When descending, when to switch to local altimeter

[scandinavian13 5,192]

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.

Exactly. That's how I would put it as well.

[Chrjs 61]

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.

I gave this idea some thoughts as well for quite some time, and i don't think the FMC takes the entered QNH on the Des/Forecast page into consideration for his descent calculation. For example on my descent today from FL380 down to PANC, local altimeter 2908 (QNH983). After switching to local pressure right at FL180 i was around 800ft below the calculated GP and entered VNAV-Speed until catching up again with VNAV-Path some miles later. In this case not a problem, but in the opposite direction (high altimeter and low transition level) - and this situation is quite common in Europe - this might be a "serious" problem shortly before the final approach fix. So here are my questions:- Does VNAV take the local pressure (entered anywhere in the CDU) into consideration for the descent calculation?- if not, how do especially European pilots cope with this "problem"?Thanks for your thoughts!

[Nuno Pinto 1,399]

Real pilots won't let the FMC/CDU handle the whole approach. Most are done "manually" as ATC allows further descents and that's how it should be done, including smooth transitions from altitude to FL and vice-versa. Some people prefer to let the electronics do all the work from takeoff to landing, and that's just fine, it's a way of enjoying the simulator, but it's inaccurate.After watching 8 or 9 Pilot's Eye videos, my view of the takeoff and approach procedures changed completely, and i learned very very much with those.

[davidz 26]

You should only switch over at the transition level. If ATC need to stop your descent for something they didn't foresee (e.g. RECLEARED FL160) and you have already switched to QNH, you will be maintaining 16000 FT AMSL rather than FL160.

[Fabo 213]

David: I do not agree here. For one, you get your QNH on clearance to descend to altitudes. One thing is, you might forget it, other you might forget to reset it at all.As regard stop descend clearance, you can always push the STD button there.As for VNAV des path calculation, I am pretty sure VNAV accounts for QNH, as des path always changes slightly when I input my QNH after TOD.

[vololiberista 205]

Switch to QNH when passing from FL to given altitude. (Conversely switch to FL 1013.2mb when passing from an altitude to a given FL). Switch to QFE once established on the glide slope.Simple really!!vololiberista

[davidz 26]

David: I do not agree here. For one, you get your QNH on clearance to descend to altitudes. One thing is, you might forget it, other you might forget to reset it at all.

In modern airliners, you can easily preset the QNH. Also, you should be monitoring your descent and be making a mental note of when you pass transition level. In fact, it is standard practice at airlines to callout something like "Transition, 1010 set".With the logic of "forgetting", the same could be said about forgetting to return to Standard for a stop descend instruction. The difference is that setting QNH when passing transition is a normal item, something that happens every flight. A stop descend is less common and it is easy to forget to do something when you don't often do it, especially if you are busy doing something else, like rebriefing for a runway change.It is less likely for the pilots to set the altimeter when passing transition than when issued a stop climb/descend instruction, so it is better practice to do this at transition.

[Fabo 213]

vololiberista: I personally find switching to QFE on primary altimeter detrimental on flight safety, for reasons incl. the facts that all altitudes - alt check on GS, MSAs, hill altitudes etc. are MSL. But it is nice to have it on secondary altimeter, if there is one.David: Fair points.

[veldthui 0]

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.

The XPDR usually has its own altimeter and is always set to to standard so that they all give the same altitude regardless of what QNH is set for. This avoids pilots setting incorrect QNH and the XPDR then sending wrong altitude data.

[ontheair 3]

this is french but obvious !in France (to be checked for other European countries) : TL = first IFR level just above the altitude = TA - 28 * (QNH - 1013,25)0 ft <= transition layer < 1000 ft

[skysurfer 76]

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.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.

Hi guys.You cannot reset altimeter before TL. The reason why is because ATC can cancel descend clearance and assign different (above TL) altitude again if situation dictate. When I was air controller I did that a lot and it is not because ATC screws up and try to recover. There are times when another crew does not follow correct procedures. We had lots of problems with Asiana, Korean Air, Turkish, Iran Air crews. Especially when you fly in foreign airspace like Russia, where metric system and QFE below TL is still in force. Crews have to use QFE, so they have to do some calculations to match QNH to QFE. Sometimes they get stuck between two levels and that’s the time when air controller becomes mad and has to do something. That’s the real world.

[Spin737 102]

Crews have to use QFE, so they have to do some calculations to match QNH to QFE. Sometimes they get stuck between two levels and that’s the time when air controller becomes mad and has to do something. That’s the real world.

I did that for 1.5 years, and it was a pain.I've flown at 3 airlines, and SOP at all three was when approaching the transition, you change, not before.

[vololiberista 205]

vololiberista: I personally find switching to QFE on primary altimeter detrimental on flight safety, for reasons incl. the facts that all altitudes - alt check on GS, MSAs, hill altitudes etc. are MSL. But it is nice to have it on secondary altimeter, if there is one.David: Fair points.

Usually the standby is on QNH and the Captain's co-pilot's set to QFE. It is detrimental to flight safety to not set QFE!! I can think of at least two fatal accidents and a few near misses at Biggin Hill London because QFE had not been set!vololiberista

[Fabo 213]

Are you definitely not mixing up QNH and QFE? Fatal accident because QFE has not been set, that sounds fundamentaly wrong.

[vololiberista 205]

Are you definitely not mixing up QNH and QFE? Fatal accident because QFE has not been set, that sounds fundamentaly wrong.

Certainly not! Do you know the difference between QNH and QFE? The airport in question is at a height of 600ft. The circuit is 1,000ft agl.Whoever flies that on a QNH setting will be at the wrong height. And, as I have said has caused fatal accidents.vololiberista