Working out QNH?

[Mike12421]

This might sound daft, but I am having real difficulties with working out the correct values of QNH.:ugh::ugh::ugh:
For example, I have been using AirQuiz.com (which by the way I have found very useful), however I am having difficulties with the following types of questions;

28) Your aerodrome elevation is 57 ft, and the QFE you have just been given is 1029 millibars. What would you expect the aerodrome QNH to be?
A 1008 millibars
B 1028 millibars
C 1031 millibars
D 1032 millibars
You did not attempt this question. The correct answer was C.

&

35) You are intending to fly in accordance with the quadrantal rule on a magnetic track of 120. The QNH is 996 mbs. Which of the following is your correct flight level?
A FL 30
B FL 35
C FL 55
D FL 60
You chose A, but the correct answer was C.

Any help would be very much appreciated. Thanks.

[EGBKFLYER]

Two things:

:ok: 1mb = 28ft. Not all the way through the atmosphere but definitely down at around sea level where these questions are based.

:ok: Pressure REDUCES as you INCREASE your height/ altitude (and vice versa!). I think of it as the weight of air on my head - the lower I go in the atmosphere, the more weight of air above my head so the greater the pressure...

In your first example - 57ft = 2 x 28 roughly = 2mb

Now - to add or take away? The QFE is the pressure on the ground at the airfield, which is ABOVE sea level (the question told you). The QNH is the pressure at sea level. To go from the airfield (QFE) to sea level (QNH) you need to go DOWN, so the pressure INCREASES.
QFE = 1029 so QNH = 1029 + 2 = 1031. Answer C.

In your second example, it mentions FL - flight levels. All flight levels use 1013mb by definition.

996mb to 1013mb is a 17mb difference = 17 x 28ft = 476ft

You must be above the transition altitude to use flight levels, which is mostly 3000' QNH in the UK (the question should really define this).
Quadrantal rule for track 120 = ODDS +500, so A,C are out straight away. If you flew at FL35, that would be 3500' on 1013mb = (3500-476)' on 996mb, i.e. nearly 3000'. This would give inadequate separation between you on your FL and some other poor s*d flying at 2999' on the QNH! (Look up the definition and calculation of the Transition Layer - it solves this problem).

So the lowest safe level you could cruise at is FL55 - answer C.

[Mike12421]

Thank you very much! I wish Trevor Thom made it that easy.

[EGBKFLYER]

It also helps if you draw the problem. Ask a flying instructor to show you - we all had to do lots of this for the exams!

[spekesoftly]

--LAND--QFE1029mb--- ^
----------------------- |
----------------------- |
----------------------- |
----------------------- 57ft
----------------------- |
----------------------- |
----------------------- |
----------------------- v
-----------------------~~~~~~SEA~~QNH1031mb~~~~~~~

[Mike12421]

Thank you, both of you. One less thing to struggle on.

[411A]

I usually just listened to the ATIS, and set the altimeter to QNH at the proper time.:E

I did go to work for one small airline that used QFE for landing.

I mentioned to the chief pilot that I thought this to be a daft idea, whereupon they changed straightaway to QNH.

Ahhh, much better!

This is a true story...however I expect the change was done just as I arrived...thankfully!

[Megaton]

411A,

Despite your vast and enviable experience, have you never been to a country where it is QFE that is given by ATC and one then has to convert to QNH?

[411A]

Oddly enough, HamPhisted, no.

Where would this be then?

[145qrh]

Russian ATIS gives Qfe, ATC also give you height clearances, so you have to work out QNH and the appropriate altitude to descend to..:=

[GlueBall]

I have travelled a great deal, and I am still travelling and I have never been given a QFE setting at any of the world's airports. :ooh: