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-   -   Current QNH? (https://www.pprune.org/atc-issues/632441-current-qnh.html)

FinnishATCO 13th May 2020 12:19

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!

chevvron 14th May 2020 09:49


Originally Posted by FinnishATCO (Post 10781347)
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.

ShyTorque 14th May 2020 10:04

Seems like a reasonable question to me. Sounds like the lockdown is getting to some....

Gonzo 14th May 2020 14:10


Originally Posted by FinnishATCO (Post 10781347)
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 it’s a QNH change then we pass that to aircraft via RT.

Kooim00 14th May 2020 21:17

Here we have a live QNH feed displayed on a dedicated met screen with the feed coming from an on airport AWS

jmmoric 15th May 2020 11:35

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.

oneo 18th May 2020 09:41

We have an AWOS screen, updated instantly for pressure, temp, rvr etc changes.

FlightDetent 18th May 2020 10:27

What is the rounding algorithm? 1 hPa equals 30 feet, give or take, and that is formally relevant.

jmmoric 18th May 2020 11:02


Originally Posted by FlightDetent (Post 10785824)
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)

FlightDetent 18th May 2020 13:36


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.


jmmoric 18th May 2020 15:41


Originally Posted by FlightDetent (Post 10786004)
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.

FlightDetent 18th May 2020 16:00

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. :ok:

https://cimg5.ibsrv.net/gimg/pprune....b345911371.png


42go 18th May 2020 21:04

I guess chevvron is feeling the pressure?

jmmoric 19th May 2020 12:26


Originally Posted by LookingForAJob (Post 10786269)
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...

Jim59 19th May 2020 21:27

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

Islandlad 20th May 2020 05:24


Originally Posted by jmmoric (Post 10785852)
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?

LastStandards 20th May 2020 22:40


Originally Posted by Jim59 (Post 10787223)
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.

grizzled 21st May 2020 20:53


Originally Posted by 42go (Post 10786329)
I guess chevvron is feeling the pressure?

His reaction is a barometer of these difficult times...

DaveReidUK 21st May 2020 21:52

I've always though he was a bit of a mercurial character.


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