QNH - practical application?

[ayavner]

Hi guys,

 

I do know what QNH is and how to get it, how to apply it, what to expect when it changes, etc.

 

What I am *kinda* wondering is this... yes, I can see why you would want to have it for various relevant calculations. But, I wonder how much of a difference it really makes when dialing it up on the altimeter.

 

Seems like you get ATIS, dial up the QNH, and your needle moves a foot or 2 in altitude?? I can see wanting to be as accurate as possible, but really when you are that close to the ground (as in landing) at some point it seems like you aren't looking at your altimeter and counting down the feet, you are looking at where you are going!

 

Can someone help me with that? Is it just me?

 

adam

 

 

[nathan_c]

If the following information is incorrect, someone please correct me, I have only learnt this stuff in the last couple of weeks from an ATC perspective, and I am trying to apply to the situation what I can remember without re looking up the regs (ive been having to learn alot in a short period of time lately so I may have some figures mixed up.)

 

A change of 1 hectopascal is equal to roughly 30 feet. Therefore changing your QNH by 1 hpa will cause you to be about 30 ft out, if you are out by 5 hpa then you will be misreading by 150ft, which can obviously cause an issue. You are correct in stating that you should be looking outside when VFR, especially during landing, but it can effect things like the altitude you fly your circuit at, which will affect other aircraft in the circuit as well obviously due to altitude differences (say if call joining downwind they will expect you to be at 1000ft, not 850ft or 1150ft and they may not spot you) . If you are talking to atc, and giving them your level, or are cruising at a cruising level then if your QNH is set wrong you wont be at the correct altitude which is also bad :P In saying all that, you have to bare in mind though that a VFR altimeter is only legally required to misread by + or - 100ft, so you cant always guarentee even with the right QNH your altimeter will read accurately anyway.

 

Its alot more relevent if you are IFR though (Im assuming your not, but for informations sake I will say this anyway) where your altimeter really does need to be correct due to things like your lowest safe altitudes, cruising levels, traffic avoidance, approaches in IMC conditions etc etc. Due to this IFR altimeters have to be more accurate and are only allowed to misread by + or - 60 ft (obviously you would want as close as possible, but they are the legal limits).

 

 

[ayavner]

thank you, that makes perfect sense :-)

 

science in the real world, kids!

 

:-)

 

 

[facthunter]

At the moment we fly pressure levels. We are separated on the basis of them so there has to be a system where we are all referencing a common datum (or as near to it as possible.) If you are at a known height, ie an aerodrome that is level and the height of that drome is available, setting that height on your altimeter will hopefully have the QNH reading in the Kollsman scale. Conversly setting the area QNH will result in the alt reading the aerodrome height when you land. This does result in you doing a fair amount of mental gymnastics when you have to work out the altitudes to overfly, fly downwind and base legs etc but that's altimetry. You use forecast QNH figures for the area and time appropriate to your operation and aerodrome ATIS where applicable. This all asumes that your ALT is calibrated and working correctly. GPS heights are not the same unless it is co-incidental. The altimeter assumes a STANDARD atmosphere exists. It can't be calibrated to consider anything more IF you are flying above 10,000 feet (Varies with local QNH) You revert to "standard" MSL pressure of 1013.2. So everybody is still using a common reference at those "levels" They are referred to as LEVELS above that, not altitudes...Nev

 

 

[Ryanm]

If we have the pressure altimeters 1hpa out the wrong way, we could very well be below minimums on an instrument approach!

 

 

[shags_j]

That's why there is less error allowed on IFR than VFR.

 

 

[Guest Howard Hughes]

If we have the pressure altimeters 1hpa out the wrong way, we could very well be below minimums on an instrument approach!

But your altimeter(s) are allowed to be up to 60 ft out at departure, that's already two hector pascals!

 

 

[shags_j]

Well IFR is allowed to be 75ft out till first landing then if not 60ft out is u/s ;)

 

 

[jackc]

I know I am recycling an old thread here. Question I have is, if you were landing at a property airstrip and you made contact by radio to someone at that airstrip that could give you a barometric pressure reading you could set your altimeter to reflect that and have an accurate height above ground at that location? I think it’s called a QFE? Not QNH?

Is this correct please?

 

Cheers,

 

Jack.

[Nobody]

I know I am recycling an old thread here. Question I have is, if you were landing at a property airstrip and you made contact by radio to someone at that airstrip that could give you a barometric pressure reading you could set your altimeter to reflect that and have an accurate height above ground at that location? I think it’s called a QFE? Not QNH?

Is this correct please?

 

Cheers,

 

Jack.

Yes and no....

 

It would only give you an accurate height above the ground under standard atmosphere conditions. That is, if your altimeter has been set to QFE when you land at the airfield it will show 0ft. If you are in the circuit and the altimiter shows 1000ft you probably aren't at 1000ft exactly.

 

Many mechanical altimeters can't have a very wide range of QFE set in the scale. This can mean that at some high locations you just can't set the altimeter this way. It isn't commonly done in Australia but is in places overseas.

[Old Koreelah]

...if you were landing at a property airstrip and you made contact by radio to someone at that airstrip that could give you a barometric pressure reading...

 

Jack if your flight planning was up to scratch, I'd expect you'd know the elevation of the strip you're going to land on, plus the height of nearby terrains and towers, etc.

[facthunter]

I think QFE is used in some countries but it won't provide separation with other traffic except the particular aerodrome you are operating in reference to. It would simplify circuits for early students but once "out of the circuit" something better has to operate. Incorrect setting of a subscale can cause accidents, particularly in IFR conditions, as can misreading the altimeter also.

The 3 needle altimeter has caused problems for a long time and some use a radio altimeter as a back up. (nice feeling to know where the ground is).. You should get information under ALTIMETRY as to how it all works and it's part of your normal knowledgebase. You must know in advance just what the height of your destination aerodrome IS and add that height to all the circuit levels for manoeuvering which are normal procedure based on forecast or advised QNH figures. If those figures are accurate your alt will read aerodrome height at the aerodrome reference point. Some aerodromes slope a bit and you should allow for that if you are serious.. The QNH may vary with time as weather systems move across the country.. Nev

[poteroo]

If you were taught to fly your downwind leg with the destination strip/runway 'cutting' the strut or aileron/wingtip at a set point - then it doesn't really matter, (from your flying point of view), whether you are a couple 100 feet high/low: your aircraft will be in it's correct position in relation to the strip/runway. Get your head out of the cockpit and watch for the correct visual cues in your flying, and stop stressing over what an inherently inaccurate instrument reads.

happy days,

[Yenn]

I would agree with poteroo, except for the keeping a point on the plane aligned with a point on the ground. All you need is to watch what is happening, there is no need to be watching the altimeter, nor any other instrument if you are competent.

[old man emu]

I know I am recycling an old thread here. Question I have is, if you were landing at a property airstrip and you made contact by radio to someone at that airstrip that could give you a barometric pressure reading you could set your altimeter to reflect that and have an accurate height above ground at that location? I think it’s called a QFE? Not QNH?

Is this correct please? Jack.

 

The simple answer to your question is: Yes.

 

Within the calibration error of your altimeter, if you knew what was displayed in the sub-scale window of an altimeter reading zero feet altitude on the ground, then you could set the sub-scale to the same number and your altimeter would show you your height above the location of the altimeter on the ground. That sub-scale value is the QFE.

 

If you were going to be flying into the same strip a few times, then the first time you land, write down the current Area QNH, then wind the sub-scale down until the altimeter reads zero. That's the air strip's QFE. Now subtract the QFE value from the Area QNH and you will get a number in millibars. Multiply that number by 30 to get the strip's height AMSL. Jot that height down in your records.

 

Next time you fly into the strip, keep the altimeter on Area QNH and plan joining the circuit at (strip altitude + 1000). After that, do as poteroo advised.

 

Don't forget that when cruising below 10,000 there are "air tunnels" 1000' thick that you have to fly within.

 

 

[TABLE]

[TR]

[TH]Magnetic tracks[/TH][TH]From 000° through East to 179°[/TH][TH]From 180° through West to 359°[/TH]

[/TR]

[TR]

[TD]Cruising altitudes  (Area QNH)[/TD][TD]1500

3500

5500

7500

9500[/TD]

[TD]2500

4500

6500

8500[/TD]

[TD][/TD]

[TD][/TD]

[/TR]

[/TABLE]

[jackc]

Thanks everyone:-). All advice taken on board!

 

Cheers,

 

Jack

[APenNameAndThatA]

If you were taught to fly your downwind leg with the destination strip/runway 'cutting' the strut or aileron/wingtip at a set point - then it doesn't really matter, (from your flying point of view), whether you are a couple 100 feet high/low: your aircraft will be in it's correct position in relation to the strip/runway. Get your head out of the cockpit and watch for the correct visual cues in your flying, and stop stressing over what an inherently inaccurate instrument reads.

happy days,

 

I thought it could matter a lot. If someone was 200 feet high in the circuit and someone was 200 ft low overflying, then they could collide. Also, if planes in the circuit are not at the same height then it will make collisions more possible, especially as two planes that have not seen each other are approaching the threshold. Disclaimer: I have 50 hours.

[Old Koreelah]

I thought it could matter a lot. If someone was 200 feet high in the circuit and someone was 200 ft low overflying, then they could collide. Also, if planes in the circuit are not at the same height then it will make collisions more possible, especially as two planes that have not seen each other are approaching the threshold. Disclaimer: I have 50 hours.

I think you're both right. I guess Potty was suggesting you don't spend too much time looking at your Altimeter.

[facthunter]

I doubt many use it for landing, in a precise way. Under a certain height on approach you get pretty visually orientated with the "plane" of the runway . (projected flatness of it) and things like runway centreline. and your relationship with it. Calls are automatically made on some modern jets based on the radio altimeter height to reinforce flare height and rate of sink awareness by call rate. Nev