There is an intense high pressure area over southern Britain this morning and is expected to strengthen to 1050hPa by 00:00Z 20 January. Current pressure here close to Manchester Airport (EGCC) is 1046hPa and still rising.

The difference in altitude from STD to QNH is a massive 877 feet! Take advantage and fly over southern UK today as this is an extremely rare weather event.

That's 31 in of merc. Wow, that's like a strong Canadian High.  If we get those here in southeast MO, it almost always means very cold temps and clear skies.

2 hours ago, Mace said:

That's 31 in of merc. Wow, that's like a strong Canadian High.  If we get those here in southeast MO, it almost always means very cold temps and clear skies.

Highest pressure in the British Isles since 1959 but as it’s right over us that could still change. Its position means it’s not pulling in cold continental air. And the proximity of the Atlantic helps. A couple of hundred miles further north and things would be very different. Midnight here and it’s just -0.1°C. 😁

I was working the last couple of nights, when we started running the flightplans for the arrivals into LHR and LGW the other staff in OCC couldnt work out the gasps coming from the Flightplannimg desk of 'what the hell is going on with the TAFs' and 'is saber playing up tonight with the tafs' 

We had to double check the qnh was correct 

@fluffyflops, interesting to hear from those directly involved. A rare event indeed. It would be nice to hear from any pilots who have flown in the last 48 hours. Did you have to take any different actions because of the intense high?

From a purely FSX:SE perspective, I was "flying" an MD-11 into Doncaster and whilst travelling up the UK, on vectors for the approach, the altimeter was moving around a heck of a lot with FSOpen Clouds pulling in real METARS.

Practically every time I switched centre and approach controllers, the default ATC would give me a different QNH. (So much so, I kept pressing "B" occasionally to obtain the "instant" QNH  )

Hope you don't mind the armchair pilot's take on this, Ray.

I really don't fully understand the mechanics behind all of this - but what does happen with this type of rare event? - lots of snow?

Nothing special when flying in unusual high or low pressure conditions. There are procedures how and when to apply temperature and pressure corrections. That's it.

The only thing that usually happens in such cases is that you confirm the QNH and/or make sure that you haven't misread or written down a wrong QNH. 

Edited January 20, 20206 yr by FDEdev

8 minutes ago, FDEdev said:

Nothing special when flying in unusual high or low pressure conditions. There are procedures how and when to apply temperature and pressure corrections. That's it.

The only thing that usually happens in such cases is that you confirm the QNH and/or make sure that you haven't misread or written down a wrong QNH. 

oh okay....

thanks for the explanation

3 hours ago, HighBypass said:

From a purely FSX:SE perspective, I was "flying" an MD-11 into Doncaster and whilst travelling up the UK, on vectors for the approach, the altimeter was moving around a heck of a lot with FSOpen Clouds pulling in real METARS.

Practically every time I switched centre and approach controllers, the default ATC would give me a different QNH. (So much so, I kept pressing "B" occasionally to obtain the "instant" QNH  )

Hope you don't mind the armchair pilot's take on this, Ray.

Not at all although you would get far better instructions if you used Radar Contact (free) and not the dreadful default ATC. Strange you should have so many variations of QNH given pressure would have been uniform over large areas. For realism please don’t go near the B key. It only works correctly in areas where the Transition Altitude is 18000ft. 😉

2 hours ago, Skywolf said:

I really don't fully understand the mechanics behind all of this - but what does happen with this type of rare event? - lots of snow?

Depends on the type of flight. If you stay below the Transition Altitude of either 5000 or 6000ft in the U.K. you’ll just dial QNH on your altimeter and that’s the end of it. Snow is not a factor.

However, if you are flying above the TA then you have to switch to QNH when descending through it. If QNH was 29.92 / 1013.2 then there would be no change in indicated altitude when you switch. But the bigger the difference between QNH and STD the bigger the difference in indicated altitude when you do. With QNH of up to 1050hPa / 31” that is a massive difference.

With pressure as high as it’s been in the last 48 hours that would result in an indicated altitude difference of approximately 900ft. Hence why I was interested in the views of actual pilots and whether they changed from standard procedures.

1hPa = 33ft so 1050-1013 = 37.

33 * 37 = 1,221ft. That’s a massive difference.

Edited January 20, 20206 yr by Ray Proudfoot
Additional info

The 747-400 has an anomaly mentioned as a non AFM limitation whereby in a climb or decent and planning to level off within 2000ft of switching from QNH to standard (QNE) or vice versa FLCH should be used instead of VNAV when the QNH is 1006hpa or below.

I can’t think of anything else pressure related from a procedural point of view.

I think ATC have revised minimum vector altitudes at times of low pressure, and the TL/TA shifts with pressure as well.

@jon b, thanks Jon. That suggests very low pressure is of greater concern than high pressure. Is that just because with high pressure you could be higher than normal whereas with low altitude could be a safety factor? But it sounds like the TL/TA is increased for very high pressure.

4 hours ago, jon b said:

I can’t think of anything else pressure related from a procedural point of view.

It's been too many decades ago, but AFAIR a hpa correction was required when flying to Innsbruck at gross pressure deviations.

Edited January 21, 20206 yr by FDEdev

4 hours ago, Ray Proudfoot said:

But it sounds like the TL/TA is increased for very high pressure.

The other way round.  As soon as the pressure drops below standard, the TL is increased to maintain a minimum separation of 1000ft for aircraft flying at TA.

TA doesn't change, it's a fixed value.

Edited January 21, 20206 yr by FDEdev

11 hours ago, Skywolf said:

- but what does happen with this type of rare event? 

Our  takeoff calculator goes nuts with warnings