Altimeter Setting

[plasmarb]

I know this has been asked a million times, and apologies in advance but I've researched this question and confused myself silly.

My understanding is that QFE - Height above aerodrome (0' on ground)

QNH - Altitude above aerodrome (Set sub scale with current MB reading) will display aerodromes elevation above MSL.

QNE - Altitude above MSL at standard setting of 1013 (used when above 3000ft, only if dealing with ATC)

As a microlight pilot I know we'll mostly deal with QFE & QNH.

My question is, how do you set QNH from QFE?

Thank you!

[Aerials]

Well, you know QFE as being zero feet when your aircraft is on the ground. Most if not all airfields have their elevation datum shown in the AIP or local web pages. At 30 feet per hecto-Pascal, you work out how many 30 foot (increments) there are in the airfield altitude. This gives the number of hPa which you then wind on to the altimeter. Your pressure sub-scale should now show the airfield elevation. Some airfields helpfully give a hPa difference as well as elevation, an example is here, not mathematically perfect but good enough for our use: Welshpool (http://airfieldcards.com/index.php/airfields-v-z/welshpoolairfield)

[engineno9]

Just to add to this, although it's something you can work out easily enough (as above) you'll probably find you just fly to whatever you're given. You wouldn't normally make a conversion.

For example, the airfield I fly from is at en elevation of around 200ft AMSL, circuit height is 1200ft AMSL. On the ATIS recording you get given the QNH, but the tower often give the QFE verbally. So you have the choice to set QNH and fly at 1200ft in the circuit, or set QFE and fly at 1000ft, as indicated on the altimeter. It makes no difference.

If you're only given one of them at any given airfield, you'd probably just work with that as opposed to converting.

[BillieBob]

QNE - Altitude above MSL at standard setting of 1013 (used when above 3000ft, only if dealing with ATC)QNE is not an altimeter setting, it is the indication on the altimeter when landing at an aerodrome with the Standard Pressure Setting set on the sub scale. It is used at high altitude airports where QFE is too low to be set on the sub scale. QNE has nothing whatever to do with 'Altitude above MSL', it is not 'used when above 3000ft' or 'only if dealing with ATC'.

QFE is the observed pressure at a reference datum (usually the aerodrome reference point or landing threshold) which, when set on the altimeter sub scale, will indicate height above the reference datum.

QNH is the observed pressure at a location adjusted, using the standard atmosphere, to mean sea level which, when set on an altimeter sub scale, will indicate altitude above MSL.

When set to a pressure of 1013.2 hPa an altimeter will indicate flight level

[Falcon99]

A simple way to determine QNH. If you are on the ground set the airfield elevation on the altimeter, the subscale now reads QNH. Obviously you cannot do this in flight. On the ground, set the altimeter to read zero feet and the subscale reads QFE.

I must disagree with BillieBob, QNE is most definitely an altimeter setting and is used by all aircraft operating above the Transition Level.

[farsouth]

Suggest you look up the definition of QNE Falcon99 - BillieBob has it correct.....

[Alex Whittingham]

Q Codes from the British Admiralty, the 1912 International Radiotelegraph Convention et seq (https://en.wikipedia.org/wiki/Q_code#Aeronautical_Code_signals_.28QAA.E2.80.93QNZ.3B_ICAO. 29) originally designed for morse to shorten transmission of repeated and predictable phrases. The letter Q being used at the start of a three letter code because it is relatively distinctive and not much otherwise used, the two subsequent letters (sometimes) having some attached meaning. For instance 'FE' allegedly relates to 'Field Elevation', 'NH' to 'Newlyn Harbour', 'DM' "Direction magnetic' (or 'magnetique') although I can't find any evidence for this and they may just be later mnemonics.

[Piltdown Man]

How? Just turn the knob. But are you asking where you get the value of QNH? If that is the case you can get it from the nearest airfield or London Information both of whom who will pass you the Regional QNH or the one closest to you. The next question is when. Personally, I'd do that on the ground and update it as either the pressure changes locally or I fly elsewhere. The benefit of doing so means your charts and altimeter match. Both your altimeter and your charts have the same datum.

[eckhard]

BillieBob has it correct.....

Not quite:

When set to a pressure of 1013.2 hPa an altimeter will indicate flight level

The indication is 'Pressure Altitude'.

