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Current QNH?

Old 13th May 2020, 12:19
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Current QNH?

A question to fellow controllers around the world.

What means of updating altimeter/QNH are being used in your country/at your unit? Do you have a display showing actual altimeter/QNH values for setting altimeters available in your unit? If not, what kind of routine is used for letting pilots know the actual QNH, and what is the update frequency for the values?

Thank you for any replies in advance!
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Old 14th May 2020, 09:49
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Originally Posted by FinnishATCO View Post
A question to fellow controllers around the world.

What means of updating altimeter/QNH are being used in your country/at your unit? Do you have a display showing actual altimeter/QNH values for setting altimeters available in your unit? If not, what kind of routine is used for letting pilots know the actual QNH, and what is the update frequency for the values?

Thank you for any replies in advance!
You're an ATCO and you have to ask a question like that?
I think not.
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Old 14th May 2020, 10:04
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Seems like a reasonable question to me. Sounds like the lockdown is getting to some....
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Old 14th May 2020, 14:10
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Originally Posted by FinnishATCO View Post
A question to fellow controllers around the world.

What means of updating altimeter/QNH are being used in your country/at your unit? Do you have a display showing actual altimeter/QNH values for setting altimeters available in your unit? If not, what kind of routine is used for letting pilots know the actual QNH, and what is the update frequency for the values?

Thank you for any replies in advance!
We have a live QNH/QFE readout which gives pressure to 0.1hPa.

As soon as the QNH or QFE moves up or down by 1hPa, our met page alerts and if itís a QNH change then we pass that to aircraft via RT.
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Old 14th May 2020, 21:17
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Here we have a live QNH feed displayed on a dedicated met screen with the feed coming from an on airport AWS
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Old 15th May 2020, 11:35
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QNH/QFE with the option for an inHg-setting, for the US pilots if they require it.

Normally without any decimals, if we want to see that, it can be selected.... but the system does the rounding down just fine for us.

Should start blinking with changes, but that does not work... so we'll just keep an eye on it. Latest QNH is put on the strips when given, so rather straight forward.
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Old 18th May 2020, 09:41
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We have an AWOS screen, updated instantly for pressure, temp, rvr etc changes.
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Old 18th May 2020, 10:27
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What is the rounding algorithm? 1 hPa equals 30 feet, give or take, and that is formally relevant.
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Old 18th May 2020, 11:02
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Originally Posted by FlightDetent View Post
What is the rounding algorithm? 1 hPa equals 30 feet, give or take, and that is formally relevant.
Rounding algorithm?? For what?

If it's the QNH/QFE, the decimal is always rounded down... so 1013.9 is given as 1013.

Oh, and the 1 hPa = appx. 30 feet.... that is only at ground level, due to the way an altimeter is set up, at 30.000 feet 1 hPa is roughly 70 feet. Which is the reason vertical separation is 2000 feet above FL290 (but is reduced in RVSM airspace with requirements on the altimeters of the aircraft flying there)
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Old 18th May 2020, 13:36
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QNH/QFE, the decimal is always rounded down... so 1013.9 is given as 1013.
Thanks for confirming. Sorry for the simplification, it's understood that 1 hPa is around 27 feet on ISA SL but increasing going up, and thus around 30' below TA/L where pilots need it.

Going deeper, if the value as displayed in single digits decimals fluctuates between x.9 and x+1.0 what is the filtering logic? So you do not have to issue a new report for every instant change. Genuine geeky question.

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Old 18th May 2020, 15:41
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Originally Posted by FlightDetent View Post
Thanks for confirming. Sorry for the simplification, it's understood that 1 hPa is around 27 feet on ISA SL but increasing going up, and thus around 30' below TA/L where pilots need it.
Technically, I cannot come up with any situation where you need to convert hPa into altitude.... the 27 feet is something you learn at the academy, and in most cases never use it again (as a pilot). As a controller there are more situations where it may be relevant, but since most transition levels are now (or should be) at least 1000 feet from the transition altitude, and those calculations are done automatically, that one is not often required anymore at most ATC units.

But yes, if it keeps flipping between 1013 and 1014 (because it's flipping the decimal 3,9 and 4,0), you'd have to pass it. But those situations are relatively rare.... at least where I work.
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Old 18th May 2020, 16:00
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Things getting lost in translation, thanks for the insights provided.

As a regular (weekly basis) visitor to QFE run ATS theatres with a QNH built machine it becomes second nature and a good tool to have in the box. Not to mention some back-office calculations or the annual assessments of technical competence.




