aviation question related to vatsim/real world ATC

SeanGa wrote:

When the ATC gives you a pressure setting for the altimeter (e.g QNH1022) do you keep this setting all the time, or only until you pass 18000 ft and then put it back on when you descend past 18000ft?

thanks in advance

When you leave an airport, the current ATIS will give you the QNH (altitude AMSL), which you put into the aircraft's altimeter. ATC may give you the QNH when you are talking to them requesting departure clearence, but it isn't essential. Whilst you are in their airspace, you will use this QNH.

If you ascend through 3000ft, then you change the altimeter subscale setting to QNE (1013mb) which is the universal datum for aircraft above transition altitude. Note, that in the USA, transition altitude is 18000ft and the pressure subscale is 29.92in HG and that in Europe, the transition altitude differs between European countries.

When you descend back down through transition altitude, you will be in contact with the destination airport if you are flying IFR and you will be given the QNH and possibly QFE (uncommon) to key into the altimeter subscale, which must be read back to confirm accuracy. If you are flying VFR and aren't yet in contact with somebody( i.e. don't have a VFR flight following (US) or a FIS (UK), then you will listen to ATIS and get the current information for the destination airport, and will notify ATC that you have this information. When flying in / out and around airspace below transition altitude, you always use QNH - QFE is just for MATZ penetration and circuits if the traffic density is low.

In summary, ATC / ATIS gives you the QNH which you use until directed otherwise. QNE is used above transition altitude at all times.

Ahhh... lovely! pleasure sweet pleasure. great response.

another question though, what is QNE?

QNE is the universal altimeter subscale datum that is used at or above transition altitude (including the transition layer and any transition levels). i.e. 29.92 InHg for American altimeters and 1013.2hPa for European altimeters.

That means all aircraft have the same point at which to measure their current altitude from which avoids collisions and is a whole lot safer!

thanks again 🙂

When you leave an airport, the current ATIS will give you the QNH (altitude AMSL), which you put into the aircraft's altimeter. ATC may give you the QNH when you are talking to them requesting departure clearence, but it isn't essential. Whilst you are in their airspace, you will use this QNH.

If you ascend through 3000ft, then you change the altimeter subscale setting to QNE (1013mb) which is the universal datum for aircraft above transition altitude. Note, that in the USA, transition altitude is 18000ft and the pressure subscale is 29.92in HG and that in Europe, the transition altitude differs between European countries.

When you descend back down through transition altitude, you will be in contact with the destination airport if you are flying IFR and you will be given the QNH and possibly QFE (uncommon) to key into the altimeter subscale, which must be read back to confirm accuracy. If you are flying VFR and aren't yet in contact with somebody( i.e. don't have a VFR flight following (US) or a FIS (UK), then you will listen to ATIS and get the current information for the destination airport, and will notify ATC that you have this information. When flying in / out and around airspace below transition altitude, you always use QNH

A small clarification on a correct reply (details really). As noted, when you leave an airport you set your altimeter to local QNH (this is usually given along with your engine startup clearance and is of course contained in the ATIS).

Ascending aircraft turn altimeter to 29.92 (1013) crossing the Transition Altitude, while descending aircraft turn to local QNH crossing the Transition Level.

I did not understand what you meant by "If you ascend through 3000ft, then you change the altimeter subscale setting to QNE (1013mb)"

Bindolaf wrote:

A small clarification on a correct reply (details really). As noted, when you leave an airport you set your altimeter to local QNH (this is usually given along with your engine startup clearance and is of course contained in the ATIS).

Ascending aircraft turn altimeter to 29.92 (1013) crossing the Transition Altitude, while descending aircraft turn to local QNH crossing the Transition Level.

I did not understand what you meant by "If you ascend through 3000ft, then you change the altimeter subscale setting to QNE (1013mb)"

I meant if you ascend through 3000ft (i.e. the Transition Altitude in the UK; you don't have to if you are flying VFR happily at 2000ft) then you need to change the altimeter subscale setting (i.e. from the three subscale settings to choose from, QNH, QFE, QNE) to QNE which is 29.92 / 1013.

I hope that clears up what I meant

TA is 3000 across the UK? Really? Wow, seems low 🙂 Thanks for clarifying.

Bindolaf wrote:

TA is 3000 across the UK? Really? Wow, seems low 🙂 Thanks for clarifying.

Except they decided to make it a bit more complicated. TA in the London TMA is 6000ft, around the controlled airspace around Manchester TA is 5000ft and around Birmingham controlled airspace, the TA is 4000ft etc.

That's how it is usually, each airport has its own TA. Germany is an exception I know, with a TA of 5000 across the board (and the US of course with 18000).