'Flight Level' is an ATC and operational shorthand term for describing the availability of cruising altitudes above the Transition Altitude.

After all, if you set 1013.2 hPa on the ground at a sea-level airfield with a QNH of 1003, the altimeter will indicate about 300ft. Your pressure altitude is 300ft, but no-one would describe it as 'Flight Level zero zero three', would they?

I totally agree with BillieBob's definition of QNE though! It's amazing how often this is mis-understood. Not terribly relevant today, I guess......

[Falcon99]

My apologies to BillieBob.

[BillieBob]

The indication is 'Pressure Altitude'.Apologies for a less than robust attempt at paraphrasing ICAO Annex 2 which actually states that an altimeter, when set to a pressure of 1013.2hPa, "may be used to indicate flight levels".

To be strictly accurate, an altimeter with its sub scale set to 1013.2 would, according to the Annex 2 definition, only indicate pressure altitude in a standard atmosphere. How many nits do we wish to pick?

[plasmarb]

QNE is not an altimeter setting, it is the indication on the altimeter when landing at an aerodrome with the Standard Pressure Setting set on the sub scale. It is used at high altitude airports where QFE is too low to be set on the sub scale. QNE has nothing whatever to do with 'Altitude above MSL', it is not 'used when above 3000ft' or 'only if dealing with ATC'.
Thanks BillieBob

Question; you have to set 1013.2 on the sub scale so surely this is a setting procedure? What I meant by 3000ft was the UK's transition altitude to use QNE over QNH.

[eckhard]

Plasmarb,

I think you still don't quite get it.

If I were you, I would forget about 'QNE'.

If you're planning to fly a microlight around the UK, you will definitely need to understand QFE and QNH, and you seem to have some idea about those two.

Here is a brief outline of altimeter setting procedures that you may find useful:

Pre-flight check
Set the aerodrome QNH. The altimeter should read within 50ft of the published elevation.

If you like, you can set the QFE and the altimeter should read within 50ft of zero.

(If you want a more detailed check of the inner workings, for an altimeter that is new to you, or has recently come out of maintenance, you can verify that adding 10hPa to the setting shows a 270ft climb and vice-versa, and that there is no unconnected movement of the needles or sub-scale, especially if you pull or push gently on the setting knob while you turn it.)

Departure
Set the aerodrome QNH.*

*If you are staying in the circuit, some aerodromes might prefer you to set QFE.

Cruising flight
Below the Transition Altitude (generally 3000ft in the U.K.), use either the QNH of a nearby aerodrome or the Regional Pressure Setting (RPS).

If you plan to cruise above 3000ft in class G airspace, you will be above the transition altitude and you will set 'Standard' or 'ten-thirteen'.
You will then fly at a Flight Level that is suitable for your magnetic track (semi-circular level).

MATZ penetration
The controller will give you a clearance based on the aerodrome QFE. He/she will pass the RPS once you exit the MATZ.

Approach and landing
You can use the destination aerodrome's QNH or QFE, as you prefer.
Bigger airfields tend to just pass you the QNH but QFE is available on request.
Small airfields might just pass you the QFE. Again, QNH should be available on request.

Nobody will mention 'QNE' as it is not an altimeter setting. It is the reading of the altimeter set to 1013.2hPa when the aircraft is on an aerodrome. Put simply, it (QNE) is not relevant unless the QNH is abnormally low, in which case it is probably blowing a gale and your microlight should be in a hangar.

If you want to derive QNH from QFE, a quick way is to divide the airfield elevation by 30 and add that to the QFE.

Similarly, you can derive a QFE by subtracting 1hPa from the QNH for every 30ft of airfield elevation.

Most airfields in the U.K. have an elevation of 600ft or lower, so the conversion between QFE and QNH should be fairly easy.

Having said that, I think you can see that there is a big potential threat here for making a cock-up and setting the wrong number on the sub-scale, especially in a noisy, draughty environment. That is why IFR procedures need to be much more prescriptive.

[eckhard]

BillieBob:

To be strictly accurate, an altimeter with its sub scale set to 1013.2 would, according to the Annex 2 definition, only indicate pressure altitude in a standard atmosphere. How many nits do we wish to pick?

