QNH, QFE?, density & pressure alts.

[Lump Jockey]

A couple of questions for you, that I'd like some help with! I've read, but have now forgotten unfortunately, that it's possible to calculate the airfield's elevation by taking the QNH and QFE (is that right?) away from each other then multiplying by a figure...? Can anyone clarify please?
Secondly, when do you need pressure and density altitudes? I've seen a map of the UK where it's split into loads of regions, does this have something to do with it? (Each area having its own value.) If this is so, does each region keep that value for "life"? or is it a guideline or something?
Thanks ever so!:)
LJ

[Fly Stimulator]

RTFM (http://www.amazon.co.uk/exec/obidos/ASIN/1840372672/thelakelanlooptr/202-6368519-5981413)

[Pilot16]

Hi

to answer your first question:

1 millibar of preassure equals roughly 30 feet.

If for example if QNH (sea level preassure) is 1012 millibars
and QFE (preassure on aerodrome) is 1009 millibars
the difference is 3 millibars (1012-1009=3)

since 1 millibar equals roughly 30ft
3 X 30 = 90ft...aerodrome elevation is therefore 90ft feet above mean sea level.


thats the answer to your first question, if i said anything incorrect feel free to correct me anyone.

P16

[Speedbird252]

Hi Lumpjockey.

Understanding the relationship between Airfield elevation and Mean Sea Level is the key.

QNH is the barometric Above Mean Sea Level (AMSL) pressure in Hectopascals. On this setting, the aircraft altimeter reads altitude.

QFE is corrected for altitude pressure at the airfield, on this setting the altimeter in the aircraft reads Feet AGL (Above ground level). (Usually above the highest fixed point on the runway)

If, say the airfield you fly at is above or below sea level, then the QNH and QFE figures will naturally be different.

One millibar or hectopascal = 27ft. Once you have the QNH and the QFE reading it is then possible to work out the difference in airfield elevation and sea level.

If the QNH is 1010mb and the QFE, quoted to you at the airfield is 1019, the difference is 9hPA. Again using the 27ft per mB, that would equate to 9mB X 27 = 243FT.

Assuming that you are, from your question attempting to calculate the field elevation, this would give you an Airfield elevation of 243FT AMSL.

In answer to your question, 1019 minus 1010 = 9 x 27 = 243.

So I guess it works.


Im not so sure what you are asking with your second question, The UK has 20 (twenty) ASR`s (Alimeter setting regions) which are ares that on-route Pilots can use as triggers for updating their altimeter setting. As you know that here can be some very nasty implications in flying distances without updating you altimeter.

Pressure and density altitudes are something that you need to go through int he Trevor Thom or Jeremy Pratt books.

In basic, once you have climbed through the Transistion Altitude, (which is usually 3500ft in the UK) you are then referencing your height to a pressure altitude, which is expressed as a Flight Level. IE - If you are at 4000ft, the you are at FL40, if you are at 7500ft you are at FL75. You also need to understand the the relationship between transition level and transition altitude, which is also rather boring, yet rather relevant!

It would take ages to explain, and Im sure you have the Trevor Thom book (Air Law) - if you dont go get it!!

It would be real difficult to explin it here, and the book does it well.

For the record, Im a 29 hour PPL and ive answered the above on what I think I know, please be gentle with me if I have got it wrong!!

Regards all,

Speedy










:p

[Lump Jockey]

Thanks PILOT 16 , that's what I read! Good man!

Thanks to all of you. Good solid answers! I will get the book, but it will be for information only, as I don't intend on pursuing a flying career, or hobby as of yet! Cheers again.

[Speedbird252]

missed a bit -

Each ASR does have its own value. It is the lowest available QNH taken within that area, that then becomes a reference for everyone flying on the QNH in that ASR.

Ive also got to agree with Fly Stimulator, I answered this as it was alomost like an exam question, but this sort of stuff is available to everyone,

RTFM! - We did!

Speedy.

[Whipping Boy's SATCO]

Just a minor correction to Speedbird252, the Regional QNH (ASR) is the lowest forecast QNH within a particular region.

[Speedbird252]

Well pointed out Whipped SATCO,

Forecast QNH is indeed correct Sir. ( i could admit to getting it wrong, but realistically i just didnt know the correct definition!)

Does the rest sound right?

Cheers mate

Speedy

[noelwj]

Hey you guys, excellent answers and advise, however why not simply come on over here to Northern Ireland, both Newtownards and City of Derry/ Derry City/ Eglinton Airport are situated on the waters edge and enjoy the handiness of identical pressure settings.

[drauk]

What a lot of responses. Then again, I wouldn't invest too much time in trying to answer Lump Jockey's "question". If you (metaphorically) like the sound of your own voice then go ahead, but Lump Jockey is some kind of wind-up merchant. Elsewhere on pprune he has claimed to be a builder and then elsewhere again a car sprayer. He asks questions about different flap designs on heavy aircraft, INS navigation, different categories of automated landing systems and yet claims not to understand the relationship between ground speed, wind and airspeed (i.e. addition). Some of his questions are quite amusing though, so no complaints from me.

