When do I change pressure form STD to QNH?

[Edward Berkley]

Just wondering what altitude would be the right height to change from STD to QNH.

[Sebastien Bartosz]

At the Transition Level. Can be found on approach plates.

[Andreas Fuchs]

Errrrr, well, no! Legally it is "correct" to change from STD to QNH/local altimeter setting when p[Mod - Happy Thoughts]ing transition level. Practically this will NOT be done by most professional pilots, because the chance of forgetting it is just too high. The best technique (in my opinion) and actual SOP (Standard Operating Procedure) with almost all operators and real world pilots is to set QNH/local altimeter setting right away when being instructed to descend to and altitude, coming from a flight level.

[Randy Tyndall 1087023]

Seek and ye shall find...straight out of the Pilot resource Center that should be read by every new member and reread by existing members from time to time.

 

Altitude versus flight level

 

This is one area that is commonly misunderstood. Simply, altitudes are in reference to an altimeter setting from a local weather station, and flight levels are in reference to a standard altimeter setting. The world has agreed that a standard altimeter setting is 29.92"Hg or 1013 mb. As was discussed above, the point at which a pilot changes from a local altimeter setting to the standard setting is called the "transition altitude"; the point at which a pilot changes from the standard altimeter setting to the local altimeter setting is called the "transition level". When ATC [Mod - Happy Thoughts]igns an altitude below the transition level, it will be expressed in thousands/hundreds of feet/meters. For example, if a pilot is [Mod - Happy Thoughts]igned an altitude of 10,000' and that altitude is below the transition altitude, the controller will express it as "one zero thousand". If that same pilot was in a different country and above the transition altitude at 10,000', the controller would express it as "flight level one zero zero".

 

Randy

[Thimo Koolen]

At the Transition Level. Can be found on approach plates.

 

Not always. In Europe, often the transition altitude (ft to FL) is determined and on the charts, but the transition level (FL to ft) is determined in the ATIS, because it changes. In The Netherlands, TA is 3000ft and TL is often between FL40 and FL45, depending on teperature and QNH.

 

 

EHAM TL sheet.

[Sebastien Bartosz]

At the Transition Level. Can be found on approach plates.

 

Not always. In Europe, often the transition altitude (ft to FL) is determined and on the charts, but the transition level (FL to ft) is determined in the ATIS, because it changes. In The Netherlands, TA is 3000ft and TL is often between FL40 and FL45, depending on teperature and QNH.

 

 

EHAM TL sheet.

 

My point exactly. It will be given by ATC = the ATIS

[Magnus Meese]

Errrrr, well, no! Legally it is "correct" to change from STD to QNH/local altimeter setting when p[Mod - Happy Thoughts]ing transition level. Practically this will NOT be done by most professional pilots, because the chance of forgetting it is just too high. The best technique (in my opinion) and actual SOP (Standard Operating Procedure) with almost all operators and real world pilots is to set QNH/local altimeter setting right away when being instructed to descend to and altitude, coming from a flight level.

I agree with this method when you're close to the transition layer, but if I fly in the US and get cleared to a FL while in the low altitudes, I'm not going to swap for STD 10 000' below TA. If something occurs that requires a level-off, the chance of inadvertently being on the wrong altimeter setting is too large in my opinion.

[Simon Kelsey]

Errrrr, well, no! Legally it is "correct" to change from STD to QNH/local altimeter setting when p[Mod - Happy Thoughts]ing transition level. Practically this will NOT be done by most professional pilots, because the chance of forgetting it is just too high. The best technique (in my opinion) and actual SOP (Standard Operating Procedure) with almost all operators and real world pilots is to set QNH/local altimeter setting right away when being instructed to descend to and altitude, coming from a flight level.

I agree with this method when you're close to the transition layer, but if I fly in the US and get cleared to a FL while in the low altitudes, I'm not going to swap for STD 10 000' below TA. If something occurs that requires a level-off, the chance of inadvertently being on the wrong altimeter setting is too large in my opinion.

 

On the face of it that sounds logical. However...

 

How often are you cleared to a FL and then suddenly asked to stop at an altitude on the way up? What is the greater risk, forgetting to set standard p[Mod - Happy Thoughts]ing 18,000 on the 99.9% of times that an unplanned level off is not required, or forgetting to set QNH the 0.1% of times that ATC screw up and have to get you to level off suddenly?

 

Andreas' method is SOP at all the airlines whose procedures I'm familiar with.

[Andreas Fuchs]

Exactly. The cases where you need to level off at level during descent or at an altitude during climb and would have to revert to STD/QNH are very rare. I have operated 203 flights in the past 12 months (lucky me, I do not get overworked!) and on exactly one flight we had to revert from QNH to STD, because an ATCO told us to stop our descent at a flight level instead of the altitude that had instructed us before.

 

The risk of forgetting to change to local pressure or to standard pressure during normal operations is way too high and we regularly see it here on VATSIM. For example people cruising along at FL362 indicated. When being asked about their altimeter settings, they correct it and attain the correct level. During descent to an altitude it is even more critical, especially if there's terrain around that you need to keep clear off.

 

That is the reason why all operators that I know about have it in their SOPs as described earlier in this thread: change your altimeter setting when instructed to descend/climb to an altitude/flight level. It is the safer approach to this "problem", you can consider that airlines have made a proper risk-[Mod - Happy Thoughts]essment before implementing this rule.

[Trent Hopkinson]

If you got cleared to a Flight level - STD

If you got cleared to an altitude - QNH.

 

"Descend to Flight level 210" = STD (29.92/1013)

"Descend to eleventy thousand, QNH 1022" = QNH 1022.

