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Old 4th Oct 2008, 19:02   #1 (permalink)
 
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TCAS RA at Aircraft's Certified Ceiling

Hi, I am just wondering what would happen if you are cruising at or just below the certified maximum altitude and a tcas ra goes off? You obviously can't climb higher so what do you do?

Thanks
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Old 4th Oct 2008, 19:13   #2 (permalink)
 
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Compared to doing nothing climbing is the better option is it not? Its not the absolute ceiling so there will be some room left to manoeuvre within and maybe 10 - 20 knots of speed to trade in short term, possibly a good reason not to climb to the maximum possible altitude straight away? If its that tight a margin on performance then perhaps the operator should mandate you put the transponder to TA only? Just a few thoughts, but I reckon following it as best you can would be the best bet.
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Old 4th Oct 2008, 19:23   #3 (permalink)
 
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ACAS Bulletins

Like the previous poster replied - follow TCAS command, taking whatever performance the aircraft gives to you.

Matters like this are addressed in an excellent way in the ACAS bulletins published by Eurocontrol, use the following link

EUROCONTROL - ACAS II Bulletins and Safety Messages

Underlying address is as follows, should the link work incorrectly from your browser. On the page that you reach with the address below, you will find a link to the 10 bulletins that have been published to date.
http://www.eurocontrol.int/msa

edit: corrected address specification
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Old 4th Oct 2008, 21:10   #4 (permalink)
 
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Most often max certified crz level is not an absolute level as someone above stated. It's often a certified altitude of a 2 % buffet margin or sometimes higher depending on the aircraft. Also governing the max crz alt is the maximum cabin pressure differential. But there is always a safety margin built into that so the aircraft most often can climb all the way up to an absolute altitude...

so always follow TCAS what ever it might say.

As a note, you should be aware of the new communication prescribed in case of a TCAS RA issued by ICAO in May 2008.

Safe flying!
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Old 4th Oct 2008, 22:01   #5 (permalink)
 
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A review of this thread may be useful. TCAS is inhibited where the aircraft performance is inadequate to respond to the RA, this would apply for this case too.
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Old 5th Oct 2008, 09:55   #6 (permalink)
JAR
 
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Embraer 195 ceiling 41000' TCAS:

"No Climb commands or Increased Climb commands are issued at or
above 34000 ft MSL."
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Old 5th Oct 2008, 14:59   #7 (permalink)
 
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Hi Mad (Flt) Scientist

Only because some things stick in the mind, whilst others don't, would I have posted that TCAS will not issue RA's outside of the aircraft's current performance capabilities to achieve the RA regardless of specified aircraft limitations so spot on.

Then begs the question - if TCAS recognises danger but also recognises a lack of performance ability what happens ??
Does it re-work any possible options to avoid the conflict - or does it default to (literally) throwing it's arms in the air and saying "too hard, not my problem!!"

Just wondering.
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Old 5th Oct 2008, 15:59   #8 (permalink)
 
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Cooperative solutions

If both aircraft involved in a conflict have TCAS, then between the two TCAS systems they will agree a solution. For example, if you are at Max Altitude, and someone is climbing to your level, although you may not be able to go up, they will still be told to reduce or stop their climb.

If TCAS cannot give you a resolution because of inhibit logic, it will give you a Traffic alert instead and let you figure something out. Actual inhibit inputs are airframe dependent, but can include air data inputs (altitude), gear position, flap position, and so on.

Slight change of thread - is it just my imagination, or have there been quite a lot of fairly basic TCAS questions on PPRuNe? I don't want to be rude, but isn't "everything you ever needed to know about TCAS" taught in pilot school any more?

Although there are lots of clever algorithms inside the box, the end result is actually pretty simple, and (at least I always thought) completely understandable in about 5 minutes.
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Old 6th Oct 2008, 04:14   #9 (permalink)
 
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TCAS has priority

Hi folk,
If the question is from a 320 "Busdriver", the answer is that you will follow the RA. The 320 alt limit is more a result of Engine thrust avail than the aerodynamics of the aircraft. The aircraft is able to momentarily climb above without a problem.
Follow the RA in all cases - except one. That is if you get an additional GPWS warning of "Terrain Pull Up". Then the GPWS warning takes priority. You may miss the traffic but if you don't follow the GPWS you won't miss the ground.
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Old 6th Oct 2008, 19:51   #10 (permalink)
 
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Max Certified aircraft altitude is setup with hardwired jumpers into the back of the Tcas computer. When I have tested aircraft on ground with the test box using a scenario of two aircraft at or near Max altitude and a conflict the RA is co-ordinated with sufficient time to prevent aircraft having to climb.
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Old 6th Oct 2008, 20:37   #11 (permalink)
 
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Hi...this is from a paragraph in a book titeled AIRCRAFT DIGITAL AND ELECTRONIC COMPUTER SYSTEMS
quote: It is important to be aware that TCAS provides only vertical guidance...TCAS also ignores performance limitations, in other words, when flying at maximum altitudes, TCAS may still generate a climb command unquote:

now the term max altitudes was not elaborated upon so i don't know what to make of it
bf
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Old 6th Oct 2008, 21:23   #12 (permalink)
 
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I believe that you have 1.3g to the buffet margin at max cert alt. That gives you plenty of room to manouvre at the height if you really need it. I would suggest in the case of an RA you need it. It is perfectly safe and reasonable, however your AFM is the authority.

