Airfield QNH - what's the point?
 

Why do Airfields not use the QNH of the surrounding Region?

If I'm inbound, the airfield's QFE is useful. If we didn't use QFE as per the US, then airfield QNH would be useful as an alternative. But I can't see any reason to have both.

If I'm flying past and getting a basic service, I have to conform to the airfield QNH for a few miles and then change back to the Region.

Obviously all aircraft in the area need to be using the same QNH, but why have a different one to the surrounding area?

I'm sure there must be a good reason, but I'm blowed if I know what it is.

Then perhaps a little more study of altimeter setting procedures is required.

Personally I never use QFE.

IFR traffic only uses QHN with a good knowledge of airfield elevation and MSA, however a visual circuit will likely be based on QFE.

If anything they could do away with QFE and then visual traffic would have to be more aware of airfield elevation.

Regional QNH is something completely different again. An airfield QNH is based on the pressure MEASURED at that airfield, and will be very accurate so when you touch down you will have airfield elevation indicated. Regional QNH is the lowest FORECAST pressure for a region, giving a worst case terrain clearance based on altitude.

By definition QNH is the pressure determined at a specific Point. A forecast Regional Pressure is based upon an estimate of the worst case (lowest) pressure and is known as the Regional Pressure Setting (RPS) it is not a QNH.

QNH is normally used for terrain avoidance, airspace avoidance, airfield procedures, and so-on within 25nm of the airfield in question.

RPS is for long routes where changing setting constantly isn't useful, and is the best case for a cluster of QNHs. It is conservative for terrain avoidance, but non-conservative for airspace, and potentially incorrect for flying an instrument approach procedure.

I use QFE, for preference, when flying visually in the circuit, but not for an IAP or terrain avoidance.

So, typical good practice is QNH on departure, RPS en-route (with a bit of fat to ensure you don't foul up and climb into the bottom edge of any airspace), then depending upon preference and procedure, QFE or QNH on arrival.

Not many countries use QFE, but it's useful and convenient in the UK.

G

with a bit of fat to ensure you don't foul up

Joking surely. With the tolerances some VFR pilots fly to "a bit of fat" reduces the safety margin between you and another aircraft with the correct QNH.

QFE...useless

QNH...why not use the closest properly reported altimeter setting to enhance accuracy?

Gee, how lazy are you...having to reach all the way up from the side of your body to reach the altimeter and actually move a knob.

These days I tend to set the nearest airfield QNH because it's more accurate than the RPS, which is by definition subject to inaccuracy.

The QNH can be obtained via the ATIS or ATC if speaking to them).

Humaround

There is no good reason to Keep QFE going there are good reasons to get rid of it.
Often aviation is steeped in tradition on the principal of this is how its always been done.
The overhead join another example from the past when aircraft had no radio, needed to see the airport details, needed to make sure they were over the right airfield etc.
I am against QFE because it adds another point of confusion. Flying IFR we use QNH only! Most of the world do not use QFE and there is a potential dangerous mistake in the making having two altimeter settings operating near the ground.

Pace

QFE is an anachronism beloved of the military for reasons unknown, airfield QNH is the really useful item, RPS is a nuisance and could be eliminated in favour of using nearby major airport e.g London Birmingham Manchester Edinburgh

A lot of airfields do use QNH rather than QFE - have a listen to the ATIS of any large airfield or look at the METARs. It is more relevant when flying pilot-interpreted instrument approaches, where the procedures are always referenced to QNH.
QFE is used more by training/smaller/VFR airfields and is easier for students/PPLs to use as you can just fly a 1000ft circuit rather than 1345 ft (or whatever). QNH is normally given as well. The RAF also use QFE (or used to, not 100% sure now), as their procedures are generally much more ground-controlled i.e. PAR, and ground controllers/PAR screens naturally work with reference to the runway threshold.
Regional QNH on the other hand is fairly meaningless and I personally never use it, preferring to use (and teach) use of any airfield QNH within 25nm. If, however, I was using regional QNH I wouldn't bother setting airfield QNH when flying past on a basic service, only if I was entering the ATZ.

Ah the old "what is the use of QFE?" debate. What larks, Pip.

Of course its of no use whatsoever (except one see below) and we should all use QNH. But I confess I do like it when the altimeter reads circuit height in the circuit and zero on the ground, its kinda comforting.

By the way I thought RPS didn't exist beneath CAS like the London TMA?:=

One fun use for QFE - confusing your students to see if they understand how things like altimeter setting, terrain clearance, base of controlled airspace, and the effects of changes in pressure as you fly towards a depression, all work. Seriously, this is important stuff and if they can't grasp QFE and QNH then they need some more help in this area.:rolleyes:

H

Could be interesting winding that little knob to set the QFE for Addis Ababa (HAAB) at 7,656' ASL...

Which is why we have QNE (which is not, of course, a pressure setting)

The assertion that is a fair personal point of view. It remains, though, a point of view. Personally, I rather like QFE for my simple puddle jumping activities.

In the olden days, the early '70s, it was normal at Ringway aerodrome to pass the QFE and QNH. Perhaps rather quaintly, Speedbird and Beeline drivers used to set it for landing. Being vastly superior and advanced, Clipper ones didn't.

http://images.ibsrv.net/ibsrv/res/sr...ns/mpangel.gif

Don't you get the airfield QNH by refering to the QFE?

RPS What's the use?
 

More to the point what's the use of a RPS? When the area is too big.

The Portree ASR covers about 27000 square miles. Imagine the possible pressure gradient over that area.

D.O.

Personally, I think we should all use QFF.

I had to look that one up.

"QFF is a Q code. It is the MSL pressure derived from the barometric pressure at the station location by calculating the weight of an imaginary air column, extending from the location to sea level, assuming the temperature and relative humidity at the location are the long term monthly mean, the temperature lapse rate is according to ISA and the relative humidity lapse rate is zero.
QFF is the location value plotted on surface synoptic chart and is closer to reality than QNH, though it is only indirectly used in aviation."

I'm inclined to agree that airfield QNH is more useful than QFE. The point of the post was to question the wisdom of aircraft in a small area using different QNH settings.

(I DO understand altimeter setting procedures, and I'm not too lazy to change the setting...)

It was in common use many years ago so a crew could take off, fly in radio silence and still have a reasonably accurate (i.e. safe) altimeter setting to use en route. That is why you can get the hourly plus the next hour.

It's less of an issue now that radio comms are so much better and ATIS stations more widely available.