TCAS vs Alt Adjust

[bmbahia]

I did some research over the forum and couldn't find out anything related to this, so...
Can anyone enlighten me on a issue about the TCAS and the altimeter setting? I've been flying the bus for some time and never heard of this before.

During cruise flight, while performing the approach briefing, I momentarily pushed the altimeter setting knob to adjust the QNH at destination, as I always do.
The captain told me that I should never do that again, because doing so I can cause a RA on another aircraft nearby.
I know that I can make the altimeter alert sing, but that's just annoying and won't do any warm, but I wasn't aware that I could make the TCAS shout.
Does anyone know if it's true?? I learnt that the transponder uses the standard setting to work out the TCAS and changing the QNH won't change anything on that matter.
Where I can find some reading about it??

Thanks.

[NSEU]

I agree with you, but finding reading material is difficult.

Which aircraft do you fly?

My wiring schematics for the Boeing 767 show the label "Alt (29.92)" coming from the Air Data Computers to the ATC Transponders (which the TCAS uses).

Wiring schematics for the 744 and 737NG show both Altitude and Baro-corrected Altitude data on the data busses coming from the ADC's/ADIRU's, but the label going into the ATC is simply "Altitude". Not really conclusive, however.

I must admit I was surprised to see, in the 744's schematics, baro correction coming from the Standby Altimeter going to all the ADC's. I have no idea what it's used for.

Rgds.
NSEU

[bmbahia]

I've been flying the 320 and now the 330.
I know that we have a link between the ADIRS and the Transponder, but that's for altitude reporting to ATC. It's used to send some information (ie. IAS, Mach, Vert Speed, Mag Hdg and some more) to ATC when it's interrogated.
But how about the TCAS?

Thanks Again.

[NSEU]

Broadly speaking, a TCAS system acts like an ATC ground station... The TCAS antennae transmit ATC-like interrogation signals and the other(intruder) aircraft respond via their ATC systems, giving, hopefully, altitude information. The other(intruder) aircraft's ATC systems get their altitude(29.92) information from their ADC's or ADIRU's. In your scenario, you are the intruder.

When two TCAS-equipped aircraft are about to collide, the airplanes talk to each other via their ATC systems to co-ordinate avoidance manoeuvres.

Hope this makes sense.
Rgds.
NSEU

[Chipmunkpac]

QUOTE
I know that I can make the altimeter alert sing, but that's just annoying and won't do any warm,

I know for sure on the 777 and 744, the aircraft will maintain the FL based on the QNH if you change it from STD, in other word, will climb or descend depending upon the QNH and which autopilot engaged. Not sure how the TCAS will react when two aircraft flying in proximity one on STD and the other on local QNH.

[john_tullamarine]

I have no particular knowledge of the innards of TCAS .. but one would consider that the mathematics would be based on standard pressure as the aim is to look at deltas rather than absolutes. If QNH, then setting errors (and most of us have been guilty of the odd oversight in setting the altimeter) would confuse the issue.

[bmbahia]

That's why I change the AP to the other side before changing the altimeter. To avoid such clb/des, although it's so brief that you will only change 10 or 20ft.
On the 330 it is "standard" to change to the other AP in order to avoid the problem.
But it's all about altitude maintenance and ATC, and I'm not sure if it will make a difference on the TCAS system.

[Jumbo Driver]

TCAS uses Transponder Mode C (Altitude), which is ALWAYS referenced to 1013mb. This is because, for both TCAS and ground SSR systems, comparisons between levels of different aircraft need always to be referenced to the same datum.

Therefore, the only way that changing an altimeter subscale setting can affect TCAS is if, by varying the subscale, the aircraft actually reacts, i.e. it climbs or descends because ALT CAPTURE is engaged to that altimeter (ADC) system. In this case, the actual change or trend, if it is significant, may prompt a TA or RA.

However, if the AP is not coupled to the altimeter (ADC) in question, or if ALT HOLD or manometric locks are engaged, then TCAS cannot possibly be affected by changing a subscale.


JD

[bmbahia]

Jumbo Driver,

That is exactly what I think, but still looking foward to confirm it.
Do you know if I can find some reading about it???

Thank you.

[OPEN DES]

Your captain is wrong and you are right. TCAS looks at pressure altitude (i.e. 1013.25/2992) independent of QNH selections.

Also there is no need to change the AP in command when pre-selecting QNH!