I would have thought that the indication would be pressure altitude whether or not standard conditions pertained.

If true altitude or density altitude were required, then a correction for non-standard conditions would have to be made.

But now I too have run out of nits to pick!

[engineno9]

For instance 'FE' allegedly relates to 'Field Elevation', 'NH' to 'Newlyn Harbour', 'DM' "Direction magnetic' (or 'magnetique') although I can't find any evidence for this and they may just be later mnemonics.

I initially remembered these as QFE = Field Elevation and QNH = Nautical Height which served me quite well.

I also recall seeing the following explanations which helped me, the first of which you've mentioned already. Although I'm just passing on what I've read elsewhere (or possibly on here) so I've no idea whether this is how they were originally derived:

QDM = Direction Magnetic (to the station)
QDR = Direction of Radial (thus magnetic bearing from the station)
QTE = True Emanation (true bearing from the station)

Where on earth we get QUJ from I don't know...!

[Sanger]

If you want to derive QNH from QFE, a quick way is to divide the airfield elevation by 30 and add that to the QFE.

Similarly, you can derive a QFE by subtracting 1hPa from the QNH for every 30ft of airfield elevation.

I've been googling this question and found your post, however could you give me a example?

Ie; Aiefield elevation of 60ft AMSL ÷ 30 feet = 20hpa or mb? So you add 20mb onto the subscale or add 20ft to the altimeter?

[eckhard]

Ie; Aiefield elevation of 60ft AMSL ÷ 30 feet = 20hpa or mb? So you add 20mb onto the subscale or add 20ft to the altimeter?

60 divided by 30 is 2.

2hPa is added to the QFE to convert to QNH
2hPa is subtracted from the QNH to convert to QFE

2mb is the same thing as 2hPa.

If you meant to say that the airfield elevation is 600ft, then the conversion would be 20hPa (or 20mb).

Because air is compressible, and the atmosphere is approximately 100km thick, the air at sea-level is compressed by the huge weight of air above it. The air at 10km altitude has a lighter weight of air above it; the air at 20 km even less; and so on. This variation of compression with altitude means that the reduction of atmospheric pressure with altitude is not linear.

At lower levels, the pressure reduces by about 1hPa for every 27ft.
At 40,000ft it takes about 70ft for the pressure to reduce by 1hPa.

So, for most airfields, a rate of 1hPa per 30ft is accurate enough.

The slow rate of change at high altitude is the reason that RVSM levels cannot be used above FL410. Pressure altimeters (that use the change of atmospheric pressure to indicate altitude) are not accurate enough at these higher levels to ensure that 1000ft separation is sufficient.

[Sanger]

Eckhard thanks for that.

So an example;
Sywell Aerdrome has a elevation of 424ft which ÷ 30ft = 14.13mb which means we add 14mb (rounded) to the subscale to give us QNH.

To get the QFE we do 424ft ÷ 30ft = 14.13mb which means we take 14mb (rounded) off the subscale to give us QFE.

Is that correct?

[eckhard]

Well yes, except you have to know the first value from which to start.

If you are airborne and you are given the QFE, you could add 14mb to determine the QNH.

If you were given the QNH and wanted to determine the QFE, you could subtract 14mb.

If you know neither the QFE, nor the QNH, you will be a bit stuck.

If you are on the ground at Sywell, you can set the altimeter to read zero ft. The subscale will show the QFE.

If you set the altimeter to read 424ft, the subscale will show the QNH.

The difference between the two should be 14mb.

[Sanger]

Spot on, do believe I have it!

On aviation charts they list the AMSL of obstructions however how do they know that a mast is 300ft AMSL if the pressure is fluctuating each day?

[tech log]

300ft AMSL means 300ft above metric sea level.

Metric sea level is a constant.

Elevation is a constant.

The height of the mast above ground is constant.

The mast will always be 300ft above metric sea level.

The atmospheric pressure may change every day, the only effect this will have will be to change the QNH....i.e. changing the sub scale you have to dial into the altimeter to get the correct readings.

Altitude is relative to MSL. MSL is constant. Pressure changes. So you have to change the sub scale to 'restore' the correct relationship between altitude and MSL.

[eckhard]

Excellent explanation from tech log.

