Originally Posted by
BDAttitude
you are using the left arrow for your calculation where you should be using the right one. If you dug a hole at your airfield and sent your QNH set altimeter down, it would not necessarily show 0 at sea level, because it is using standard atmosphere.
I.e. You mustn’t use the geographic elevation but the density altitude above QNH of your airfield for conversion.
Ahh. If only they'd drawn that diagram on the article I read about QFF. What I understand from that is that the ACTUAL atmosphere on any day doesn't exhibit that nice round 0.12hPa/m gradient. Thank you!
p.s. Unless you're flying towards me
, I think you may have got left and right reversed. As you point out, my calculation uses ISA (right) rather than reality.
Originally Posted by
Sir Niall Dementia
If you are flying QFE or QNH why do you want to know the elevation in metres?
It's that lovely thing they call SI units. That pressure reduction "constant" is per metre. Concorde engineers apparently got VERY good at swapping back and forth between metric and imperial without having to down tools and take a 2 hour lunch break. Thanks anyway Niall.
Originally Posted by
AAKEE
One other affecting part is that at least QNH is rounded down to the nearest whole hPa.
Calculation may give more than 3 hpa, but when its rounded down it comes closer to the QFE.
At lower levels it around 27 feet / hPa.
3.74 hpa, after rounding in most cases 3.
Does it have an instrument approach and does the threshold height differ from ARP ?
Edit:
Just looked at the plates:
Airport elevation 101’
Runway 27 = 87’
Runway 09 =. 74’
Problem solved.
Stop edit.
Try calling TWR and get both QFE and QNH in exact value ( including one tenth) and youll see :-)
All excellent and informative points - the last being the deal maker. I need to stop looking at the ARP and start looking at the threshold elevations.
This has brought out a wicked thought; it's very tempting when inbound to ask a controller for the QFE by runway just to gauge their sense of humour. I suspect if it were a TWR, they'd have me orbiting for a few dozen minutes whilst they sent someone to measure it to the nearest Pascal for me.
