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Obviously for aircraft arriving to or departing from an airport/airspace (as relevant), they listen to the ATIS and/or are given the QNH.

But what happens if that pressure changes? Even a fairly small change can make a big difference when we're dealing with 1000ft of vertical separation (which can be much less by the time we account for human margins for error).

Is the new QNH announced to the entire frequency, or similarly the change of ATIS, which each aircraft would then be expected to check? Or would it just be updated when assigning a new altitude to any aircraft already in the airspace?

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  • $\begingroup$ Bear in mind that in terms of separation between aircraft, the actual QNH doesn't matter so long as everyone is using the same value. $\endgroup$ – Dan May 17 '16 at 8:03
  • $\begingroup$ @dan that's my point - I'm asking how everyone is updated when the value changes to avoid people using different values and therefore being closer than they expected $\endgroup$ – Jon Story May 17 '16 at 9:52
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In the US, ATC simply broadcasts a notification on whatever frequency is needed. For the terminal environment, section 2-9-2 of the ATC orders says:

Broadcast on all appropriate frequencies to advise aircraft of a change in the ATIS code/message.

The phraseology is usually something like this:

Attention all aircraft, Bowman information Kilo is now current.

For en route operations (see section 2-7-2) the ARTCC gives altimeter setting updates directly:

Attention all aircraft, the Nashville altimeter now 2992

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    $\begingroup$ Similar to UK although the broadcast call sign is "All stations". $\endgroup$ – Simon May 16 '16 at 18:38
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    $\begingroup$ PLEASE NOTE: This answer is only correct for FAA rules! In all other countries which work according to ICAO rules a broadcast of QNH changed is PROHIBITED! So eg. in european airspace we have to address every aircraft to provide the revised QNH‘ $\endgroup$ – pcfreakxx Feb 11 at 16:21
  • $\begingroup$ @pcfreakxx That seems excessive. What about if it's a change in ATIS? Does that have to be given individually? $\endgroup$ – DJClayworth Feb 11 at 19:55
  • $\begingroup$ @DJClayworth I don’t get exactly what you mean, but if the ATIS changes we will tell the aircraft to „check info G“ or whatever letter it might be $\endgroup$ – pcfreakxx Feb 11 at 19:57
  • $\begingroup$ @pcfreakxx I mean do you have to tell each aircraft individually that "Information Golf" is now current? $\endgroup$ – DJClayworth Feb 11 at 20:08
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The simple answer is, usually the altimeter setting does not change fast enough for it to be an issue.

Near ground level, one-hundredth of an inch of mercury (00.01 inHg) corresponds to roughly ten feet of pressure altitude. Thus one inch of mercury corresponds to one thousand feet of altitude. (This rough rule of thumb can be seen by looking at a lowest-usable-flight-level table: Below 2992 FL180 is unusable, below 2892 FL190 unusable, below 2792 FL200 unusable.) This is close to negligible for separation purposes, given that minimum IFR-IFR vertical separation is 1000 feet and ATC scopes only display altitude to the nearest 100 feet. Of course arrival aircraft are more concerned with having the latest altimeter setting, especially if the ceilings are close to instrument approach minimums!

If the atmospheric pressure is changing quickly, the weather station will note that with the remark PRESRR (rising rapidly) or PRESFR (falling rapidly). This provides a heads-up to pilots and ATC to keep an eye on the setting. What is "rapidly"? 00.06 inHg (sixty feet) per hour! Not a gigantic difference. It's not super rare to see this at my facility but it's not an everyday occurrence by any means.

When aircraft are enroute below the flight levels a controller should advise them of the nearby altimeter setting "at least one time when operating within [their] area of jurisdiction" (7110.65 2–7–2 c1). Essentially every time an aircraft is switched to a new facility they will get the local altimeter setting, and by listening when new arrivals come onto the frequency they can update as needed. If an aircraft is showing drastically off (300+ feet) from their assigned altitude, ATC will re-issue them the altimeter and ask them to confirm their indicated altitude.

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Where ICAO rules apply (e.g. most of the world), broadcasting a change in QNH to the entire frequency is not sufficient. The QNH is considered (for good reason) to be safety critical information, and so each aircrew most positively acknowledge that they have correctly received it. This means informing each aircraft on the frequency of the new QNH, and wait for them to read back. I'm sure this is not always actually done, but it is what the rules say.

Reference: ICAO document 9432, 2.8.3.5:

The following information shall always be read back:

[...]

c) runway-in-use, altimeter settings, SSR codes, level instructions, heading and speed instructions and, whether issued by the controller or contained in ATIS broadcasts, transition levels.

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  • $\begingroup$ Do you have a reference for the need to inform each aircraft individually? $\endgroup$ – Jon Story May 18 '16 at 13:19
  • $\begingroup$ I may be wrong, but it looks to me like section 2.8.3 is specifically about clearances? It makes sense to require a full readback after issuing a clearance to an individual aircraft, but that seems like a different scenario from informing all aircraft on frequency about an altimeter change. $\endgroup$ – Pondlife May 18 '16 at 13:34
  • $\begingroup$ @Pondlife You do have a point. Document 4444 and Annex 11 contain similar paragraphs requiring readback of altimeter settings, but those are also contained in sections specific to ATC clearances. The requirement to read back QNH is one thing though, another point is do controllers have to inform of a change in QNH at all? The obvious answer is yes, but I can't seem to find and source for this in ICAO documents. $\endgroup$ – expeditedescent May 18 '16 at 16:26
  • $\begingroup$ From document 4444: "7.4.1.2.1 Prior to taxiing for take-off, aircraft shall be advised of the following elements of information, in the order listed, with the exception of such elements which it is known the aircraft has already received: [...] c) the QNH altimeter setting and, either on a regular basis in accordance with local arrangements or if so requested by the aircraft, the QFE altimeter setting;" - this suggests that crews should recieve regular updates of QFE settings, but not necessarily QNH. $\endgroup$ – expeditedescent May 18 '16 at 16:28

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