View Full Version : TCAS RA at Aircraft's Certified Ceiling

4th Oct 2008, 19:02
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?


Port Strobe
4th Oct 2008, 19:13
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.

4th Oct 2008, 19:23
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 (http://www.eurocontrol.int/msa/public/standard_page/ACAS_Bulletins_Safety_Messages.html)

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.

edit: corrected address specification

4th Oct 2008, 21:10
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!

Mad (Flt) Scientist
4th Oct 2008, 22:01
A review of this thread (http://www.pprune.org/questions/337186-does-tcas-know-aeroplanes-performance-limitations.html) may be useful. TCAS is inhibited where the aircraft performance is inadequate to respond to the RA, this would apply for this case too.

5th Oct 2008, 09:55
Embraer 195 ceiling 41000' TCAS:

"No Climb commands or Increased Climb commands are issued at or
above 34000 ft MSL."

5th Oct 2008, 14:59
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. :ok:

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.

CJ Driver
5th Oct 2008, 15:59
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.

6th Oct 2008, 04:14
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.

6th Oct 2008, 19:51
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.

6th Oct 2008, 20:37
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

Miles Magister
6th Oct 2008, 21:23
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.


10th Oct 2008, 09:55
A320 is certified to 39800 feet so if you are flying U.S. or Europe, not a problem.

10th Oct 2008, 13:20
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.

10th Oct 2008, 15:08
PEI 3721......is the 1.25 pullup limit fixed for all altitude or does it apply to a certain level only?

10th Oct 2008, 18:56
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 (http://www.eurocontrol.int/msa/gallery/content/public/documents/Doc9863_ACAS_Pilot_Training_chp5.pdf) 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. (http://www.eurocontrol.int/msa/public/standard_page/ACAS_Training.html)

10th Oct 2008, 20:30
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.

11th Oct 2008, 03:20
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.

11th Oct 2008, 13:30
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.

11th Oct 2008, 17:56
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.

11th Oct 2008, 20:08
Lookforshooter, any aircraft fitted with ACAS – commercial or GA – will only have vertical, climb or descend type commands. There is no turning flight associated with ACAS in any aircraft type.
ACAS is designed not ‘to fly you in to trouble’, indeed it does the very opposite and provides information of how to avoid trouble.
All aircraft have essentially the same system, thus whether a large or small aircraft, commercial or GA, all pilots must follow the commands; if someone does not, then there is an accident waiting to happen.

Commercial aircraft are flown by professionals who have the required airmanship / discipline to obey ACAS.
History may identify the one dominant accident involving ACAS as a problem of worldwide communication – ensuring that all crews, operators, and national authorities work to the same set of rules. The lessons have been learnt from that accident; the international rules are now firmly established.
A remaining weakness is with those humans who do not seek the necessary knowledge to understand both the principles of the system and the rules for its operation. Yet even with these, the individual must overcome bias and mis-belief, and follow the ACAS commands.

I and probably other pilots would have concerns about your apparent weaknesses in knowledge and resultant attitude about ACAS, but the most worrying aspect is that if you don’t correct these, you might be in the same airspace as us.

Re brakes; apart from mixing up anti skid with autobrake – functionally different systems, but usually combined in the same box; you are correct about the effectiveness of manual braking. In fact any pilot can better the braking distance achievable with RTO/Max autobrake providing that s/he applies the brakes quickly and maintains maximum pressure until stopped, i.e. follows the rules.

11th Oct 2008, 20:38
While I understand the robotic airline flight crew approach that is taught to blindly go where the TCAS box tells them to go, I don't think most passengers will..it didn't help the pax on the mid air in Europe. As far as weakness in knowledge ....ahhh another aviation expert to to tell us how it is....your not qualified to make any judgement on aviation knowledge or experience based on a few internet posts, no one is. If you have to resort to insults in the face of a rational logical discusion...that means you can't discuss the facts presented. Insults are the refuge for the weak minded.

Mad (Flt) Scientist
11th Oct 2008, 20:58
"Blindly" following TCAS guidance is no different to "blindly" following windshear escape guidance, or flight director guidance in autoland, or any of a number of other cases. Yes, there may be one chance in a billion that the systems have gone u/s and are giving you misleading guidance. That's the defined certification risk, after all. But there's a great deal more than one chance in a billion that someone doing their own thing by the seat of their pants will, in fact, do the wrong thing (or a worse thing) and will create more trouble than they were in to start with.

