QNH altimeter setting

[CaptJackG]

When I set the QNH altimeter it reads about 150 below field elevation at Nellis.

[Blech]

Hi,

 

Created a Ramp Start for a test

The 28.18 OFE in the briefing is quite ok (should be 28.16)

[CaptJackG]

set QNH altimeter and see what that reads. Should show field elevation. QNH is used in the US and your approach chart altitudes are base on QNH.

[Frederf]

The "QNH" listed on the briefing is:

1. Only for static weather. It is not valid for dynamic weather.

2. Actually QFF which is QNH for a theoretical sea-level station.

 

You cannot apply the "QNH" from the briefing accurately to any airfield except those at exactly sea level and static weather is used. Because your airfield is not at sea level the briefing QFF won't be equal to that airfield's QNH. QNH is a calibration at a particular location and will be wrong below and above that elevation depending on the temperature anomaly.

[Nealius]

Adding useful information others may not have found. I was experiencing some large "errors" running at realistic weather settings for NTTR and did some investigating, and learned a bit about altimeters in the process.

Temperature effect on altimeter settings is roughly thus: 4ft x (difference in temp from ISA) x (indicated altitude notated as 1.xx for 1,xxx ft)

For example, on a 43C day my altimeter reads 1,690ft. So:

4ft x (43 - 15) x 1.69 = 189ft

Add 189ft to my indicated altitude of 1,690ft and I get 1,879ft, which is within FAA +/-75ft tolerance.

Now on a 0C day, my altimeter reads 1,950ft. So:

4ft (0 - 15) x 1.95 = -117ft

Subtract 117ft from my indicated altitude of 1,950ft and I get 1,833ft, which again is within FAA tolerance.

Note that if you set mission temp to 15C, the altimeters are accurate to within +/-10ft, so the QFF issue may have been resolved. 

Edited June 17, 2024 by Nealius