QNH question
 

hi folks,

the question below is one i came across when googling for a diagram that explains qnh and it's wrecking my head!

could anyone explain the answer in simple terms as i've totally blocked out on it.

thanks in advance.

k :)

"A plane leaves an aerodrome "A" which is 500' above sea level with a qnh of 990 mb and flies to another aerodrome "B" which is 200 ft above sea level with a QNH Of 1010. Assuming the pilot doesnt recalibrate altimeter from QNH at aerodrome "A" and maintains a height of 500 feet after leaving aerodrome "A" what is his actual height above runway level of airport B ? Whats his height above mean sea lever ?

No, I'm not going to answer it but I am going to tell you something which one of my first instructors told me 40 years ago. It's the answer to just about every altimeter question so just remember this:

When you turn the subscale setting up (ie increase the millibar reading) the altitude increases. When you turn the subscale down the altitude decreases. That's the indicated altitude on the altimeter we're talking about...

Now apply that to your question because it will provide the answer.

[QUOTE
"A plane leaves an aerodrome "A" which is 500' above sea level with a qnh of 990 mb and flies to another aerodrome "B" which is 200 ft above sea level with a QNH Of 1010. Assuming the pilot doesnt recalibrate altimeter from QNH at aerodrome "A" and maintains a height of 500 feet after leaving aerodrome "A" what is his actual height above runway level of airport B ? Whats his height above mean sea lever ?[/QUOTE]

Poorly worded question but here is my take on it. He leaves airport A and climbs to an indicated 500 feet, the datum on his altimeter set to 990mb QNH. As he flies towards an area of higher pressure, he effectively descends although his alt will still be reading an indicated 500 feet. The difference between the two QNHs is 11mb; about 330 feet. Therefore his vertical distance from sea level has reduced by that much. So if he is flying along with an indicated 500 feet on his alt, given airport B is 200 feet above sea level, he would be 30 feet underground by the time he gets there as he would only be 170 feet above sea level.

The question maybe implies that he is flys out of airport A at 500' above ground level; but to know that he would have set QFE on departure.....

Makes perfect sense after a bottle of red.

Obviously that bottle of red is stronger than you thought bottom rung!:} ;)

Agreed, badly worded question. I assume he climbs 500ft on leaving A, which would give :

20mb difference = 600ft.

500ft aal at aerodrome A = 1000ft amsl

Low pressure to high pressure means he effectively descends so:

at B : 400ft amsl and 200ft above B.

If he just maintains 500ft on his altimeter at A, he theoretically doesn't get off the ground until the pressure begins to rise, in which case he would be 100ft below sea level and 300ft below B.

I wish I could blame the above on a bottle of wine but if I'm talking out of my ar$e it's all my own doing!:p

I reckon you're on the right track Strepsil, but wouldn't he climb from a low to high pressure?

His datum of 990mb is 600ft above 1010mb so this would place him 1400ft above aerodrome B and 1600ft amsl.

If I'm wrong I'll have another couple of these rum and cokes!

[quote=bottom rung;2912087]Put the cork back in the bottle ;)


mind how you go when you fly towards low

Just as well I'm not working tomorrow. Just checking you were all paying attention!:)

QNH
 

Hi Kraggy,

Just few things to be clear. If you set QNH at airport A, you can not read a height during a flight. Having QNH set, you will read altitude. If you depart an aerodrome A and fly with QNH set 990 mb(hPa) and want to fly at 500 ft AAL, it means you will fly at altitude 1000 ft (500 ft you fly above aerodrome A + aerodrome elevation of 500 ft). You will be higher for QNH difference (1010-990=20 mb= 20x28ft=560 ft, 1 mb =28 ft for precise calculation). With the same QNH of 990 ft within area of QNH of 1010 mb, it means you will be higher for that 560 ft (1000 ft altitude in the begining + 560 ft = 1560 ft altitude (AMSL). that means you will be above aerodrome B 1360 ft (1360+200=1560 ft).
Remeber this phrase "From high to low watch out below", what means if you enter area with lower QNH, you have to climb because you are lower than your altimeter reads. In your case it is oposite, you are higher.

