Over 100 flights in Northern Norway has been canceled on Monday. Not due to high winds but due to QNH being to low(!).

According to Norways NRK the QNH was around 946 hPa. But regional carrier Wideroes DHC8:s does not seem to be certified to fly with this low airpressure.

Via google translate: https://translate.google.com/translate? ... 1.14896456

Probably lower than 950hPa cannot be selected on the altimeters. Have seen that on a number of airplanes.

arcticcruiser wrote:

Probably lower than 950hPa cannot be selected on the altimeters. Have seen that on a number of airplanes.

There is a pretty simple procedure to correct for QNH that is off the scale. We have it in our manuals, would be surprised Wideroe wouldn’t have it. Of course, the bigger the difference the larger the error will be so there is a limit to its usefulness.

But I’m about to go fly with QNH 943 hPa.

arcticcruiser wrote:

Probably lower than 950hPa cannot be selected on the altimeters. Have seen that on a number of airplanes.

In practice, how much would 4hPa affect runway performance?

As long as you're aware of it and compensate for it I don't see why it should be a problem. It seems strange that the meters aren't calibrated to take into all possible conditions.

BWIAirport wrote:

arcticcruiser wrote:

Probably lower than 950hPa cannot be selected on the altimeters. Have seen that on a number of airplanes.

In practice, how much would 4hPa affect runway performance?

Negligible in regards of performance I'd say.

But you'll get a ~100 ft mismatch on the altimeter. This could affect a couple of procedures in the well-regulated world of aviation.

arcticcruiser wrote:

Probably lower than 950hPa cannot be selected on the altimeters. Have seen that on a number of airplanes.

AFAIK it is 948hPa on their small Dash-8s

peterinlisbon wrote:

As long as you're aware of it and compensate for it I don't see why it should be a problem. It seems strange that the meters aren't calibrated to take into all possible conditions.

It is more like the meters on their old aircraft stopp at 948hPa. Same with the 737-Classic, which also would have struggled today

While you can always compensate for it, it would require certification, procedures and a lot of other things that would increase their cost. For something that seldom happen. Read somwehere there are 27 years sine the QNH was this low in Norway

Just because the scale stops doesn’t mean you can’t adjust. You add the compensation (~30ft/hPa) to any published altitudes. Should not require any certification, just some mental arithmetic skills.

The only reason not to use compensation like that is if your performance data is not available. We have data down to 930hPa.

What nobody is mentioning is that a drop in air pressure also results in a restricted take off weight as engine performance is affected.

When you take into account that 1013 hPA is viewed as average air pressure at sea level then 945 hPa is extremely low. In usual performance calculations each point below 1013 hPA gives a small weight penalty that has to be taken into consideration when calculating the take off performance. Going from 1013 hPa to 945 hPA can be a huge limitation in your allowed take off weight.

So apart from what the instruments will allow to be set it can also be that take off weight is penalized so much that you can not carry a useful payload.

intrance wrote:

Just because the scale stops doesn’t mean you can’t adjust. You add the compensation (~30ft/hPa) to any published altitudes. Should not require any certification, just some mental arithmetic skills.

The only reason not to use compensation like that is if your performance data is not available. We have data down to 930hPa.

That's the kind of place where they fly: https://youtu.be/DNaX41m9vlM
When weather gets crappy there (btw, it's northern norway!) you don't want to have anything to do with your altimeter, except reading it!
And since when operating in IFR with a non correct altimeter setting considered even remotely acceptable anyway?

intrance wrote:

Just because the scale stops doesn’t mean you can’t adjust. You add the compensation (~30ft/hPa) to any published altitudes. Should not require any certification, just some mental arithmetic skills.

The only reason not to use compensation like that is if your performance data is not available. We have data down to 930hPa.

But, the airline has to approved to use that procedure—they just can’t tell the crew robust mental arithmetic

9MMPQ wrote:

What nobody is mentioning is that a drop in air pressure also results in a restricted take off weight as engine performance is affected.

When you take into account that 1013 hPA is viewed as average air pressure at sea level then 945 hPa is extremely low. In usual performance calculations each point below 1013 hPA gives a small weight penalty that has to be taken into consideration when calculating the take off performance. Going from 1013 hPa to 945 hPA can be a huge limitation in your allowed take off weight.

So apart from what the instruments will allow to be set it can also be that take off weight is penalized so much that you can not carry a useful payload.

this pressure is equivalent to about 2000 feet elevation normal conditions.Trondheim is close to sea level, as far as I understand, and it should be pretty cold in Norway in January.
Since Dash-8 can operate in Denver, which is much higher - I don't expect serious performance problems.

I think I would rather wait on the ground till conditions in the atmosphere are a bit more within range than hope someone gets the mental math right on the fly....

GalaxyFlyer wrote:

But, the airline has to approved to use that procedure—they just can’t tell the crew robust mental arithmetic

They want the information published in the QRah or a QRC so mental mantis is nit required

“ In order to standardise the method of which pilots correct their altitudes, the operator shall promulgate tables, for instance in their QRH (Quick Reference Handbook) or QRC (Quick Reference Card), for easy access. The tables should reflect the following:”

From https://luftfartstilsynet.no/aktorer/re ... ing-scale/

9MMPQ wrote:

What nobody is mentioning is that a drop in air pressure also results in a restricted take off weight as engine performance is affected.

When you take into account that 1013 hPA is viewed as average air pressure at sea level then 945 hPa is extremely low. In usual performance calculations each point below 1013 hPA gives a small weight penalty that has to be taken into consideration when calculating the take off performance. Going from 1013 hPa to 945 hPA can be a huge limitation in your allowed take off weight.

So apart from what the instruments will allow to be set it can also be that take off weight is penalized so much that you can not carry a useful payload.

I thought the same until I checked: a QNH of 945 hPa is equal to al altitude of 584 meters or 1917 feet; plenty of airports are much higher than that.

9MMPQ wrote:

What nobody is mentioning is that a drop in air pressure also results in a restricted take off weight as engine performance is affected.

When you take into account that 1013 hPA is viewed as average air pressure at sea level then 945 hPa is extremely low. In usual performance calculations each point below 1013 hPA gives a small weight penalty that has to be taken into consideration when calculating the take off performance. Going from 1013 hPa to 945 hPA can be a huge limitation in your allowed take off weight.

So apart from what the instruments will allow to be set it can also be that take off weight is penalized so much that you can not carry a useful payload.

The Q400 is certified for take-off and landing up to altitudes of 10,000 ft, where the actual pressure is around the 700 hPa mark. The type's performance tables for 10,000 ft and an Outside Air Temperature of -5 C (standard atmosphere) say the performance limited Take Off Mass is approx 31,000 kg - some 2,000 in excess of the maximum structural Take Off Mass. If runway length, slope and contamination is not an issue, you can get out with no weight penalty even then.

EDIT: indeed, Take Off Mass penalties at 10,000 ft start only at temperatures of +7 C... so summer at altitude.

CeddP wrote:

intrance wrote:

Just because the scale stops doesn’t mean you can’t adjust. You add the compensation (~30ft/hPa) to any published altitudes. Should not require any certification, just some mental arithmetic skills.

The only reason not to use compensation like that is if your performance data is not available. We have data down to 930hPa.

That's the kind of place where they fly: https://youtu.be/DNaX41m9vlM
When weather gets crappy there (btw, it's northern norway!) you don't want to have anything to do with your altimeter, except reading it!
And since when operating in IFR with a non correct altimeter setting considered even remotely acceptable anyway?

I know exactly where they fly and if you saw my earlier post you might have deduced that I fly in the same area . I have years of experience flying around Scandinavia in the crappiest weather it can throw around. I have a fairly decent idea what I am talking about .

There is nothing super special to do, you are not constantly adding and subtracting. You are flying the procedure that is in your FMS but you simply correct the published altitudes by the difference in QNH. Proper briefing and off you go. Then again, I fly a type where there is in practical use no lower limit on the QNH setting, so I haven't needed to use this procedure in a while.

GalaxyFlyer wrote:

But, the airline has to approved to use that procedure—they just can’t tell the crew robust mental arithmetic

Procedure is in our OM-A, that OM-A gets approved by CAA, et voila. In our manual it is actually a piece of leftover procedure from previous aircraft types I believe, all our current types have electronic QNH setting.

Anyway, my best guess is that they didn't have the performance data for the low QNH.

Sounds similar to a quandary that took place in Phoenix, AZ in the summer of 1990. Temperature went over 120 degrees F and the takeoff data for the runways there for a B-727 only went up to 120 degrees. NW ended up cancelling a few flights. The next day Boeing faxed to all aircrews tabulated data that reflect temperatures up to 130 degrees. Yes, additional weight restrictions, but everybody got off.

