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• Posts: 57

### Flightsurgeon created the topic: QNH enquiry please

Dear Everyone, please may I have some commentary on my paragraph and see if it is correct and answer my queries.

On the altimeter 1013 in Kollsman window means mainscale is Pressure Altitude.
On the altimeter QNH in Kollsman window means the mainscale is the altitude.
If main scale = 0. What reads on the Kollsman is QFE. If then QNH is known ( QNH-QFE) x 30 is the elevation. This makes the assumption that an elevated aerodrome will have a lower pressure than the sea level if it is above ground because as you elevate the pressure drops.
If elevation is known and entered in the main scale, then kollsman gives you the local QNH.

Please let me know if my assumptions are correct

Does the pressure barometer work along the same principle that PA = Elevation + [1013-QNH]30?

Kind regards

Ravi Mahajani

• John.Heddles
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• ATPL/consulting aero engineer
• Posts: 843

### John.Heddles replied the topic: QNH enquiry please

That's the story, pretty well.

Some thoughts -

(a) If main scale = 0. What reads on the Kollsman is QFE. ... and, at higher levels, as well. The zero reference is the reading if you happen to be on the runway at the relevant airfield.

(b) the equations are a bit more complicated than what we use for pilot stuff so the actual calculations involve a bit more than the usual pilot approach.

(c) be a little wary when using 30 ft/hPa for pressure/height variations. The actual relation is an exponential expression which can be seen as a graph in the following thread -

bobtait.com.au/forum/performance/6777-altimetry-rates#13257

As can be seen, by reference to the first graph, 30 ft/hPa is correct at a pressure height of around 4000 ft but is incorrect elsewhere. For pragmatic pilot calculations, 30 ft/hPa is fit for purpose in the lower levels (say MSL to 8000 ft Hp) where we might be interested in running some back of a fag packet calculations on the fly, as it were.

We routinely use 30 ft/hPa but that is only a convenient rough and ready calculation, albeit fit for purpose at the lower levels. If you run such calculations and then compare your answers to ISA tables (which are available on the net) you will observe that the calculations differ, a tad, from the tables.

(d) Does the pressure barometer work along the same principle that PA = Elevation + [1013-QNH]30? The altimeter is just a specialised barometer. The barometer on the kitchen wall is calibrated in a similar manner.

Engineering specialist in aircraft performance and weight control.