I hope it helps,

Best regards

letMfly - Knew I should have drunk something to give me a get-out clause!:O

Hey, what you doing in here :suspect:


:p

The easy way I've found to remember which way this goes (moreheight/less height) is that qnh provided by ATC is always rounded DOWN to the nearest hectopascal.(mb)
The reason is safety. If an aircraft is 30ft too low on approach, the results in a worst case scenario are potentialy more dire than if it's 30ft too high. Rounding the Q down provides up to 30ft MORE actual height/altitude. So, the lower the subscale is set, the higher the aircraft will be.
Try it on the ground. Winding the subscale down will show you at progressively lower altitude.

The way i remember it is
High Pressure to Low Pressure LOOK OUT :eek: BELOW
In other words as you transit from an area of high pressure to an area of low pressure (without changing the subscale) you 'sink' closer to the ground. :ok:

thanks for all the helpful answers folks.

very much appreciated.

have to agree that the wording of the question wasn't the best so thanks alot!

OK Kraggy... Well forget all the stuff about the effects of flying from high to low pressure, etc., these are just smoke screens in altimeter questions. Apply what I told you earlier and you'll have the answer!

Good luck

As a pilot I find some of the answers given here quite worrying if they have come from controllers. And lay off the red wine!

As a controller, I tend to agree with you fone_effect, but I am sure it will all change when NATS open its new training "Centre for Excellence".
By the way I can heartily recommend a good brand of Barbados rum for getting the brain cells working. :ok:
Kraggy, If you are doing altimetry calculations, 30ft per millibar is generally considered close enough, unless the question specifies more accuracy.

Altimetry
 

Nice to see that these quetsions are still around! Historically, quite a few pilots cam a cropper when the did not reset their altimeters and flew from an area of high atmospheric pressure to one of low pressure. Regional QNH values - (the LOWEST QNH forecast) - were introduced to meet this problem. "High to low watch out below" sums it up well. Hope the new "Centre of Excellence" is able to write better questions!

QFE vs QNH
 

Hey Kraggy and other altimetry buffs,
Spare a thought for some of your military brethren. At Newquay Cornwall Airport/RAF St Mawgan we are authorised to control in the Director's pattern, vis cct and departures with mixed ac ops on QFE and QNH. It works out quite simply provided you get your head around the coordination problems early. Some of the ATCOs stick to Flight Levels whenever possible! Seriously, it does work very well and the separation (No RVS on QNH ac) and obstacle clearance (No ROC on QNH ac) rules do subtly differ. The whole thing arose because of electric jets being unable to set QFE on their primary altimeter - leap in technology eh!? Guess who got the original (30 page!) Safety Management task?

Kraggy and to the pilot(s) who expressed his concern

I think one reason why the answers above are diverse or unspecific is that this is a poorly worded question, so badly worded it is unanswerable without making assumptions. I agree with ATCO 2 (although I was taught to use 1mb = 30' not 28'... I think for simplicity?). Placing neck & reputation on the block...

If you take off from A (elev 500'amsl) and climb to a height of 500' (height = distance above a datum, and in this case I will assume the general usage that the datum in question is the airfield?). Height + elevation means you will have climbed to 1000'amsl.
You fly straight and level and keep 990mbs set on your altimeter until you arrive at B. Here the QNH is 1010mbs, so your vertical position is not being measured above amsl, but from the point in space at which the atmospheric pressure is 990mbs.

The difference between QNH(A) and QNH(B) is 20mbs, which represents a vertical distance of 600' (20 X 30 = 600). Remembering that pressure decreases as you rise through the atmosphere, that point will be 600' above sea level, thus your true altitude will be 1000 + 600 = 1600'.

With an airfield elevation of 200', your height above Runway B will be 1400'

Have I passed?

Yep correct....
i read the answers immediately after the question and started to question myself, but this last answer is how i figured it....

heres the question for all u fellow radar controllers, if the QNH is 1013, is a FL seperated from an Altitude?, e.g. 6A from FL70

i work off the principal as it is not (because of the .25 in a FL pressure) i know its only 7ft difference, but none the less, its LESS than the standard. what do u guys do?

and YES i understand that pilots dont select the decimal when setting the altimeter....
:rolleyes: