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A320 preselecting QNH

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A320 preselecting QNH

Old 7th Nov 2021, 14:40
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A320 preselecting QNH

Anybody see an issue with preselecting QHN on the altimeter in cruise?
will the flight guidance start to chase the temporarily selected QNH ?

thanks
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Old 7th Nov 2021, 17:11
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No, it wont chase the level as you set QNH and I personally can see no problem with that being done. Some people mention it may worry ATC as they see you changing settings briefly, but Im not even convinced the short period it takes to set QNH and go back to standard again is long enough for that happen. Maybe others know better.
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Old 7th Nov 2021, 17:23
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ATC? They only see altitude reported based on STD, they do not care at all whatever you set. That said, usually ATC updating is only once every few seconds, so they don't see it anyway. Preselecting QNH has been SOP in all airlines i have worked in so far, no problem with that.
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Old 7th Nov 2021, 19:22
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Agreed - personally I believe it’s nonsense, but whomever it was that I heard spout the suggestion was intimating it was something to do with controllers being able to see your selections via mode S or ADSB or something. I don’t know enough to say whether it’s true or not, nor have the inclination to find out. It never caused me any problems in the past.
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Old 7th Nov 2021, 21:40
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The selected altitude = FCU value is being transmitted. Temporary moving from STD to QNH does not change that.

The ASI read-out on PFD is affected, but that one is not sent out (STD-based value is, hardwired).
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Old 8th Nov 2021, 03:57
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And when it comes to guidance, why does it not capture the QNH altitude?
is it stated in the FCOM?
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Old 8th Nov 2021, 07:57
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Obviously because it is the correct thing to do. Don't think I saw a written explanation.

Technically, the ALT(CRZ) should hold the isobaric surface, and changing the Kollman's window does not alter the readout from the static port. Speculating again, it could be engineered this way to achieve the goal.

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Old 10th Nov 2021, 11:04
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Brilliant, thank you
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Old 10th Nov 2021, 21:28
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There are a few misconceptions here. Yes, your current altitude is sent out with reference to STD and corrected by ATC's systems for QNH. An erroneous QNH setting will have no effect on ATC's end. However the selected altitude that some ATC systems can see through the Mode S ES is referenced to the QNH set on the FCU even if the altimeter is in STD mode.

So there is no problem with going to QNH on the altimeter, preselecting your QNH, and returning to STD. The only caveat is that if the QNH setting is very different from standard and ATC happens to look at your selected altitude, then they could query you as to what altitude you are going to level off at. In which case you can just set the QNH to 1013 / 29.92 and everyone's happy.

Personally I set the QNH where the SOP says to set the QNH (just before transition level). However until this point the QNH set on the altimeter is some random setting from 7 hours ago and ATC has never complained so the preset method suggested above really isn't a problem.
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Old 11th Nov 2021, 07:02
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However the selected altitude that some ATC systems can see through the Mode S ES is referenced to the QNH set on the FCU even if the altimeter is in STD mode.
Is that the bug reported about 5 years ago?

I understood so far 2 values are transmitted. The FCU selected ALT and the baro-pressure setting. For some weird technical reason, the S mode transmits the last QNH at all times. ATC systems at the moment ignore the selected pressure value, so no harm is being done yet.

Not sure where to find the original Information Telex from Airbus, sure someone has access to it ...?

note: mode C transmits the STD-referenced readout of the ASI at all times.
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Old 12th Nov 2021, 04:56
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ATC systems at the moment ignore the selected pressure value
Our SOP says differently but it's got other mistakes. Do you have a reference for that statement? Thank you.
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Old 16th Nov 2021, 23:30
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Still struggle to find the OIT, it's gotten pointless without it.

My other memory is the STAC visit and a 10 minutes seat next to one of the Essex Radar Magicians. I actually observed the readout on the screen being different and wrong. His explanation was 'dont care'
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Old 17th Nov 2021, 00:35
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That was my understanding as well.
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Old 23rd Dec 2021, 16:21
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What is the impact on TCAS of this? Say we happen to pre-select a lower QNH whilst directly above opposite traffic passing a thousand feet below? A trainer from Asia (nice guy but not sure he was correct) had this banned via the SOP on fear it could trigger a TCAS RA. In fact, it was a remark on my line check.
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Old 23rd Dec 2021, 16:43
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TCAS should work with XPDR mode C altitude.
This should be hardwired to STD.
Remember, there is only one static pressure. Changing the QNH is calibrating the scale on the altimeter, not changing the source pressure itself.

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Old 23rd Dec 2021, 17:35
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Very interesting thread. Thanks flightdetent.
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