PPL air law question QNH & QFE

[conk757]

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

[Air Soul]

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

[liam548]

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

[Whirlygig]

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!!

[Cusco]

Anybody spot air soul's deliberate mistook?

Cusco

[Mach Jump]

:confused:No, what is it?

MJ

[Pitts2112]

Anybody spot air soul's deliberate mistook?

Cusco

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)

[Tone]

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

[Pitts2112]

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. :(

[S-Works]

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

[conk757]

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

[S-Works]

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.

[Fright Level]

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).

[Air Soul]

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

[S-Works]

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

[conk757]

ooooooo i get it now :ok:

thanks for the help hopefully ill pass the exam now!

[BackPacker]

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

Texel is 2 feet above.

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

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.

[Cusco]

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

[C172 Hawk XP]

You just need to do what Pitts forgot to do and read the question properly

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

[BladeM3]

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:

[Air Soul]

Ok - you are at an altitude of 1230' on 1015.

1015-987=28mb

1mb =30' approx

Therefore 28x30 = 840' - ie your altimeter will indicate 840 feet lower than it did.

New indicated height is 1230-840=390'

AS

[BladeM3]

bollix, i see where I went wrong now, schoolboy errors:ugh:

[BladeM3]

You are approaching an aerodrome at an altitude of 1150ft, with QNH 1018 millibars set. You set 1001 millibars QFE for landing. What will your altimeter read now?

How I would work this one..

1018-1001=17

17*30= 510

1150ft-510ft= 640ft

Answer =640ft

[Air Soul]

Looks good to me.

AS

[BladeM3]

:ok:...........................

[sycamore]

If in any doubt about the maths ,draw a little diagram on scrap paper,remembering that pressure increases as you descend;same for met,warm air /cold air columns...BEST OF LUCK..

[False Capture]

Gross error method:

WINDING ON MILLIBARS = WINDING ON HEIGHT

In other words, if you're increasing the value in the subscale setting window by 1mb then you'll be increasing the indicated value (ie. height/altitude) by about 30ft.

Best way to tackle questions about QFE/QNH/1013mb is to start by visualising your aeroplane with the correct QNH set.

In CONC757’s first example the altimeter was reading minus 31ft with a QNH of 1029mb set. To display QFE (the altimeter would indicate zero feet with the aircraft on the ground) you have to ‘wind on’ one millibar which means the value in the subscale pressure window (or QFE as it is now) is 1030mb.

In CONC757’s second example approaching an aerodrome at an altitude of 1230ft with QNH of 1015mb set and you then set a QFE of 987 millibars for landing. In this case you have ‘wound off’ 28mb (1015mb minus 987mb) worth of height which results in the altimeter indicating 840ft (ie. 28 x 30ft) less than it did before. The altimeter would now be indicating 390ft (ie. 1230ft minus 840ft).

In the CAA questions, there’s always an answer for someone who has correctly worked out the value but applied it the wrong way round (ala Pitts2112 and Cusco).

[Lister Noble]

In the CAA questions, there’s always an answer for someone who has correctly worked out the value but applied it the wrong way round (ala Pitts2112 and Cusco).

The above is the best bit of advice given.


Read the question twice/three times,do the answer,when you have finished the paper go back and check everything again.You will have the time to do this.
I didn't on the first exam,Airlaw,and made a couple of ridiculous mistakes which cost me enough marks to fail.
Good luck
Lister:)

[Cusco]

In the CAA questions, there’s always an answer for someone who has correctly worked out the value but applied it the wrong way round (ala Pitts2112 and Cusco).

It wasn't the calculation but the definitions which caught my attention..............

Cusco;)

[LukeAir2009]

This is a practical way of doing it. If your altimeter is set to QNH 1009mb (pressure at mean sea level) and you land on a runway which is 90ft above sea level your altimeter will show 90ft. Now, suppose you want your altimeter to show 0 ft so you know that you're on the runway (QFE) - what do you do? You would have to wind off your altimeter. You would wind off 90ft which would be 3mb of pressure. Wind off pressure - wind off height. Your altimeter would then show 1006mb. If the airfield elevation is above sea level then QFE will always be less than QNH. If airfield elevation is below sea level QFE will be greater than QNH. :ugh:

Thats all folks!

[PhilD]

It's a mystery to me why we are trained to go through all this error-prone nonsense with QFE in the first place. Why not just train everyone to work on QNH as in the US, and eliminate this altimeter-twiddling?

I stopped setting QFE and decided to just work off QNH after my first US flying trip, when the instructor doing my checkout couldn't understand why I wanted to set QFE, and showed me how simple it is to just work off QNH

Having said that, I then flew from near sea level to an airfield elev 6752ft in less than 1 hour, so setting QFE might have been somewhat difficult anyway....

[India Four Two]

so setting QFE might have been somewhat difficult anyway....

.... if not impossible, as I proved to myself out of idle curiosity one day at Calgary (3550'). You run out of sub-scale and your fingers have knurled marks on them :eek:

[Crash one]

With the confusing bit being "does my altimeter go up or down?"
If you change from QNH (sea level readings) to QFE (ground level readings) you don't go up, but the ground comes up to meet you, (most of the time), so your "clearance distance" reduces. By about 30ft per millibar.
I wonder if I could have made that any more confusing than it looks.

[Talkdownman]

Death to QFE.
Misunderstandings on this thread alone should highlight why QFE should be dispensed with.
Thank you, RAF. Thank you SO much..... :*