MM
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Old 10th Oct 2008, 09:55   #13 (permalink)
 
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Chirpie

A320 is certified to 39800 feet so if you are flying U.S. or Europe, not a problem.

Last edited by Meek; 10th Oct 2008 at 09:58. Reason: title
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Old 10th Oct 2008, 13:20   #14 (permalink)
 
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Miles Magister, a reminder to avoid any misunderstanding, the 1.3 buffet margin is a turning flight requirement. TCAS manoeuvres are only vertical and are approximately defined by pull up (1.25 g), reaction time, and the required altitude/rate change; these factors (and configuration if applicable) define the TCAS limit at maximum altitude.
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Old 10th Oct 2008, 15:08   #15 (permalink)
 
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PEI 3721......is the 1.25 pullup limit fixed for all altitude or does it apply to a certain level only?
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Old 10th Oct 2008, 18:56   #16 (permalink)
 
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bflyer, the figures that I quoted were approximate, I do not know for sure that theyapply to all altitudes.

The Eurocontrol hosted document Operational use and Pilot training guidelines indicates that the maneuver requirements assume increments between 1/4g to 1/3g (1.25 – 1.33g) depending on the alert, and that the required VS is achieved in 2.5 sec; the information suggests that these apply at all altitudes. The technical information in this document should be crosschecked with the latest ACAS technical spec as some performance aspects may have changed with recent updates.

ACAS Training materials.
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Old 10th Oct 2008, 20:30   #17 (permalink)
 
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Not quite right PEI_3721.
The required VS will be reached in however long it takes using the specified acceleration. The times quoted in the guidelines are the delay from the issue of the RA to the pilot moving the stick.
For an initial RA the required acceleration is 1/4g with the manouevre starting no longer than 5 seconds after the RA is issued.
For a subsequent change in RA the required acceleration is 1/4g (for weakening RAs or "normal" strengthening RAs) or 1/3g (for RA reversals and RAs strengthening to "INCREASE CLIMB"/"INCREASE DESCENT") in both cases starting no longer than 2.5 seconds after the change in RA is issued.
These accelerations and times are indeed applicable at all altitudes.
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Old 11th Oct 2008, 03:20   #18 (permalink)
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This is all assumes of course that the robots flying the plane do exactly with the RA tells them to do. This is where judgement takes precident over a box.Your right in the coffin corner, do you climb because the RA tells you to? Your on approach at the MDA, IMC, do you decend because the RA tells you too? Your VMC right below the clouds on a visual approach...do you climb into the soup because the RA tells you to? I bought a TCAS I unit and chose not to go TCAS II for the extra $100k...I figured, the pilot should know when to climb or descend.
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Old 11th Oct 2008, 13:30   #19 (permalink)
 
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Thanks for the clarification fellman. Are you quoting from the same training reference that I used?
The Eurocontrol training materials link has another reference stating ‘5 sec’, but without specific application, i.e. 5 sec from alert to achieved VS, or 5 sec during the maneuver – its unclear.

My recollection during testing a system many years ago was of 2.5 sec ‘surprise’ (reaction) time plus 2.5 sec maneuvering.
The use of ‘g’ in a specification is fine providing the aircraft has an accelerometer, but in practical terms, a pilot requires specific guidance as to what a ‘smart’ maneuver is. Some of the training materials provide values of pitch change for speed, but the rate of pitch and feel of the maneuver will be aircraft specific, so practice is required.

Lookforshooter, the main point which has been made in many threads on ACAS is that pilots must follow the RA. This is a matter of discipline – airmanship.
As already stated, when an aircraft is at it’s maximum altitude, no Climb instruction will be given so the pilot does not have to consider performance, nor weather. There are similar safeguards at low altitude so that ACAS will not demand hazardous flight towards the ground.

A danger in aviation is from those pilots who figure that they know better than the system. If you do not follow a RA then you may cause an accident – it will be your accident as you will be part of the collision; no thinking, no judgment, just follow the RA.
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Old 11th Oct 2008, 17:56   #20 (permalink)
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PEI_3721: This reminds me of how some people look at anti-lock brakes...they trust the box to do the braking. In fact, on one of the planes I fly, I had a chance meeting with a guy that was part of the certification process. He stated the test pilots could beat the braking distances about 35% of the time. That said, they didn't beat it was by much, so just mashing down on the brakes and letting the box do the work is the defacto braking method for most pilots. Does Nascar install anti-lock brakes: No. I have landed on very slick runways, snow, ice...gravel, ect where the box just coldn't figure it out, and I just kept going. TCAS: I gave some exampes where listening and doing what the RA tells you do, could get you in trouble. I can't speak for airliners, but TCAS installed in the planes I fly, can only tell you to climb or descend...not turn right or left, or slow down, or wait and see, or whether there is a radio tower just below you..on and on. Again, if the pilot can't make a judgement a judgement call, you might as well, have my Pomeranian flying the plane.
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