Early transition to QNH vs TCAS

[737incognito]

We have a procedure to select QNH the moment we are cleared to descend to an altitude. It works well in respect that it's hard to forget to select QNH but we are worried that it my cause problems with tcas ta and ra. With big QNE/QNH difference two aircraft my be closer vertically than calculated by the tcas computers since one can be on QNH above TL while descending and other in cruise by QNE. Or there can be unnecessary warnings when difference is in different direction.
I would like to know if your SOPs allow such procedure (maybe under some conditions) or explicitly forbids it.
Thanks

[Orp Tolip]

Seems common sense to me, thats exactly what we do too, and did at my previous outfit.
Not sure about the TCAS thing, will have to look into the manuals etc more closely, but I was under the impression that the Transponder was always referenced to 1013 regardless of what was set on the altimeter subscale.
There are far more knowledgable people out there who will be able to enlighten you (and me for that matter).

[Hotel Mode]

TCAS is independent of altimeter systems and is always set to 1013.25. It goes onto TA only and ground mode with the Radio Altimeter.

[737incognito]

It sounds logical but I must admit that this is first time I hear it's independent of altimeters setting. I checked my 737 manual, some training material and some tcas stuff I've got from various sources on the net and I still couldn't find any reference to 1013.25mb. It says transponder must be operative for tcas to work, and correct me if I'm wrong, transponder is giving altitude based on present alt. setting (1013 or 0999).
Anyway, what I don't know - I don''t know. However I would appreciate very much if you give me some reference to read. Thanks

[Chilli Monster]

737

Transponder ALWAYS transmits levels based on 1013.25. What you have set on your altimeter has nothing to do with it.

(ATC radar displays convert to altitude below Transition Altitude by having the QNH input into the radar processing).

So - with all Transponders emitting the data based on the same datum, there will be no discrepancy in the TCAS information shown no matter what is set on your altimeter. TCAS WILL NOT be affected by setting QNH early, when descending to an altitude (in fact, it's better during low pressure to do so - otherwise you risk a level bust in the descent).

[Clarence Oveur]

The transponder always gives altitude information based on 1013.25.

[The Snake]

A TA or RA is only triggered if an intruder enters a specific 'airvolume' around your aircraft. The size of this volume is NOT fixed. It depends on airspeed and flightpath of intruder and your aircraft.

The radius of the volumes are expressed in 'closest point of approach' (CPA) and has noting to do with the altimeter settings of both aircraft.

In my opinion, no problem to select QNH or FL asap.

Ciao
The Snake

[411A]

Yep, the transponder outputs (references) 1013.25mb/29.92 inches, regardless of the individual altimeter Kollsman window setting.
Twist away, makes no difference.
Now, it's not always been this way.
When mode Charlie was first introduced in the USA ( a very long time ago), some altimeters had encoders contained within (not to be confused with present day encoding altimeters), and the reference was to the altimeter setting.
Remember 64 code transponders with mode C?
Yep, that's the time.
This was changed PDQ when it was realised the problems that it might cause.
Having said this, as the transistion layer in the USA is quite high, it made no practical difference, but was changed nevertheless.

[Kit d'Rection KG]

Regarding 'radius of volumes', in fact, TCAS considers 'range by range rate'.

[737incognito]

I did a search and I've found the reference regarding transponder transmission on 1013.25mb http://www.faa.gov/airports_airtraffic/air_traffic/publications/atpubs/aim/Chap4/aim0401.html#4-1-19
so I have to admit that all the information I received from you guys were correct 100%.
I really dont know how I didn't come to that info before, but better ever than never.
Thank you

[alexban]

737 : the procedure to set the QNH when cleared to an altitude it's a standard now,and the ATC is fully aware about this,and eventual separation issues.
About the transponder preset at 1013.25 , I have some questions:
- why is there a select source switch ? (Altitude SOURCE selector :enables altitude reporting from air data computer no.1 or no2. ) . We have very new planes,so it's not an old system.
-do you remember the accident involving a 757 (I'm not 100% sure ), and unreliable speed , where the crew asked the ATc about transponder altitude information, dead wrong , and they crushed in the sea?
Is it possible that the transponder reports an altitude from a selected altimeter system (cpt's or fo's ) and the TCAS uses a built-in altimeter, just for TCAS reasons?
I know that most of the time the transponder and TCAS are thought to be the same, and they are not. TCAS is a different system, incorporated and making use of the transponder system.The controlling panel is called a TCAS/ATC transponder panel.
I'm pretty sure that the altitude info the ATC gets is from the selected altimeter source ,and maybe ,the TCAS computation of the advisory/RA is done using a built-in 1013.25 preset altimeter.This preset makes some more sense, cause it will eliminate crew error (forgetting to change QNH/ STD )

[212man]

(Some) Transponders with digital readouts will also show the current Pressure Altitude when tested, normally as 10ths of a FL (e.g 100 ft will be 0.1).

[jumpuFOKKERjump]

Is it possible that the transponder reports an altitude from a selected altimeter system? It better not, because ATC radar will apply a correction assuming it is sending reference 1013.2. Our QNH setting usually comes digitally from the Met, but we enter it manually in bypass mode. If you change the QNH setting on the console in this mode you can see the levels on aircraft operating at altitudes change, not those at flight levels.
Believe you are wrong, but I had held a radar rating (high level sector) for a number of years with the same wrong idea so don't feel bad.
Back to the original question, if I instruct you to descend to 9,000 ft and reach FL140 by a point in space will you still be on 1013.2 when you pass FL140?

[Graybeard]

TCAS has been scabbed onto the old IFF system that became known commercially as the ATC transponder. Until Mode S was adopted in 1987, all transponders were called ATCRaBS, for ATC Rotating Beacon System. The transponder output is always pressure altitude, not baro-corrected. The TCAS processor uses the output from only the transponder that is replying to ATC.

ALTitude selection on the ATC/TCAS control panel is for redundancy, the same reason the plane has dual Air Data Computers and dual transponders, from back when they weren't so reliable. ALT OFF is for when the transponder might be reporting wrong altitude, or when ATC would get overloaded in the early days and request ALT be turned off. Back then, no plane had more than one transponder.

The Aero Peru 757 was a particularly tragic case. It was pressed into service in the middle of the night at Lima while being polished. The polishers neglected to uncover the tape from the static ports, and the walkaround failed to notice. Once they lifted off, none of their air data instruments, and those systems depending on air data, made sense. Indicated airspeed went down as they ascended, while indicated altitude remained at field elevation. They finally lost control over the ocean while trying to sort it out.

ATC would have believed and reported them at field altitude, of course. Some people blamed the crew for incompetency. I defy you to do any better. Nothing makes sense in that situation, and they were IMC. Their only source of speed was the IRU or DME, and they had only indication of field altitude, once above 2500' radio altitude.

Only weeks earlier, the very first 757 crash was a German charter out of a Caribbean island, where the plane had sat for two weeks. They stalled at low altitude due to an air data problem. I don't remember the details.

I don't believe tragedies come in threes, but not too long after, an AA 757 hit a mountain in Colombia by descending too soon. I don't remember another 757 crash since. The 757 has been instrumental in at least one other crash, however, due to its nasty wake on approach.

Back to selecting QNH at top of descent: it doesn't matter, as neither ATC, nor other aircraft will know.

I got involved in a case in about 1992, with a small fleet of 727-100 that flew only LAX-JFK. Sometimes NY ATC would complain they were off their cleared altitude by up to 500', especially during times of high baro pressure. Turned out that the FAA Designated Engineering Representative who designed the retrofit of the Mode S transponder (and TCAS) was unaware the transponder had to have a pressure altitude source and not baro corrected. They had to rewire the airplanes. Last I knew, that guy was still in business.

GB

[bookworm]

Is it possible that the transponder reports an altitude from a selected altimeter system (cpt's or fo's ) and the TCAS uses a built-in altimeter, just for TCAS reasons?

Even when an altimeter is used to encode the altitude (an encoding altimeter) rather than a dedicated encoder, the value encoded is still with reference to 1013.25. Changing the subscale setting only moves the hands on the face of the instrument itself (or does the equivalent in software for you folks with tapes). It doesn't affect the encoder output in any way.

[Kit d'Rection KG]

As an interesting aside, Mode S parameters displayed to ATC (when selected) include selected altitude (that is, what's in the MCP altitude window), but not (despite NATS protestations) selected pressure setting.

