When descending, when to switch to local altimeter

[tjeuten 6]

Ok all you NGX pilots, here's a little question:In Europe, TL is often quite low (FL050 feet is no exception).If I understood correctly you are supposed to switch to local QNH when you cross the TL.But when descending with the NGX, especially in VNAV, and there is a big difference between standard pressure and local QNH, when switching to local that low often results in the plane being a few hundreds of feet off the scheduled descend path, which often results in me having to go to V/S mode to make it descend faster (which in turn increases speed again, not good) in order to capture the scheduled descend path again to line up properly with the approach glide slope.Is this the way it's done in real life too ? Or can you switch to local QNH once the controllers clears you to an altitude below TL ?So for instance say TL is FL060 and I'm at FL100. The controller tells me "descend to altitude 5000 feet, QNH 1033". Do I effectively need to wait until I cross FL060 before switching to QNH or can I switch before it (for instance crossing FL090) in order to have more time to compensate on the descend path for the pressure deviation ?Just want to know if that would be legal or not, and what is done in the real world ?ThanksTJ

[Guest dlrk]

You can't change the altimeter setting until the transition level, that can cause a collision or loss of separation, since two aircraft in the same airspace will be using different altimeter settings. The solution to your problem is to enter the transition level and destination QNH and the DES FORECAST page, allowing VNAV to compensate automatically.

[badderjet 181]

So for instance say TL is FL060 and I'm at FL100. The controller tells me "descend to altitude 5000 feet, QNH 1033". Do I effectively need to wait until I cross FL060 before switching to QNH or can I switch before it (for instance crossing FL090) in order to have more time to compensate on the descend path for the pressure deviation ?

TBH, I must admit I'm not aware of how more or less 'legal' the one or other way might me. However, since you're effectively cleared to an altitude, I would suspect you can safely switch to local QNH even if still above TL. In fact I seem to remember r/w crews to have done it that way. Any NG driver is welcome to chime in here in order to confirm that, as I'd be curious as well.

You can't change the altimeter setting until the transition level, that can cause a collision or loss of separation, since two aircraft in the same airspace will be using different altimeter settings. The solution to your problem is to enter the transition level and destination QNH and the DES FORECAST page, allowing VNAV to compensate automatically.

Ah yes, your solution might be the correct way of handling things on the NG. However I question you'd risk a loss of separation - the change in altimeter setting, whenever it is performed, doesn't quite change your descent path.

[tjeuten 6]

You can't change the altimeter setting until the transition level, that can cause a collision or loss of separation, since two aircraft in the same airspace will be using different altimeter settings. The solution to your problem is to enter the transition level and destination QNH and the DES FORECAST page, allowing VNAV to compensate automatically.

True, but I'm already doing that...edit: ow, just realizing you can actually enter the destination QNH too on the DES FORECAST page. Good point ! Up until now I was only entering the TL but not the QNH... will try tonight ! Thanks

Ah yes, your solution might be the correct way of handling things on the NG. However I question you'd risk a loss of separation - the change in altimeter setting, whenever it is performed, doesn't quite change your descent path.

Also true

[GAJ52 54]

You can't change the altimeter setting until the transition level, that can cause a collision or loss of separation, since two aircraft in the same airspace will be using different altimeter settings

Thats not strictly true because you have been cleared to an Altitude not a Flight Level and should be descending anyway so there is no danger of meeting another aircraft unless ATC has made a mistake. I believe its perfectly acceptable to select QNH when cleared to an Altitude, thats why ATC will say "Cleared to 5000 ft QNH 1028". The only danger here is if ATC stops your descent prematurely and levels you off at a FL again in which case you would have to select 'Standard' pressure again.Glen

[Jetstream96 20]

I consider you should only switch to local QNH when passing TL. If I am correct, the XPDR send altitude data to ATC. If your altimeter setting is incorrect, ATC gets wrong altitude data if you. That can be really dangerous. Don't try to do this.Anyway we can enter QNH in DES Forecast page. That is helpful.

[simbio 56]

If you are few hundreds of feet above the vnav path is not dramatic you can always fix that later.You change QNH on follow the chart of your airport destination, but every airport have it's own TL.They usually vector you and let you bleed the speed by following an heading then when you match the correct altitude they ask to turn to intercept the normal descent path.If you ask for a particular procedure then is up you to judge if you can do it.Do you fly on Vatsim ?

[GAJ52 54]

The original question was about Europe, here is an exreact from the UK AIP

5.3.1 When an aircraft is descended from a Flight Level to an Altitude preparatory to commencing approach for landing, ATC will

pass the appropriate aerodrome QNH. On vacating the Flight Level, the pilot will change to the aerodrome QNH unless further Flight

Level vacating reports have been requested by ATC, in which case, the aerodrome QNH will be set following the final Flight Level

vacating report You select the given QNH when Leaving the Last reported Flight level i.e. Leaving FL090 for 3000ft you would set QNH as you leave FL090 not passing the TL Sorry if the text is small it wouldn't respond to a font size change.