Hope this helps

[vapourer]

bmbahia - try this link. Your specific question is answered under the "ACAS Principles" section EUROCONTROL - Frequently Asked Questions (FAQ)

[Pilot Pete]

Exactly what I was going to link to vapourer! Here is the question and answer from there;


How does ACAS II work if one aircraft is using the standard altimeter setting and the other QNH?


ACAS II (TCAS II) always utilises pressure altitude information which relates to the standard 1013.2 hPa. It does not affect ACAS II operation if aircraft are flying Flight Levels on the standard pressure setting, altitude on QNH, or height on QFE. The pressure setting input by the flight crew does not affect the ACAS system at all. Additionally, below 1750 feet, ACAS also uses radio altimeter data.

There you go bmbahia. Note the other comments about changing the subscale of the altimeter which is feeding the coupled autopilot though, as the aircraft WILL climb or descend to maintain the cruise set in the altitude window!

PP

[bmbahia]

That is what I was looking for.
Thank you Pete and Vapourer for the info.

[CONF iture]

bmbahia, thanks for asking that very good question on TCAS, I had never thought about that aspect before.

But I'd would like to disagree on the following :

That's why I change the AP to the other side before changing the altimeter. To avoid such clb/des, although it's so brief that you will only change 10 or 20ft

If you change the altimeter setting or reference once you're in ALT CRZ GREEN or ALT GREEN, the airplane won't climb or descent by itself.

FCOM 1.22.30
The altitude that ALT mode holds is the altitude it memorized when engaged. It is not affected by a change of barometric reference or by a change in the barometric correction

[Jumbo Driver]

Originally Posted by CONF iture

If you change the altimeter setting or reference once you're in ALT CRZ GREEN or ALT GREEN, the airplane won't climb or descent by itself.

CONF iture, I guess the answer you are giving here may be type-specific, as you quote a FCOM 1.22.30 reference ... ? What is that reference, please?

Secondly, let me pose a question. If what you say above is correct, then what happens if you are in ALT GREEN at 5000' on QNH and the QNH changes. You therefore change the QNH subscale - does the aircraft re-establish at 5000' on the new QNH or, as you seem to be suggesting, does it remain at the previous "pressure level"?

JD

[CONF iture]

You're correct JD, that's for the type bmbahia is flying, A-320/30

You're also correct regarding the second point.
But as usually QNH is updated as soon as it changes, the variation is so little, therefore we almost won't notice anything on the altimeter reading after the update.
If a sudden change in the altimeter setting was possible, 29.92 to 29.42 the pilot would have to initiate a 500 feet climb to go back to 5000 feet altitude.

[Jumbo Driver]

Then it would seem that the answer to bmbahia's question is, in part, type-specific. Certainly, to the best of my recollection, Boeing system philosophy will lead to the AP immediately adjusting vertically to regain the selected altitude if there is a subscale change on the engaged AP/ADC, hence my reply earlier in this thread.

However if the A-320/330 design is such that once ALT GREEN is annunciated the AP will not react to a subscale change, whether it be large or small or from 1013 to QNH, then the answer to the original question must be that TCAS cannot be affected by momentarily setting QNH, regardless of which AP is engaged.

I do have one more question - bmbahia says in his original post that

I know that I can make the altimeter alert sing

What then does this alert denote, if the AP is not registering a vertical deviation from the correct flight path?


JD

[NSEU]

What then does this alert denote, if the AP is not registering a vertical deviation from the correct flight path?

Alt Alert is just a simple baro altitude Vs selected (MCP) altitude comparison. The processes which tell the autopilot to respond (or not) to this deviation are independent of the alert function.

Is that what you were asking?

[CONF iture]

That’s right, it is a kind of attention getter to tell you that there is now more than 250 feet difference between your indicated altitude and the preselected altitude displayed on MCP
A way to avoid that aural warning is to momentarily select the other side AP when you want to pre adjust your own side altimeter setting.

[Shore Guy]

TCAS works remarkably well for (lack of a better word) bastardized system. Transponders were never originally designed for an application of this type, and the interaction with this and air data systems for airborne collision avoidance were an afterthought.

There are still some limitations tied to the basic design and antennae…for example, one will never see a lateral escape maneuver due to the lack of accuracy in azimuth (range is very accurate). Down the road, ADS-B may offer lateral escape maneuvers, if used as a collision avoidance device.

Our airline operates with a CDTI that views TCAS targets along with correlated ADS-B/TCAS targets. The TCAS lateral “hop” vs. the rock solid ADS-B targets is very obvious at low ranges.