To elaborate along the line of thought down which I believe Sanger was headed:

If the elevation of Sywell is shown as 424ft amsl, how do they know it will be 424ft when the pressure fluctuates each day?

Day 1.

The QNH is 1000mb. The aircraft is parked on the airfield. The altimeter sub-scale is set to 1000mb. The altimeter reads 424ft.

Day 2.

The QNH has changed overnight and it is now 1010mb. The same altimeter will now read 124ft. Has the elevation of Sywell changed? No, of course it hasn't. If you reset the altimeter sub-scale to 1010mb, the correct indication of 424ft will be shown.

Day 3.

Michael Fish is on duty and an intense low pressure system has moved across Northamptonshire. The altimeter from Day 2 now reads 724ft. What is the current QNH?

[Alex Whittingham]

Pedant mode [ON]
AMSL = Above Mean Sea Level
[OFF]

[tech log]

Where the bloody hell have I gotten 'Metric' from.

Slap on the hand for that one!

[eckhard]

Well, it was still a good explanation!

[Genghis the Engineer]

I know this has been asked a million times, and apologies in advance but I've researched this question and confused myself silly.

My understanding is that QFE - Height above aerodrome (0' on ground)

QNH - Altitude above aerodrome (Set sub scale with current MB reading) will display aerodromes elevation above MSL.

QNE - Altitude above MSL at standard setting of 1013 (used when above 3000ft, only if dealing with ATC)

As a microlight pilot I know we'll mostly deal with QFE & QNH.

My question is, how do you set QNH from QFE?

Thank you!
Coming back to the original question.

(1) Technically correct answer: subtract (airfield elevation / 27) from the QNH.

(2) Most sensible answer: ask the tower at your destination on RT what QFE is.

(3) Pragmatic answer, particularly if there's no tower: leave the altimeter on your last QNH. Mentally just add the airfield elevation to circuit height (or any other critical height) and fly to that.


Most countries don't use QFE at-all, it's become a bit of a British anachronism (although personally I quite like it), so (3) is actually the right answer in most of the world, most of the time and should not get you in trouble anywhere if you choose to do it that way.

[Sanger]

If the elevation of Sywell is shown as 424ft amsl, how do they know it will be 424ft when the pressure fluctuates each day?

Day 1.

The QNH is 1000mb. The aircraft is parked on the airfield. The altimeter sub-scale is set to 1000mb. The altimeter reads 424ft.

Day 2.

The QNH has changed overnight and it is now 1010mb. The same altimeter will now read 124ft. Has the elevation of Sywell changed? No, of course it hasn't. If you reset the altimeter sub-scale to 1010mb, the correct indication of 424ft will be shown.

Day 3.

Michael Fish is on duty and an intense low pressure system has moved across Northamptonshire. The altimeter from Day 2 now reads 724ft. What is the current QNH?

Sorry i'm not sure how to work this?

[Sanger]

Sorry to be a pain, Anyone help out with the above?

[engineno9]

Hi Sanger, I'll have a go at explaining if it helps.

Consider the altimeter a barometer, that's all it is. It senses pressure wherever it is located. To get a readout from it, you need to tell it where the ground is. You do this by telling it the pressure on the ground, either at the airfield (QFE) or at sea level (QNH). It then works out the difference between the two and gives a readout of approx 30ft for every 1 mb.

You must not lie to the altimeter about where the ground is, or the readout it generates will be incorrect.

If on Day 2, the pressure on the ground at sea level has changed to 1010mb, but the subscale is still set to 1000mb. You are now lying to the altimeter. The altimeter still thinks sea level is at 1000mb. But it is actually 10mb further down in the atmosphere. 30ft x 10mb = 300ft therefore the sea level is 300ft further down than your altimeter thinks it is. It therefore under-reads by 300ft and shows 124ft.

On Day 3, the opposite has happened, the pressure has changed again and we are once again lying to the altimeter. We know that it is over reading this time by 300ft (724ft-424ft), therefore it must think the sea level is further away that it actually is. It is out by 300ft/30ft=10mb, and thinks sea level is at 1000mb, so the actual sea level pressure must be at 990mb, some 10mb further up in the atmosphere.

Hope this hasn't confused you further - you may find it useful to look at some diagrams on this if you can get hold of any, it may be easier to visualise.