A320 preselecting QNH
Joined: Mar 2018
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From: Home
No, it won’t 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 I’m 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.
Joined: Mar 2001
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From: I wouldn't know.
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.
Joined: Mar 2018
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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.
Only half a speed-brake
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From: Commuting not home
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).
The ASI read-out on PFD is affected, but that one is not sent out (STD-based value is, hardwired).
Only half a speed-brake
Joined: Apr 2003
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From: Commuting not home
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.
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.
Joined: Jan 2007
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From: Canada
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.
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.
Only half a speed-brake
Joined: Apr 2003
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From: Commuting not home
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.
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.
Only half a speed-brake
Joined: Apr 2003
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From: Commuting not home
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'
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'
Joined: Apr 2013
Aviation Qualifications: CPL
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From: Neither here or there
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.
Only half a speed-brake
Joined: Apr 2003
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From: Commuting not home
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.
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.
