How does ATC handle changing QNH/altimeter values?

Question

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?

Answer 1

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

Answer 2

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.

Answer 3

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.