[aentwis 61]

You are all making a fundamental error in this discussion. Do not confuse Transition Level with Transition Altitude.The TA is the set figure, 18000ft in the US, in general 6000ft in UK, 5000ft in Germany, and so on, and can deviate from airport to airport. However, the figure is pretty much always on the approach plate for the runway and airport.The transition level is defined by ATC. It is defined as the first available level above the TA and ensures a minimum spacing of 1000ft between aircraft at a local QNH and aircraft operating at 1013/29.92...Thanks Glen for posting the UK AIP info on this, makes it nice and simple to understand...Andrew

[DY-361 0]

This table might be usefull for determining the transition level, I found it on wikipediaQNH_________________Transition altitude (in feet)(in millibars)_____3,000____ 4,000_____ 5,000____ 6,000____ 18,0001032–1050_____FL025____ FL035____ FL045____ FL055____ FL1751014–1032_____FL030____ FL040____ FL050____ FL060____ FL180996-1013______ FL035____ FL045____ FL055____ FL065____ FL185978-995_______ FL040____ FL050____ FL060____ FL070____ FL190960-977_______ FL045____ FL055____ FL065____ FL075____ FL195943-959_______ FL050____ FL060____ FL070____ FL080____ FL200

[tjeuten 6]

The original question was about Europe, here is an exreact from the UK AIP

5.3.1 When an aircraft is descended from a Flight Level to an Altitude preparatory to commencing approach for landing, ATC will

pass the appropriate aerodrome QNH. On vacating the Flight Level, the pilot will change to the aerodrome QNH unless further Flight

Level vacating reports have been requested by ATC, in which case, the aerodrome QNH will be set following the final Flight Level

vacating reportYou select the given QNH when Leaving the Last reported Flight level i.e. Leaving FL090 for 3000ft you would set QNH as you leave FL090 not passing the TLSorry if the text is small it wouldn't respond to a font size change.

Ok, so some contradicting points of view in all the replies here.Some say we should wait until effectively passing TL before switching, and here the UK AIP says it's basically OK (and maybe mandatory ??) to switch to local QNH from the moment you are cleared to an altitude below TL.Leaves me just the more confused :)

[Paul Deemer 13]

I think you guys are making this a little more complicated than it needs to be. When descending I always set the Altimeter after passing 18,000 feet unless I am in Europe somewhere where the transition is 6,000 feet? When using Activesky and Xgauge in the Cockpit its easy to see what the altimeter is at your arrival airport. Soon as you pass the trans altitude set the altimeter to the correct setting.Even if your at a level above your Trans altitude and given permission to descend to a level below it I would wait until the trans level before resetting the altimeter that way you never forget to do so. There is always something that could happen that could cause you to have to level off before the trans altitude and then maybe you forget to set it back to standard. If you always wait until the trans altitude your reflexes will kick in at the appropriate time by habit. This makes the most sense to me. If you randomly change procedures and mix them up your more likely to forget something.

[Airbus Commander 563]

You set the local QNH upon being cleared to an altitude below the transition altitude. Lets take an example familiar to me, say on the approach into DUB.*Shamrock 10E, is level @FL080.Approach: "Shamrock 10E, descend to altitude 3,000', direct LAPMO and you are cleared for the ILS approach, report intercepting the localiser, QNH 1020."Me: "Direct LAPMO, 3,000 feet, cleared on the ILS, QNH 1020, and will report on the localiser. Shamrock 10E"At this point, you select 3,000' on the FCU, [open descent or V/S, whatever takes your fancy] , set 1020 on the altimeter, and then get direct LAPMO in the FMGS. Arm the app mode on the FCU as well if reasonable.The reason we do this is to avoid confusion at a later point, ie at 5'000' [the transition altitude] trying to recall the the QNH setting as well as preparing to intercept the ILS and doing a million and one other things, it's just easier.Capt. Rónán O Cadhain.

[oskrypuch 36]

Here is a rule of thumb, that will satisfy the regs and is easy to remember. All changes of the altimeter to/from QNH should be done while in the flight levels, and ONLY WHEN in a climb or descent that straddles the transition altitude.Done.* Orest

[japascoe 98]

The reason we do this is to avoid confusion at a later point, ie at 5'000' [the transition altitude] trying to recall the the QNH setting as well as preparing to intercept the ILS and doing a million and one other things, it's just easier.Capt. Rónán O Cadhain.

On the NGX the point about avoiding confusion is negated since unlike on the Airbus you can pre-select the QNH and then just hit the button when passing TL. Not disagreeing with the rest of your post, but just wanted to point this out.