Is the TCAS RA the optimum solution to every single encounter? No, but it's a reliable solution.

Just as in the brakes case: is the anti-skid the very best you can get? No. Maybe you can beat it by a few %. But if you try to do so, you may also screw it up and fail by more than a few %. Now, if your ALD and LFL were based on "the box", there's no upside in being a couple of hundred feet short - and a huge downside in being long, if the runway is limiting.

The entire system is set up on the assumption that the person sat in the left seat is not an expert - there's a huge community behind the procedures, some of whom are experts. Following the advice of the experts, as encompassed in SOPs, is rarely a bad idea.

11th Oct 2008, 21:30
Mad - I don't trust my FD to tell me to turn left, but the other 5 things set up, in disparate equipment and systems to verify that I should turn left. Not to mention a sense of dead reckoning, and understanding of where I am at in the flight, what ATC is doing, and how they are equiped. I don't sit back and just hope my F/D is working. I sure hope pilots aren't just sitting back, crossing thier arms, hoping the autoland works...???!! !@!$

11th Oct 2008, 21:32
Does Nascar install anti-lock brakes: No.
And Formula 1 banned traction control. Hint: Not because F1 drivers are faster without it. Because they make more mistakes without it. And mistakes are fun to watch when they aren't in large transport aircraft. And despite the validity of MFS' statements about designing systems for non-experts, these systems were designed for experts, and outperformed those experts.

I'd love to know who you fly for so I can remember to stay clear.

12th Oct 2008, 00:59
I persevere with this thread, particularly as the issue is of great importance to all of the industry.
ACAS relies on appropriate behaviour of both parties in the event of a conflict, thus there is need for a knowledgeable, universal, well discipline approach to flight operations.
The availability of ACAS in small / GA aircraft implies that those pilots also require a similar high standard of operation (with respect to ACAS) to those in commercial operations.
Whereas a pilot of a small/GA aircraft, with poor understanding of some basic aspect or who has a macho attitude, might suffer a landing (or any other) accident, the result affects only that individual and hopefully no one else. But an inappropriate response to ACAS could result in an accident involving an unwitting third party including many passengers in a commercial aircraft.

If a GA pilot decides not to follow an RA then the commercial safety situation may be no better than it was when the need for ACAS was first noted – Cessna 172 / MD 80 collision.

I am not suggesting that all small / GA operators lack operating qualities, indeed the vast majority (as have commercial pilots before hand) have demonstrated the necessary skills, and strive to improve them before moving on to larger aircraft.
The issues are the immaturity of some people, their reluctance to learn, or poor attitudes in an operational climate where there is little or no oversight, operator / regulatory control, or a second crew member to monitor. Worst still is that a few, hopefully very few, might progress to the very light jets with opportunity to really mix with the larger aircraft.

galaxy flyer
12th Oct 2008, 03:26

If you have TCAS II on board, you are legally obligated to follow RA commands! To not do so, puts everyone at a risk far greater than not doing so. You can have any opinion you want, but you must follow the commands, PERIOD. Yes, at this stage, only up or down plus "monitor Vertical Speed". Follow for my family's sake

SSG V1.10 lives on, to the endangerment of all, it seems.

Terrific..some more holier then thou pontification from the airline sector...which has caused more loss of life in aviation related incompetence then any other sector could in the next hundred years.

Care to produce a few facts to back up that outrageous claim?

12th Oct 2008, 09:12
Add up all the GA deaths vs Airline deaths, see who wins.

Between 1987 and 2007 the GA (nonscheduled air taxy operation, CFR 135) caused 1030 deaths and in the same timeframe scheduled airline operation (CFR121) caused 49 deaths. That is taken directly from the NTSB database. Since GA flies a lot less hours as well scheduled airline operation is several hundred times more unlikely to kill someone than GA is.

We had that little piece of statistics allready ssg, however you still spout the same nonsense. Time to wake up and leave that little cozy dreamworld of yours.

Capt Pit Bull
12th Oct 2008, 12:41
Forum search FTW.

Go back and read the previous thread quoted by Glueball. In detail. There is too much dogma on this thread.

RAs should not be ignored. But if you think that any RA that is given to you is gaurunteed to be achieveable, you're cruising for a bruising.

If you have TCAS II on board, you are legally obligated to follow RA commands! To not do so, puts everyone at a risk far greater than not doing so.

Actually, that's not true.

There is an important distinction to be made between 'not following' (which is riskier, but only slightly) and 'manouvering opposite' (which is far far riskier).