 

Easy.

[Jonathan Fong]

As pretty much everyone here has said...

 

Officially, you should change from STD to QNH and vice versa when p[Mod - Happy Thoughts]ing the transition level/altitude (whichever applies). However, many pilots tend to forget to do that (especially on older/less advanced planes which lack the visual alerts that more modern planes tend to have), meaning that it's common practice to set the QNH when given a descent to an altitude (and vice versa for STD and ascent to a FL) and to simply reset the altimeter if it is necessary to stop a climb/descent for some reason before the TA/TL.

[Trent Hopkinson]

As pretty much everyone here has said...

 

Officially, you should change from STD to QNH and vice versa when p[Mod - Happy Thoughts]ing the transition level/altitude (whichever applies). However, many pilots tend to forget to do that (especially on older/less advanced planes which lack the visual alerts that more modern planes tend to have), meaning that it's common practice to set the QNH when given a descent to an altitude (and vice versa for STD and ascent to a FL) and to simply reset the altimeter if it is necessary to stop a climb/descent for some reason before the TA/TL.

 

Officially you should follow the SOP as defined by the FCOM of the company you operate for. Of course on Vatsim not everyone has access to such things, but there are some real world companies which contain a SOP to change from STD to QNH on receiving a clearance to an altitude. and to change from QNH to STD when receiving a clearance to a Flight Level.

 

In China you get cleared to xxxx metres QNH xxxx, or "cleared to xxxx metres "on standard" " - which always makes me think I heard "Cleared to xxxx metres non-standard" which is a different ICAO meaning entirely, and makes me think I either filed the wrong RVSM direction or got cleared to an altitude that wasn't what I filed due to traffic. But that's only a thing in China.

 

Back in the good old Metric days of Russia pre-2012, they used to just say "Climb to Flight Level xxxx Metres, which was great and easy to understand.

[David Zhong]

I would argue that you should guard against error in the non-normal situation rather than the normal situation.

 

The error in the normal situation is managed by the use of standard procedures, checklists and good habits - part of that could be briefing the transition altitude/level prior to departure/descent and noting it again when cleared through the transition layer. Do this often enough and you'll always have in the back of your mind, "have we p[Mod - Happy Thoughts]ed transition yet? Do we need to reset the altimeter?"

 

Doing this the other way will mean you have no controls to mitigate the risk of wrong altimeter setting in an early level-off situation.

[Trent Hopkinson]

I would argue that you should guard against error in the non-normal situation rather than the normal situation.

 

The error in the normal situation is managed by the use of standard procedures, checklists and good habits - part of that could be briefing the transition altitude/level prior to departure/descent and noting it again when cleared through the transition layer. Do this often enough and you'll always have in the back of your mind, "have we p[Mod - Happy Thoughts]ed transition yet? Do we need to reset the altimeter?"

 

Doing this the other way will mean you have no controls to mitigate the risk of wrong altimeter setting in an early level-off situation.

 

Which is why it is pretty strange that real world airlines are training and doing it the other way in their SOP's.

[Simon Kelsey]

Let's look at risk.

 

Risk is the likelihood of an event multiplied by the severity of the consequences.

 

So on Andreas' 203 flights, 202 were normal and one involved an unexpected level off.

 

What are the consequences of failing to set the correct pressure?

 

Well the severity of the error depends largely on the ambient pressure. 1hPa = (roughly) 30ft, so whilst there will be an error any time the QNH is different to 1013, to have what you might term a significant error of ~200ft or more the QNH would have to be below 1006 or above 1020. Not that uncommon but depends where you are in the world and the time of year/weather conditions

 

For there to be a significant error you would have to be instructed to level off unexpectedly at an altitude (a very rare event) on a day when the QNH is particularly high or low (a moderately rare event).

 

The consequences are primarily mid-air collision or CFIT. It is unlikely that a controller will have asked you to level off unexpectedly at a level that is so marginal for terrain clearance that a couple of hundred feet of altimeter error will be the difference between impact or not, so the most likely issue is mid-air collision.

 

The consequences of this are very high, but there is a fair bit of risk mitigation; controllers will generally be working to 1000ft separation standards, so for the altimeter error alone to result in a collision the QNH would have to be extremely low or high (above about 1046 or below about 980) or both aircraft would need to have made opposing altimeter setting errors (both extremely unlikely). The main consequence, therefore, is likely to be a temporary erosion of vertical separation. Not great, but not catastrophic either.

 

So in this case we have a very rare event, with relatively low consequences = fairly low risk.

 

But on 202 flights, Andreas was cleared to a flight level without any unexpected level offs.

 

The consequences of an altimeter setting error are the same, but the likelihood of the event is much higher. Therefore the risk is higher. Especially, arguably, with a higher TA where there is more time between takeoff and reaching TA and therefore more opportunity for distraction (from the cabin, dealing with paperwork etc).

 

Don't forget also that the 'conditioning' is that if you hear 'flight level' you automatically set STD and if you hear 'altitude' or 'feet' you automatically set QNH. So if you are asked to level off at "xx thousand feet" the automatic response anyway should be to set QNH.

 

A Big Airline has primary altimeters to standard once cleared to a flight level and above Aa. The standby is left on QNH until above TA and MSA. On descent, the standby is set to destination QNH as part of the approach briefing (in the cruise) and the primaries are set to QNH once cleared to an altitude.

 

The procedure mitigates against the greatest risk.

[Andreas Fuchs]

Perfect, there is nothing to add. It is all about risk-mitigation.

[Edward Berkley]

Thanks everyone for your answers. It helped me a lot!