Primarily for general aviation discussion, but other aviation topics are also welcome.
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By PaulB
#1661948
The reported QNHs are quite high at the moment. Birmingham is giving 1038 and Inverness 1042

EGPE 011750Z 01005KT 9999 FEW025 BKN045 04/M02 Q1042
By flyingdoug
#1662023
The forecast suggests it will be sticking around for another week at least. Sometimes feels like a bit of a waste getting high pressure at this time of year as it usually means low cloud and fog. Would be good if it could be rearranged for sometime during the summer instead!
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By gaznav
#1662043
If I recall correctly in the dim and distant past from my military aviation groundschool then 1051hPa (mb in those days) could not be dialled on some altimeters and so 1013 was to be used and then lots of altimetry calculations? Also, when the QNH reaches the trigger value of 1051 hPa it was theoretically possible to gain one Flight Level by using both the Transition Altitude and the Transition Level simultaneously while still having a transition layer of 1000 ft!

I don’t think I’ve ever seen 1051. [just checked for the UK and the last time was 1902!]

PS. Just found this on the internet to help explain:

For correction of pressure that is higher than what can be set into the altimeter the pilot should subtract the altitude that appears from the table.
Example: 1050 HPA is the maximum value that can be set into the altimeter and the actual pressure is 1053 HPA. The difference is 3 HPA, which equals 90 feet, which is rounded off to the nearest 100 feet. If cleared altitude is 5000 feet, the pilot should set QNH 1050 and fly at 4900 feet indicated on the altimeter (cleared altitude 5000 feet minus 100 feet correction).
#1662047
gaznav wrote:could not be dialled on some altimeters and so 1013 was to be used and then lots of altimetry calculations?

QNE: the reading in feet on an altimeter with the sub-scale set to 1013.2 hPa when the aircraft is at aerodrome or touchdown elevation.

gaznav wrote:when the QNH reaches the trigger value of 1051 hPa it was theoretically possible to gain one Flight Level by using both the Transition Altitude and the Transition Level simultaneously while still having a transition layer of 1000 ft!


The trigger QNH is now 1050*. If the TA is 6000 (as with LTMA) the TL becomes FL 60, with 6000ft and FL60 become separated by standard vertical separation. (This could catch out TMA ATCOs when 'jumping stacks...!)

(*Assumed value of 27.3 ft per hPa derived from a linear correction which is applied to corrected barometric altitudes and confirmed as being utilised in aircraft and ATS systems)
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By 2Donkeys
#1662072
The US has quite specific rules once the pressure hits 31 inches (effectively 1050)

FAA wrote:§ 91.144 Temporary restriction on flight operations during abnormally high barometric pressure conditions.

(a)Special flight restrictions. When any information indicates that barometric pressure on the route of flight currently exceeds or will exceed 31 inches of mercury, no person may operate an aircraft or initiate a flight contrary to the requirements established by the Administrator and published in a Notice to Airmen issued under this section.

(b)Waivers. The Administrator is authorized to waive any restriction issued under paragraph (a) of this section to permit emergency supply, transport, or medical services to be delivered to isolated communities, where the operation can be conducted with an acceptable level of safety.
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By rikur_
#1663136
seanxair wrote:Flew this afternoon and was surprised how unsettled the air was up to 1900ft. (Cloudbase 2300ish)

ditto - but it seemed to smooth out at 2200 ... albeit very hazy by that point .... too cold to climb for VMC on top.
The cloud base was notably ripply, varying evenly by a few hundred feet every ~ half mile. When trimmed I noticed that I was varying between +100fpm and -100fpm in a similar pattern.
Some sort of wave pattern along the bottom of the cloud base. I'm sure glider folks will understand it, but beyond my meteorology.
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