This may seem like I'm laboring a point, but if you look at the really nasty TCAS incidents (including the ones that would have been collisions if there hadn't luckily been some horizontal separation) a regular feature is people manouevering opposite. Whereas there are plenty of times where RAs have not been followed and with low risk.

When pilots are told "follow the RA, always, thats all there is too it" then they are receiving incomplete guidance. The bean conters love it - a single sentence in an ops manual is cheap; proper training isn't.

It is very possible to receive an RA that is unachievable and/or otherwise unsafe. In those circumstance crew need to know what to do to minimise the risk. In my experience as a trainer if you tell people they mist follow the RA, and then give them a situation where they can't, there is a good chance they will do something really stupid unless they have a good alround knowledge of the system and other collision avoidance techniques.

To answer the Original Poster:

You follow any RA that is generated, whilst being sure to protect the aircrafts flight envelope. If the RA is a climb, and is not achievable, do your best to achieve the best rate of climb, even if its only a few hundred feet a minute, and get as close to the green arc/segment as possible.

UNDER NO CIRCUMSTANCES think "We cant get over him, we must descend!"

If you see the other aircraft, the curvature of the Earth will probably make it appear to be well above the horizon. UNDER NO CIRCUMSTANCES discard your RA in favour of your visual perception of relative altitude and commence a descent.

In other words UNDER NO CIRCUMSTANCES manoeuvre opposite the RA.


galaxy flyer
12th Oct 2008, 20:45
:ugh: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.

The German mid-air was caused by pilots following the ATCO's instructions, rather than following the RA. This isn't brain surgery-TCAS is a "last ditch", save your butt device, not an alternative ATC device, as our friend SSG V1.10 would believe. It has saved me twice in civil aircraft and several other times in mil ops. I investigated a crew that decided they were smarter than the ATCO and the TCAS-nearly created a mid-air!! :ugh:


SSG: You have been shown to be in error and dangerous by any number of Ppruners that are experts in aviation, in all disciplines-pilots, engineers and operations types-a smarter man than you would retreat and try to learn, as opposed to continued idiotic posts.

12th Oct 2008, 21:04
Denti...your numbers are incorrect, all I have to do is find airline crash after airline crash and find more then 49 deaths, in that ten year period.

Actually, you are right (of course, its 19 years, not 10, but basic math isn't your forte), i did use the wrong table. In that period scheduled airlines caused the death of 1966, with an average rate of 0,0174 deaths per 100.000 hours, vs. 0,775 deaths per 100.000 hours in CFR 135 operation. Or to say it differently, flying GA you have 43,39 times higher chance to die than flying scheduled airline services. Airline statistic shows data per 100.000 take offs or miles as well, sadly that data is not available for GA.

The trend in both types of operation is very nice indeed as fatalities become less common in the last few years.

About the TCAS thing, i dont know how much training others get, but it is a normal piece of training every six months in our simulator sessions, and of course EGPWS has priority over TCAS.

12th Oct 2008, 21:20
The person behind Lookforshooter, is either a very misguided non pilot, or a pilot who should be identified and reported to his authority under the terms of a safety report; in either instance, s/he could be added to a Pprune ignore list.
Alternatively, granting Lookforshooter some semblance of intellect, the debate is with an individual who wishes to devil subjects with very biased views. In some circumstances there could be merit in eliciting information this way. However, there is a fine line between such arguments and the promotion of false information which the unsuspecting or unwary pilot reading Pprune might take as the truth, and thus reduce flight safety.
In this forum, this form of debate is hazardous at least, and IMHO unethical amongst such a well respected open minded group.
I do not know which of the above views is more accurate.
I still have a choice, in both what I read and what I believe; I align my understanding with the peer reviewed, expert, and majority views to help maintain a good safety record in our industry.

Blue Coyote
12th Oct 2008, 22:12
This is a seriously uneducated individual as far as TCAS/ACAS goes. Based on his input to this thread I hope he stays in Asia as I don't want to be anywhere near his accident when it eventually happens.


13th Oct 2008, 00:50
After being reasonably well behaved, Lookforshooter has overstepped the mark considerably and been consigned to a gloomy place.

Protocol wise, my concern with him was not his lack of technical and procedural wisdom (that being addressed adequately by dissenting posts - indeed, such robust discussion is good for the education process of those coming up through the ranks). However the lack of civility and manners is out of line - when any poster descends into the depths of ill-considered and persistent personal attack it is sin bin time ...

13th Oct 2008, 01:57
Well I myself felt Lookforshooter made some good points! Nice to see some non airline 'SOPS only' folks in here. One thing for sure, I never saw a TCAS unit installed that talked to the ground prox, then argued with each other over who took precidence. The fact is, each unit operates independently...one worries about traffic the other about terrain. Both can and will squak, and have squaked at me in the pattern at mountainous airports at the same time. The fact is if you have a flock of airliners above you, mountains below...the PILOT will have to make a decision, not the box. Obviously there are those in here that are very uncomfortable with that.

13th Oct 2008, 03:01
NonFlushingLav, “… I never saw a TCAS unit installed that talked to the ground prox …”

Time to look again; commercial installations have priority logic, probably in the alerting system. In very unusual and difficult circumstances the pilot is not confronted with conflicting messages and thus does not have to make a decision.

“CS 25 AMC 25.1322 Alerting Systems
6.7 There should be only one aural signal at a time. If the possibility of two or more aural signals at the same time cannot be avoided it should be shown that each signal is clearly intelligible to the crew. The order in which the signals are presented should be that in which crew action is required.”

Re SOPs, often there is need for considered debate about SOPs, the good ones, poor ones, personal SOPs, and those which are ‘rules’. ACAS SOPs are rules with very few exceptions.

13th Oct 2008, 03:31
I never saw a TCAS unit installed that talked to the ground prox, then argued with each other over who took precidence. The fact is, each unit operates independently...one worries about traffic the other about terrain. Both can and will squak, and have squaked at me in the pattern at mountainous airports at the same time. The fact is if you have a flock of airliners above you, mountains below...the PILOT will have to make a decision, not the box.

Actually, that's precisely what TCAS II does.

With respect to the recent conversation on inhibited TCAS commands, my AOM includes the following information about certain functions:

Advisory Priority: Can revert to TA ONLY or STBY when higher priority advisories (e.g., GPWS, Windshear, etc) occur.

Altitude Climb Limit: Inhibited in accordance with aircraft performance limitations.

Descend RA: Inhibited below 1,000 feet in descent, and 1,200' in climb.

Increase Descent RA: Inhibited below 1,450'.

Resolution Advisories: Inhibited below 400' in descent and below 600' in climb. (TCAS automatically reverts to TA only)

--one might hope that with the demise of lookforshooter, the professional tone of the forum might be restored, but unfortunately he's already begun posting as nonflushinglav (strangely appropriate), and like his many other pseudonyms, has started his new posting career by agreeing with his old identity.

13th Oct 2008, 04:15
Guppy that's silly...I never heard of a TCAS unit hooked up to the GPWS unit or radar altimeter. How would the TCAS unit know what altitude MSL you are? At an 8000 ft field, flying at 1400 ft in the pattern, would it know that your at 9400 ft MSL or at 1400 ft AGL?

13th Oct 2008, 04:21
I've no doubt there are many things of which you haven't heard, or with which you're not familiar...seems that most things in aviation fall into that category for you. Clearly the concept of integrated avionics is among those things...things you don't understand. Once again, despite your different screen names and claims...it only goes to show who you are, and who you aren't.

Aren't the various functions integrated on your microsoft flight simulator?

13th Oct 2008, 04:28
Guppy is on my ignore list...Certainly the challenge is out to everyone if they can show a TCAS unit coupled to Ground Prox and Radar Altimeters...the only way it would know not to decend you at a higher elevation altitude, into...you guessed it....mountains.

13th Oct 2008, 05:52
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.

Spanner Turner
13th Oct 2008, 06:29
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

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.

Capt Pit Bull
13th Oct 2008, 10:30
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.


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.


13th Oct 2008, 18:18
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.

13th Oct 2008, 19:02
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.

galaxy flyer
14th Oct 2008, 04:23
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.


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.

CJ Driver
14th Oct 2008, 23:30
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!".

Mad (Flt) Scientist
15th Oct 2008, 01:13
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.

galaxy flyer
15th Oct 2008, 03:45
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?


15th Oct 2008, 07:39
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.

15th Oct 2008, 13:34
As MFS stated many of the restrictions are type specific. IIRC the Avro RJ has engine failure detection logic, which is sent to ACAS.

CJ Driver
16th Oct 2008, 22:21
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?

Capt Pit Bull
17th Oct 2008, 12:26
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 ;)


galaxy flyer
19th Oct 2008, 00:06
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,