So, if the QNH is different to 1013, the controller will know that an aircraft in descent has selected, say, 5000ft - but, crucially, not whether the flight crew have changed from STD to QNH.

Thus is another safeguard removed by inadequate regulation.

[alexban]

Indeed ,you are right. The transponder sends altitude info compared to 1013.25 ,no matter what is selected on the main altimeters.
This is from a transponder description:
'The KT 76C displays Flight Level Altitude, marked by the letters “FL” and a number in hundreds of feet, on the left side of the display. For example, the reading “FL 065” corresponds to the altitude of 6,500 feet, referenced to 29.92 inches of mercury (or 1013 hP) at sea level. Flight Level Altitude represents “pressure altitude,” and should not be confused with true altitude.
Please note that the displayed altitude may not agree exactly with the aircraft’s altimeter when flying below 18,000 feet, because encoders are preset to 29.92 inches of mercury. An encoder’s altitude transmission is automatically corrected for proper altimeter setting by a ground based computer, to present the correct altitude to the controller."
And I think the source selector that I've talked about will select which static ram will be used in conjuntion with the barometric capsule of the transponder (preset on 1013.25 ) in order to display the pressure altitude.

[Kit d'Rection KG]

Good quote, but that wasn't my point...

Mode S downlinks the selected altitude (I'm not talking about aircraft altitude), not the datum. That's where the problem lies.

[zerozero]

Never mind, topic already covered by previous poster!
:8

[alexban]

kit : during descent ,it's not so important what pressure you've selected, most important it's when you level off. And if you level off with incorrect altimeter setting, the transponder will transmit 1013.25 ,the ATC computer will transform this with proper altimeter setting and the ATC will ask you if you forgot to change altimeter.
If ,by any chance you're instructed to stop descent at FL XX,then you should change again to STD. If you forgot to do so,the ATC will again prompt you to do it.
I have visited one ATC facility ,and I've been amazed of the amount of new,computerized technology they have there. They improved a lot since primary radar....

[Chilli Monster]

during descent ,it's not so important what pressure you've selected

No, it's not - until you HIT the mountain when you've got low pressure met conditions :rolleyes:

Basic Altimetry 101!

[alexban]

chilli ,low pressure...how low? in a mountain area..you are enroute,cleared to descent to alt X which has a minimum 2000' obstacle clearance,am I right?
And, the moment you'll go bellow actual cleared altitude ,forgetting to change from STD to QNH, the ATC will advise you about that (their computer correct automatically for the QNH)..of course,you'll say now that in a remote,3rd world country....Then I'll say you have an EGPWS onboard..:)
Better to do all your checks correctly,and follow SOP ,and you'll be ok.
Basic CRM..

[Chilli Monster]

chilli ,low pressure...how low? in a mountain area..you are enroute,cleared to descent to alt X which has a minimum 2000' obstacle clearance,am I right?

Yes, but why erode that 2000ft safety margin by incorrect Altimeter Setting Procedures. As to how low - 2000 / 30 = 166mb - = QNH of 847mb. OK, it's never going to get that low, however, if we take a more realistic figure of 975mb (which have been possible) then your 2000ft above terrain now becomes 860ft.

If the terrain is NOT mountainous and the MSA is based on 1000ft obstacle clearance then this scenario could actually put you 140ft below the level of the obstacle.

And, the moment you'll go bellow actual cleared altitude ,forgetting to change from STD to QNH, the ATC will advise you about that (their computer correct automatically for the QNH)..of course,you'll say now that in a remote,3rd world country....

Not even third world country. Mode 'C' is not an instantaneous readout. By the time the data is interrogated and transmitted back to the radar screen you may have desended another 200ft.

Better to do all your checks correctly,and follow SOP ,and you'll be ok

True, SOP being (for the above reasons) that QNH is set as soon as you are cleared to an altitude. May I remind you of your answer which triggered this:

during descent ,it's not so important what pressure you've selected, most important it's when you level off.

Which is NOT SOP for many operators, nor is it SOP for many countries Altimeter Setting Regulations, and is the total opposite of "best practice" for the reasons I have explained.

Now do you see why your reasoning is flawed?

[alexban]

chilli, regarding your quote ,I was answering the topic starter fearing about TCAS conflicts during descent ,this is why I've said 'it's not important during descent ".