Last edited by FlightDetent; 18th May 2020 at 16:10.
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Old 18th May 2020, 19:31
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Originally Posted by jmmoric
But yes, if it keeps flipping between 1013 and 1014 (because it's flipping the decimal 3,9 and 4,0), you'd have to pass it. But those situations are relatively rare.... at least where I work.
Back in my day, some 25 years ago, the QNH and/or QFE were set when the met report was issued and did not change until the next routine report was issued, usually 30 minutes later, or a 'significant change' occurred; and for QNH/QFE, a significant change meant a change in ambient pressure of 1.0 hPa or more from the last observation.

With the advent of systems which continually displayed the ambient pressure, some people seemed to get awfully excited by by changes of 0.1 hPa and wanted to change the QNH/QFE whenever the barometer changed through an integer hPa value, despite the fact that 15 seconds later it would change back to the original setting. All this did was add to workload for both ATC and pilots - and bear in mind that the difference that it make to an altimeter reading was a little under 3 feet.

When automated ATIS broadcasts were introduced there was a debate about how often the met data should be amended in the broadcast. Of course, the technology was capable of generating a message containing the values of each element that pertained at the moment the message was created. But then there were questions; does each iteration of the message get a different letter to identify it, or would two crews, each with information alpha have slightly different information, and did it matter? The reality was that it would make no difference operationally but may well cause confusion if two crews thought the had 'alpha' but found out that it was different, or more likely, if a crew picked up the ATIS and then went back a little later to see if it had been updated and got the same letter but different data. And at least one of the automated systems available at the time updated the QNH whenever it went through and integer value.

I was involved in a consultation, which included a face-to-face part for discussion and to help understanding of the issues and the equipment capability, a on how automated ATIS broadcasts should work. The general view seemed to be that updates every 30 minutes and special observations for significant changes worked OK. But then one bright-spark pilot said that if more up-to-date information was available the it was essential that it was passed to crews, and no-one was going to change his mind. I don't recall what became of the consultation, I handed the results over to someone else to take it further and went to do more interesting things....well, mostly more interesting.

Lots of things have changed since I was a boy, and there are now standards for things like D-ATIS which were just a dream when I used to talk to the aeroplanes. But, in the UK at least (CAP746), I note that the QNH is still only supposed to be updated at the next routine observation, or when the ambient pressure changes by 1.0 hPa or more since the last observation. So,hopefully, changing the QNH 'because it's flipping the decimal 3,9 and 4,0' will be a very rare occurrence.....in fact, never.
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Old 18th May 2020, 21:04
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I guess chevvron is feeling the pressure?
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Old 19th May 2020, 12:26
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Originally Posted by LookingForAJob View Post
Back in my day, some 25 years ago....
It's a shame you cannot give "upvotes" here, really appreciate the reply.

I just went through the observers course... they've found out we could handle the observing from the tower, and moved the meteorologist.... (actually they fired the observer many years ago, because the meteorologist could do that)... and since the system is automated, and certified to operate without anyone doing anything anymore.... We're only supposed to poke it if anything is really wrong...
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Old 19th May 2020, 21:27
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I understand that the QNH for an airfield can be read directly from an instrument at the airfield.

When a Regional QNH (UK) is given how is that determined and obtained by approach controllers?

PPL
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Old 20th May 2020, 05:24
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Originally Posted by jmmoric View Post
Rounding algorithm?? For what?

If it's the QNH/QFE, the decimal is always rounded down... so 1013.9 is given as 1013.

Oh, and the 1 hPa = appx. 30 feet.... that is only at ground level, due to the way an altimeter is set up, at 30.000 feet 1 hPa is roughly 70 feet. Which is the reason vertical separation is 2000 feet above FL290 (but is reduced in RVSM airspace with requirements on the altimeters of the aircraft flying there)
Doesn't that depend where your ground is?
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Old 20th May 2020, 06:53
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Area QNH based on the lowest forecast QNH in the whole region. I used to get it from GAMET.
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Old 20th May 2020, 22:40
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Originally Posted by Jim59 View Post
When a Regional QNH (UK) is given how is that determined and obtained by approach controllers?
Hopefully, never, since approaches are not based on what is a rough and ready forecast (UK IAIP defines a Regional Pressure Setting as for use only when an appropriate airfield QNH is not available).

In practice the Met Office disseminate Regional Pressure Settings as their forecast for the current hour and the next hour - originally via Afpex, most of us see them nowadays on the Met Office Aviation Met site.
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Old 21st May 2020, 10:00
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Originally Posted by LastStandards
Hopefully, never, since approaches are not based on what is a rough and ready forecast (UK IAIP defines a Regional Pressure Setting as for use only when an appropriate airfield QNH is not available).
But an approach controller (it will usually be an approach controller but in reality any controller in the UK) should be able to provide any RPS on request as part of the FIS.
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