PPL air law question QNH & QFE
Join Date: Mar 2005
Location: Home
Posts: 36
Likes: 0
Received 0 Likes
on
0 Posts
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
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
Join Date: Sep 2008
Location: Isle of Sodor
Age: 44
Posts: 15
Likes: 0
Received 0 Likes
on
0 Posts
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
How I would work this one..
1018-1001=17
17*30= 510
1150ft-510ft= 640ft
Answer =640ft
...........................
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..
Join Date: Dec 2001
Location: down-route
Posts: 413
Likes: 0
Received 0 Likes
on
0 Posts
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).
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).
Join Date: Sep 2005
Location: Norfolk UK
Age: 80
Posts: 1,200
Likes: 0
Received 0 Likes
on
0 Posts
Read the Question
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
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
Join Date: Oct 2000
Location: E Anglia
Posts: 1,102
Likes: 0
Received 0 Likes
on
0 Posts
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).
Cusco
Join Date: May 2009
Location: Tain
Posts: 3
Likes: 0
Received 0 Likes
on
0 Posts
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. 
Thats all folks!
Thats all folks!
Join Date: Jun 2001
Location: London, England
Posts: 191
Likes: 0
Received 0 Likes
on
0 Posts
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....
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....
so setting QFE might have been somewhat difficult anyway....
Join Date: Sep 2006
Location: Scotland
Age: 84
Posts: 1,434
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