^ The exact same thing happened to the CRJs in PHX a few years back. Temp hit 120, and max operating temp on the CRJ is 118.

https://www.cnbc.com/2017/06/20/theres- ... rport.html

WayexTDI wrote:

9MMPQ wrote:

[...] I checked: a QNH of 945 hPa is equal to al altitude of 584 meters or 1917 feet; plenty of airports are much higher than that.

TripleDelta wrote:

[...]The Q400 is certified for take-off and landing up to altitudes of 10,000 ft, where the actual pressure is around the 700 hPa mark.[...]

The thing is: QNH does not indicate the true absolute air pressure but the elevation-adjusted relative one. In other words: the QNH value expresses the barometric pressure that would occur at a particular point if that point would be located at sea level.

Kilopond wrote:

WayexTDI wrote:

9MMPQ wrote:

[...] I checked: a QNH of 945 hPa is equal to al altitude of 584 meters or 1917 feet; plenty of airports are much higher than that.

TripleDelta wrote:

[...]The Q400 is certified for take-off and landing up to altitudes of 10,000 ft, where the actual pressure is around the 700 hPa mark.[...]

The thing is: QNH does not indicate the true absolute air pressure but the elevation-adjusted relative one. In other words: the QNH value expresses the barometric pressure that would occur at a particular point if that point would be located at sea level.

Thanks, that's the definition of QNH, yes.

What it means is that any airport in the area of a QNH of 945 hPa will have a "simulated" altitude 1,917 feet higher; so, if an airport is at 2,000 ASL, it's "altitude" will now be 3,917 ft, and the airline will operate it like if it was operating at an airport 3,917 ft ASL (instead of 2,000 ft), with all the restrictions (if any) associated with an airport at that altitude.

WayexTDI wrote:

What it means is that any airport in the area of a QNH of 945 hPa will have a "simulated" altitude 1,917 feet higher; so, if an airport is at 2,000 ASL, it's "altitude" will now be 3,917 ft, and the airline will operate it like if it was operating at an airport 3,917 ft ASL (instead of 2,000 ft), with all the restrictions (if any) associated with an airport at that altitude.

An airport at 2000 ft elevation doesn't care about QNH. It's not flying and always is at 2000 ft MSL. It's real terrain and its elevation is never "simulated." If you try operating out of it as if it were 1917 ft higher you could collide with something very hard that you don't want to be colliding with.

kalvado wrote:

....as far as I understand, and it should be pretty cold in Norway in January.

Most of the airports in Northern Norway is near the see, so it is usually not that cold. Just a lot of bad weather

WayexTDI wrote:

What it means is that any airport in the area of a QNH of 945 hPa will have a "simulated" altitude 1,917 feet higher; so, if an airport is at 2,000 ASL, it's "altitude" will now be 3,917 ft, and the airline will operate it like if it was operating at an airport 3,917 ft ASL (instead of 2,000 ft), with all the restrictions (if any) associated with an airport at that altitude.

Take a metar for EVE elevation 85’, METAR ENEV 101320Z 00000KT 3500 -SN VV019 M00/M01 Q0945 RMK WIND 1400FT VRB01KT

That’s a density altitude of 739 ft, hardly an issue if the operator has the procedures in place.

hivue wrote:

WayexTDI wrote:

What it means is that any airport in the area of a QNH of 945 hPa will have a "simulated" altitude 1,917 feet higher; so, if an airport is at 2,000 ASL, it's "altitude" will now be 3,917 ft, and the airline will operate it like if it was operating at an airport 3,917 ft ASL (instead of 2,000 ft), with all the restrictions (if any) associated with an airport at that altitude.

An airport at 2000 ft elevation doesn't care about QNH. It's not flying and always is at 2000 ft MSL. It's real terrain and its elevation is never "simulated." If you try operating out of it as if it were 1917 ft higher you could collide with something very hard that you don't want to be colliding with.

In theory, you can set altimeter to 1020 - or whatever number would be - and consider everything exactly 2000 feet higher than it is (or about 975 and 1000 feet higher), including runway, terrain, buildings etc. But, as it was said, such calculation may be very easy in the office, but confusing when airborne. "Climb and maintain 5000" - do I need to add 2000? Oops, it was the other way, to bad, so sad..
Besides, it is a no go until procedures are written and approved.