[Lump Jockey]

Oi, Drauk.......hello mate!!:O First off, are you a spy? No seriously, of course I understand a lot about flying and the mechanics of flying, but there are "silly little things", (excused?), that I'm not 100% on. That's the bottom line. My wording could be a hell of a lot better, agreed! (I'm concentrating on the punctuation you know!). I am a builder, well was a builder, but jacked it all in for a better paying job and less hours.:) Anyway, enough about me, do you come here often?!:= My questions are, contrary to what you think, 100% genuine. I'm also glad I amuse you Drauky!!:D This question with the TO speed and the other one with landing speed, all stems from a couple of pilots I know who have both told me things that either don't make sense, or contradict each other!
One said the TO speed, (forget about TOW and all that for this question..), if you on r/w 25 with wind at 210 at 20, said the GS is 44KIAS if the TO speed was 60KIAS. So, how does he find that out?
Cheers guys, love the lumpy bricky-come-sprayer-come-pilot-come-whatever you want!

[Chilli Monster]

Lump Jockey

You might want to read up on some abbreviations ;)

KIAS - Knots indicated AIR speed.

The only time that can be the same as the Ground speed is when there is zero headwind / tailwind component. Therefore the Ground speed in your post just needs to be referred to as knots.

As for pressure and density altitude. Pressure altitude is based on ISA (international Standard Atsmosphere). Air Density however is also affected by temperature - the hotter the temperature the lower the air density. Not very important in the UK but at hot and high airfields where you can have field elevations of 4000ft with a high outside air temperature your density altitude can be in the region of 5-6000ft. Makes for extremely long take off runs and very slow climb performance.

[FlyingForFun]

Though I'd add my bit.

So, Pressure altitude vs Density altitude. The ISA (International Standard Atmosphere) is a theoretical atmosphere, it doesn't exist in real life but it's supposed to be an "average" of what the atmosphere really does.

In the ISA, pressure is 1013.2mb at sea level, and decreases the higher you go. In real life, pressure always decreases the higher you go, just like the ISA, but the starting point isn't always the same as the ISA. It decreases at roughly the same rate as the ISA (which is about 1mb per 30 feet at sea level) - this will vary very slightly depending on the temperature, but not enough to worry about unless you're designing instrument approaches or doing ATPL theory exams. The "pressure altitude" is the altitude in the ISA at which the pressure is equal to the current real-life pressure. You can find it by setting your altimeter to read 1013.2 (1013 is close enough!) and reading your altitude. All "flight levels" are based on pressure altitudes - in most of the UK except the London area, that means anything above about 3000'.

Density is a little more complicated. Like pressure, the ISA has a specific starting value, and it decreases the higher you go. Like pressure, the real-life density doesn't always do exactly the same as the ISA. Here, the temperature is relevant - in fact, to find the density altitude, you take the pressure altitude then add a correction for the temperature. Very hot temperatures reduce the density, and therefore increase the density altitude. Aerodynamics all depend on the air density, since it's the density which determines how "thick" the air is, and how many molecules of air are going to be affected by your wings. So your take-off run will be much longer at high density altitudes (i.e. low densities), because there are fewer molecules and so you need a higher true airspeed before your wings will lift you off the ground. It's quite rare to see anyone calculate density altitude. Instead, performance charts in aircraft manuals usually have a bit of each graph which combines the pressure altitude with the temperature - it's much easier that way.

Hope that helps!

FFF
--------------

[Flyin'Dutch']

I know that you are not worried about some hotness but:

Air Density however is also affected by temperature - the hotter the temperature the lower the air density. Not very important in the UK

Does not justice to the issue of lacklustre performance of normally aspirated machinery on hot days over here.

Try flying an Arrow near MAUW on those hot summer's days.

:eek:

It aint funny.

FD

[MayorQuimby]

Lump Jockey:

You said:
"if you on r/w 25 with wind at 210 at 20, said the GS is 44KIAS if the TO speed was 60KIAS. So, how does he find that out"

On takeoff, the speed will be 60KIAS (airspeed). However the ground speed will only be 45 (give or take) knots, as there's a headwind. If the wind was blowing from straight ahead, you'd already have 20 KIAS (airspeed) before even starting your takeoff roll, giving you a groundspeed of 40 knots on takeoff. However, since the wind is not blowing from straight ahead it's less than 20. The calculation is:

Runway 25 = 250 degrees (give or take)
Wind from 210 degrees, so difference is 40 degrees.

Headwind component (part of the wind that is blowing from straight ahead) = COS(40 degrees) * 20 knots = 15.32 knots.

60 - 15.32 = 44.68 - call it 45 KIAS. That'll be your groundspeed on takeoff.