PPL air law question QNH & QFE
 

Hi,

I have my air law exam next week and im totally stuck on one type of question! Ive used the search funtion but with no success
Please help
The question is...

You are setting your altimeter at an aerodrome, where the elevation is -31 ft. The QNH you have been given is 1029 millibars. What would you expect the aerodrome QFE to be?
A 1015 millibars
B 1030 millibars
C 1028 millibars
D 1032 millibars


How would you work this out??
Thanks alot for any help in advance and hope this is the right section to put it in.

Cheers
Conk

Quick revision of the basics;

QNH - the pressure you set on your altimeter subscale that will result in the altitude being displayed as feet above mean sea level.

QFE - the pressure you set on your altimeter subscale that will result in the height being displayed as feet above aerodrome datum.

In a standard atmosphere the pressure decreases by 1mb (or 1HPa) for an increase in height of approximately 30 feet.

Thus at an airfield with an elevation of 330 feet above sea level the QFE will be 11mb lower than the QNH.

To answer your question - your airfield elevation is -31 feet, therefore the QFE will be 1mb higher than the QNH of 1029mb - ie 1030mb, answer B.

HTH

rule of thumb 1mb = 30ft so id go for 1030mb.This is because you lose approx 1mb per 30ft in height you gain.


Almost the same type of questions appear in MET too..

Liam

Below sea level and QFE will be greater than QNH. Not many airports are actually below sea level!!

If you set your altimeter subscale such that the "altitude" is zero, the setting should be equivalent to the QFE. Do the same for the airfield elevation, and the setting will read the QNH.

Cheers

Whirls

PS edit - Air Soul's excplanation is more accurate but I fly from Norfolk where there's v. little difference!!

Anybody spot air soul's deliberate mistook?

Cusco

:confused:No, what is it?

MJ

He added, rather than subtracted, the 1 mb. The problem airfield is 31 feet below sea level, so the QFE would be lower than the QNH. Answer should be 1028mb. (provided I've done my sums right here, of course)

Oh Pitts, this could be the start of a bad day for you.

Of course. You're right. I shouldn't just wake up and try to do math. I think a cup of coffee is required first. :(

Tone, you beat me to it......

Thanks fore the replys

According to air quiz the answer was B

Could some please post a formula up because im still struggling?

Im fine with questions like...

You are approaching an aerodrome at an altitude of 1230 ft, with QNH 1015 millibars set. You set 987 millibars QFE for landing. What will your altimeter read now?

Is it along the same principles?

Conk

conk, the formula is the same you have been using. You just need to do what Pitts forgot to do and read the question properly!

The aerodrome was MINUS 31 feet which puts BELOW sea level. In this case the QNH was going to be lower than the QFE or put it another way the sea level was above the airfield so you are going to add rather than deduct.

QNH = 1029

Airfield Elevation is 31ft so 1mb BUT it is a NEGATIVE elevation so

1029 +1 = 1030

If it was a postive elevation then it would be 1028

There are a few negative elevation airfields around, I seem to recall Texel may be.

I dislike questions like this. It's a trick question really as in over 20 years and 12,000 hours of flying, I have never landed at an airport below sea level ones.

(Some of the sea level ones were slightly lower after my landing, but not before - old aviation joke).

At the risk of trying to reply before coffee time, here's y stab at an equation;

QNH - (Airfield Elevation/30) = QFE


Airfield elevation is in feet.

AS

I have landed at Schiphol many times and I seem to recall it is minus 11ft.

ooooooo i get it now :ok:

thanks for the help hopefully ill pass the exam now!

Texel is 2 feet above.

Correct. Furthermore, Lelystad is -13ft. And my home base, Rotterdam, is even -14ft. Those are the three fields in the Netherlands that are below MSL.

But in the Netherlands we don't do QFE, just QNH so it's all a bit of a non-issue.

When I landed at Furnace Creek (Death Valley National Park Ca. -210ft) a coupla years ago, even though I knew it was below sealevel and by how much, the altimeter readings so spooked me on final that I threw the approach away, went around and sorted myself out: second circuit = perfect landing.

Cusco

exam advice
 

Still just about the best equation to use in any exam at all (maths or not ! ) :

R.T.F.Q = (at least) 1/2 T.F.A

You are approaching an aerodrome at an altitude of 1230 ft, with QNH 1015 millibars set. You set 987 millibars QFE for landing. What will your altimeter read now?


810ft?
Got my Airlaw in the next 2 weeks, bloody dreading it. This QNH & QFE throws me too.:confused: