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TCAS RA at Aircraft's Certified Ceiling

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TCAS RA at Aircraft's Certified Ceiling

Old 13th Oct 2008, 05:52
  #41 (permalink)  
 
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Certainly the challenge is out to everyone if they can show a TCAS unit coupled to Ground Prox and Radar Altimeters...
I believe I just did...having copied my information from the company Aircraft Operations Manual for our B747, that is.
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Old 13th Oct 2008, 06:29
  #42 (permalink)  
 
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Certainly the challenge is out to everyone if they can show a TCAS unit coupled to Ground Prox and Radar Altimeters...
Oh Dear, the following is from the 747-400 Maintenance Manual(my highlighting).

The dedicated components of a TCAS system consists of:
• A TCAS computer
• A top directional antenna
• A bottom antenna,
- which may be either omnidirectional or directional, depending
on the flight phase.

The TCAS system interfaces with:
• The ATC system;
- L and R ATC Mode S transponders,
- top and bottom ATC antennae and
- ATC/TCAS control panel.
• Integrated display system (IDS)
• Modularised avionics and warning electronics assembly
(MAWEA)

The system makes use of interfaces with:
• Left and Right Radio Altimeters
• Ground Proximity Warning Computer (GPWC)
• Landing gear module
• Air/ground relays
• Left and Right Distance Measuring Equipment (DME)
• L and R central maintenance computer (CMC’s)
• Data management unit (DMU)
The TCAS II computer is a receiver/transmitter unit which contains
the processors required to determine if the path of nearby airplanes
will intersect the flight path of the TCAS equipped airplanes.
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Old 13th Oct 2008, 10:30
  #43 (permalink)  
 
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The TCAS won't descend you into a mountain, EGPWS has priority. Yes, Pitbull, if you don't follow the RA, the coordinating aircraft will have a RA of greater magnitude, but following it is the correct response. The San Diego B727/C172 crash occurred when the pilot's thought they had there traffic "in sight" when the real conflict was not in their sight. If they had a TCAS II RA, decided the plane they had in sight was not really a conflict, they would have had the mid-air. Follow the RA.
I'll tell you what Galaxy. Why don't you stop putting 'head banging into wall' symbols and start actually reading posts.

The TCAS won't descend you into a mountain, EGPWS has priority.
Agreed. You should be safe against ground contact. With a caveat: Check your MEL. Can you dispatch with the E part of EGPWS inop?

Yes, Pitbull, if you don't follow the RA, the coordinating aircraft will have a RA of greater magnitude, but following it is the correct response.
Wrong.

Both aircraft will get their initial RA's. The only way aircraft B will get a strengthened RA is if aircraft A manoeuvres in the opposite sense. If aircraft A does not follow, or is slow to follow, or can not fully comply (e.g. makes +500 instead of +1750), then this will have no effect on the type of RA received by aircraft B.


The San Diego B727/C172 crash occurred when the pilot's thought they had there traffic "in sight" when the real conflict was not in their sight. If they had a TCAS II RA, decided the plane they had in sight was not really a conflict, they would have had the mid-air. Follow the RA.
I'm not sure if this is directed at my post, but it's in a continuous paragraph from where you refer to me by handle, so I presume it is.

Nowhere have I suggested that RA's should be ignored if they are considered to be unnecessary. On the contrary, I've been singing the "Think its not needed? So what, follow it anyway" tune ever since TCAS was mandated, even though the 'official' guidance didn't change to that until several years later.

The pilot has the ability to be aware of multiple hazards that TCAS simply can not analyse. i.e. anything that doesn't have an altitude encoding transponder. As such, it is possible that the act of following an RA may, in itself, be immediately hazardous. Do you dispute this? I know its a bit unpopular, but I believe that a crew might be required, from time to time, to excercise some judgement. But you know what? We could argue the toss on that one all day. The reality is that the perception of non TCAS risks depends on the type of operation. An air transport / IFR / controlled airspace / major airport pilot will have much less concern than someone who operates into places buzzing with non transponding traffic. So I suggest we just put that aside and just concentrate on the original posters issue; i.e. Performance.

The point I'm making is very simple, so I'll spell it out:

An RA may not be achievable.

Yes, TCAS has performance inhibitions. But you really need to appreciate that these are NOT comprehensive. I'd classify them as 'inhibiting the totally impossible' rather than 'guarunteeing the possible'. In addition to that, TCAS (probably: type dependant) doesn't know about engine failures / flight control malfunction. It doesn't know if you're covered in Ice.

Dispute it all you like, bottom line is that the pilots job has to be to protect the flight envelope, including if necessary not fully complying with an RA.

Don't want to take my word for it? Why not take a TCAS manufacturer's:

"The pilot must not exceed stick shaker or other stall warnings or protections when following an RA"

That's from a manufacturers manual, not just whatever sections your management pilot saw fit to put in your ops manual.

You may NOT be able to follow an RA. You MUST KNOW what to do if you can't.

pb

Last edited by Capt Pit Bull; 13th Oct 2008 at 10:31. Reason: finger trouble
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Old 13th Oct 2008, 18:18
  #44 (permalink)  
 
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Safety studies on TCAS estimate that the system improves safety in the airspace by a factor of between 3 and 5.However, it is well understood that part of the remaining risk is that TCAS may induce midair collisions: "In particular, it is dependent on the accuracy of the threat aircraft’s reported altitude and on the expectation that the threat aircraft will not make an abrupt maneuver that defeats the TCAS RA. The safety study also shows that TCAS II will induce some critical near midair collisions..." (See page 7 of Introduction to TCAS II Version 7 (PDF) in external links below).One potential problem with TCAS II is the possibility that a recommended avoidance maneuver might direct the flight crew to descend toward terrain below a safe altitude. (!!)Recent requirements for incorporation of ground proximity mitigate this risk. Ground proximity warning alerts have priority in the cockpit over TCAS alerts.Some pilots have been unsure how to act when their aircraft was requested to climb whilst flying at their maximum altitude. The accepted procedure is to follow the climb RA as best as possible, temporarily trading speed for height. The climb RA should quickly finish. In the event of a stall warning, the stall warning would take priority.[edit] TCAS LimitationsWhile the benefits of TCAS are undisputable, it can be assumed that TCAS' true technical and operational potential (and thus its possible benefits) is not yet being fully exploited because of the following limitations in current implementations:TCAS is limited to supporting only vertical separation advisories ATC isn't automatically informed about resolution advisories issued by TCAS-so that controllers may be unaware of TCAS-based resolution advisories or even issue conflicting instructions (unless ATC is explicitly informed about an issued RA during a high-workload situation), which may be a source of confusion for the affected crews In the above context, TCAS lacks automated facilities to enable pilots to easily report and acknowledge reception of a (mandatory) RA to ATC (and intention to comply with it), so that voice radio is currently the only option to do so, which however additionally increases pilot workload Today's TCAS displays do not provide information about resolution advisories issued to other (conflicting) aircraft, while resolution advisories issued to other aircraft may seem irrelevant to another aircraft, this information would enable and help crews to assess whether other aircraft (conflicting traffic) actually comply with RAs by comparing the actual rate of (altitude) change with the requested rate of change (which could be done automatically and visualized accordingly), thereby providing crucial realtime information for situational awareness during highly critical situations TCAS equipment today is often primarily range-based, as such it only displays the traffic situation within a configurable range of miles/feet, however under certain circumstances a "time-based" representation (i.e. within the next xx minutes) might be more intuitive. Lack of terrain/ground awareness information, which might be critical for creating feasible (non-dangerous, in the context of terrain clearance) and useful resolution advisories (i.e. prevent extreme descent instructions if close to terrain), to ensure that TCAS RAs never facilitate CFIT scenarios. Aircraft performance in general and current performance capabilities in particular (due to active aircraft configuration) are not taken into account during the negotiation and creation of resolution advisories, so that it is theoretically possible that resolution advisories are issued that demand climb or sink rates outside the normal/safe flight envelope of an aircraft during a certain phase of flight (i.e. due to the aircraft's current configuration), furthermore all traffic is being dealt with equally, there's basically no distinction taking place between different types of aircraft, neglecting the option of possibly exploiting aircraft-specific (performance) information to issue customized and optimized instructions for any given traffic conflict (i.e. by issuing climb instructions to those aircraft that can provide the best climb rates, while issuing descend instructions to aircraft providing comparatively better sink rates, thereby hopefully maximizing altitude change per time unit, that is separation) TCAS is primarily extrapolation-oriented, as such it is using algorithms trying to approximate 4D trajectory prediction, in order to assess and evaluate the current traffic situation within an aircraft's proximity, however the degree of data- reliability and usefulness could be significantly improved by enhancing said information with limited access to relevant flight plan information, as well as to relevant ATC instructions to get a more comprehensive picture of other traffic's (route) plans and intentions, so that flight path predictions would no longer be merely based on estimations but rather aircraft routing (FMS flight plan) and ATC instructions. For TCAS to work effectively, it needs to be fitted to all aircraft in a given airspace. However, TCAS is not fitted to many smaller aircraft mainly due to the high costs involved (between $25,000 and $150,000). Many smaller personal business jets for example, are currently not legally required to have TCAS installed, even though they fly in the same airspace as larger aircraft that are required to have proper TCAS equipment on board.
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Old 13th Oct 2008, 19:02
  #45 (permalink)  
 
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The concept of the paragraph has been working for centuries now, and while simple, is a brilliant tool for enhancing the understandability of nearly any material.

Even material one has blindly copied verbatim, with no understanding, from another source.
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Old 14th Oct 2008, 04:23
  #46 (permalink)  
 
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Capt Pit Bull

Thanks for an informative post. I learned a lot, thanks. I also learned to watch my fingers on the touchpad, I went back and looked at my post to understand your comment on "banging heads". Where they came from, I don't now.

The SAN mention was aimed at the other poster who seemed antagonistic toward following RAs as being mindless airline responses.

Thanks for the thoughtful info on TCAS. Yes, quite agree pilots must operate within the aircraft's envelope. We go to TA Only when OEI.

GF

I operate large corporate jets and did operate C-5s. Also survived a tactical military mid-air with a parachute ride, so v. interested in the subject.
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Old 14th Oct 2008, 23:30
  #47 (permalink)  
 
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I wonder why?

Galaxy Flyer - I've seen other comments similar to yours that says "we turn of TCAS RA's when we are OEI". I wonder why?

In TA only mode, the TCAS says (and I paraphrase) "Traffic! We are going to die!" and leaves it up to you to decide what to do next. If you leave the switch in RA mode, in the exactly the same aircraft in exactly the same scenario, it will say one of a small set of messages of the form (and I paraphrase again) "Descend now or we are going to die!".

Without revisiting the caveats previously posted about what performance may be available to you, and the fact that you need to exercise judgement to not turn one emergency into another - why do you prefer the "Traffic!" message to the one with some useful additional information attached? Even OEI, you can manage most of the commands (descend, maintain VS, etc), and the only one that will make you cringe is a command to increase your rate of climb - but then you are ALWAYS going to cringe if the box just said "Traffic!".
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Old 15th Oct 2008, 01:13
  #48 (permalink)  
 
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Originally Posted by Capt Pit Bull View Post
Yes, TCAS has performance inhibitions. But you really need to appreciate that these are NOT comprehensive. I'd classify them as 'inhibiting the totally impossible' rather than 'guarunteeing the possible'. In addition to that, TCAS (probably: type dependant) doesn't know about engine failures / flight control malfunction. It doesn't know if you're covered in Ice.
Just to comment on that item. I think you'll find it to be type specific. Certainly I was earlier today reviewing a certification report for an aircraft with TCAS which did specifically address the airframe icing and WAI ON implications for performance.
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Old 15th Oct 2008, 03:45
  #49 (permalink)  
 
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CJ Driver

Based I what I learned from Capt Pit Bull, I also wonder why we select TA Only. If pilots are supposed to protect the envelope, we should be responsible in the OEI case.

MfS: As the resident flight scientist, any ideas?

GF
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Old 15th Oct 2008, 07:39
  #50 (permalink)  
 
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TA ONLY prevents the generation of coordinated RAs. So any other aircraft with TCAS treats your aircraft as if it doesn't have TCAS and thus does not expect you to do any avoiding action and consequently computes its own RAs in accordance with that. This will not compromise safety in the least but will excempt you from following an RA you might not be able to achieve in the first place.

It is an item on our OEI checklist as well, as far as i know one of those items Boeing is pretty keen on.
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Old 15th Oct 2008, 13:34
  #51 (permalink)  
 
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As MFS stated many of the restrictions are type specific. IIRC the Avro RJ has engine failure detection logic, which is sent to ACAS.
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Old 16th Oct 2008, 22:21
  #52 (permalink)  
 
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How does that "not compromise safety"?

Denti - sorry but I am not convinced. In any airprox, there are many ways of solving the problem - I climb, or you climb, or I descend, or you descend, or whatever. In many encounters, the TCAS coordination means that quite mild changes - or even "no change" - can be the solution. In some of the encounters however, a guaranteed solution may only be possible with a coordinated response. Since turning off your TCAS removes those from the possible solution set, then surely you have compromised the safety net?
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Old 17th Oct 2008, 12:26
  #53 (permalink)  
 
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Sorry to be slow responding, kind of buried at work.

Galaxy, no probs mate. Regarding the whole TA only thing:

The reason its selected is a coordination thing that has most relevance to an encounter where manouevres in a particular sense are inhibited for one or both aircraft.

E.G. lets say 2 medium turboprop aircraft meet one another at high level. They will likely both have climb RA's performance inhibited.

Likewise a low level encounter both aircraft could be descent inhibited.

In these situations we can't have the aircraft manoeuvre in opposite senses, instead TCAS has to constrain one aircraft from doing anything with a preventative RA, whilst giving the other a corrective RA. This is an exercise in TCAS / TCAS coordination, and follows some established rules.

So, lets say aircraft A gets told to climb, and aircraft B gets told to stay level.

The strength of an RA, the time its issued, the response times, and the corrective RA thresholds are such that a single RA ought to be sufficient - if it weren't, the the system would be inadequate for protection against Non TCAS aircraft.

However, lets say that aircraft A has lost an engine, the crew are in the early stages of working the procedures and haven't yet gone to TA only, yet are unable to get anywhere near +1750 fpm. This is going to be a bit of a shame, because there is an aircraft present - aircraft B - that has the performance required to avoid the collision, but its being specifically told not to climb.

Whereas, if aircraft A is at TA only, aircraft B will NOT coordinate its escape manoeuvre. It'll say to itself "need to manouvre - descent inhibited - I will climb". Problem solved.

In otherwords "TA only mode" frees up the other aircraft in the encounter.

Essentially, IF you are working in an 'all tcas equipped' environment, then using TA mode is a "I have priority, you get out of my way" mode.

To that end CJ driver, use of TA mode is safer if the aircraft has a situation where you know you can not respond fully or accurately to an RA. Various aircraft I have flown have had 'TA only mode' specified in several abnormal checklists relating to engine or flying control malfunctions.

Mad Flt:

Yes, lots of type dependancy in there. For sure, the TCAS may have inputs from various systems to help it know what the aircraft can and can't do. For example, on the ATR (which I was a skipper on whan TCAS was mandated) the system had inputs by making particular ice protection selections. Setting "level 2" protection (i.e. we are entering icing conditions) changed the TCAS perf inhibitions (as well as changing a bunch of other stall protection / minimum speeds etc).

Similarly in a jet, ice protection bleeds reduce performance in a quantifiable manner and presumeably quite accurate calculations can be made regarding perf inhibitions.

However, I was alluding to actual airframe ice, rather than airframe ice protection. I know a lot of folks here will be driving jets with good performance but spare a thought for the turboprop drivers who live in the teens of thousands rather than just punching through them in a couple of minutes

pb
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Old 19th Oct 2008, 00:06
  #54 (permalink)  
 
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Capt Pit Bull

Again, much obliged; now I understand not only the reason for TA ONLY, but also the importance of the selection.

Thanks much,

GF
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