’t happen. I just hope that I’m not flying in the same airspace as you if god for bid I got an RA because I now know you would probably cause an accident that could otherwise be avoided.
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Old 23rd Dec 2003, 02:07
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Unhealthy faith

Why are you jumping on Blastoid for questioning your faith in TCAS? TCAS a great idea, and having it is certainly better than not having it, but I got the impression that this thread was started as a warning about using it for primary seperation (which appears to be the intention of a lot of the classE nonsense going on right now)?
Some of you seem to show an amazing lack of logic when you claim that you'll be safe as long as you follow the RA. How do you know what the intruder is doing? Is he following his RA or ATC, or something else? Search PPRuNe and find the threads where lots of different pilots give lots of different opinions about following RAs over ATC. One of the dissenters was a Speedbird pilot. You certainly don't need to be in the Calcutta FIR to come unstuck!
Blastoid, there is obviously no point in arguing with you on this subject as you will not concede that your opinion is faulty
Hmmm. Wonder how many that could apply to?
I just hope that I’m not flying in the same airspace as you
That's right, all you can do is hope. You will never know what the intruder will do with version 7 kit.
transponders chat to each other
That depends on the transponders.
my TCAS will compare our Altitudes
That's if the intruder is squawking alt (note- see DJ737 vs C421).
If you don’t have a TCAS, please squawk Mode C and that will allow me to see you
Two assumptions there. 1. The transponder works and 2. The info it provides is accurate. Under NAS, a VFR in classE with an inaccurate transponder could get cleaned up because you did respond to an RA.

TCAS is not cut and dried. Your own words say so...
the majority of operators will rely on and follow TCAS
the majority is not good enough. Ueberlingen proved that. Using TCAS as the major seperation tool in NAS class E is dangerous. TCAS is good, yet fallible, especially when GA transponders are involved. It is good as a backup, but it's implied use under NAS is a disgrace.
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Old 23rd Dec 2003, 04:52
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Hotdog
However rest assured, that the majority of operators will rely on and follow TCAS RAs to avoid potential midair collisions regardless of ATC errors like Ueberlingen or pilot errors. You don't have to stray too far from home, just ask any Qantas pilot who regularly operates in Calcutta FIR for instance.
Until recently, I was under the same impression. What surprised me was the following extract from the ATSB report into the Virgin/C421 incident:
Because the crew of the 737 had the C421 in sight, they chose to maintain visual separation rather than follow the TCAS RA. That action was in accordance with company policy.
I had believed that all operators had a policy of ‘follow the RA, regardless’. Is this a recent change and does it affect Qantas also? Is it only in Class E?

I also have a question about the following:
The 737 crew had identified the C421 on their TCAS display and saw it during this period.
What methods do pilots use to identify another aircraft on TCAS? In the Virgin incident, there were potentially two aircraft within relatively close proximity – a C421 and a BE20. (In the actual incident it appears that the BE20 was sufficiently far away that no confusion could have occurred.)

Surely it is not a great leap to suggest that a pilot looking for a C421 2,500FT below might see a BE20 3,000FT below and believe they have the right aircraft? (Even worse if the two aircraft are same type.)

Controllers use various methods to positively identify aircraft prior to any reliance on radar derived information. Is there any procedure to ensure that pilots have identified the correct aircraft (both visually and on TCAS) prior to ignoring a TCAS RA?
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Old 23rd Dec 2003, 13:00
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Woah there!

Hotdog and 404 Titan,

Go ahead, shoot me down. I was not trying to explain the midair at Uberlingen (which, if you have read any reports into the disaster, was the culmination of a lot of unfortunate circumstances - but as the Reason model will show us these things will eventually line up.)

Merely I was trying to present anecdotal evidence of known situations where TCAS software can fail. Clearly as ones so reliant on it for day to day separation, you do not want to believe that there could be anything wrong with the software. However, as you say, you are solely reliant on your TCAS RA and the other pilot (i) following their TCAS RA or (ii) using their transponder with (correct) mode C. I guess when it comes down to it you can't fly the intruder aircraft as well as your own

As for
Blastoid, there is obviously no point in arguing with you on this subject as you will not concede that your opinion is faulty
see above but I was not blaming Uberlingen on a TCAS fault.

I just hope that I’m not flying in the same airspace as you if god for bid I got an RA because I now know you would probably cause an accident that could otherwise be avoided
Did I say I wouldn't follow an RA? Where do you find the evidence to make a comment like that?

DirectAnywhere,
the fact is we shouldn't HAVE to rely on TCAS in Class E but guess what(?) it's the best we've got
Damn right, how TCAS got designed into the airspace model is a disgrace. Don't tell me "see and avoid" isn't infallible? If you're not happy with the new system, tell someone about it.
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