Where to get altimeter pressure settings

[No. 2]

Hello,

Could someone tell me where I obtain the actual/forcast regional altimeter pressure setting when operating out of an airfield with no ATS? I tried looking on the MetOffice web site, but no luck.

Thanks

[FlyingForFun]

No. 2.

Before you reach the aircraft, you can check the METAR for the nearest large airport, this will contain the airfield QNH, which will be pretty close to the regional QNH.

Once you're in the aircraft, just set the altimeter to 0 (for QFE) or to the airfield elevation (for QNH). This will give you a good approximation of the regional setting, although you might want to leave a bit of extra room below any controlled airspace until you've had a chance to check with your nearest FIS station, or London Information.

FFF
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[Circuit Basher]

FFF - Agree with what you're saying for QFE.

Empirical Method
What you then need to do is know the field altitude AMSL. Divide that by 30 and this will give you the number in HPa that you need to subtract from QFE to give you the field QNH (if you're at a field with -ve altitude above AMSL, then add ;) ).

For the regional QNH, you need to obtain the forecast QNH for the region.

'Proper' Method
Get on the radio and contact London / Scottish Information or your nearest ATC and they'll tell you Regional QNH.

HTH.

[Edited by Circuit Basher because he made a complete a*$e of himself on the first attempt!!] ;) ;)

[BlueLine]

This is another example of how the UK antiquated system of Regional Pressure is no use to man nor beast.

If you flew on a QNH (from a nearby aerodrome) then you would have a pressure thats pretty close at any airfield nearby (within 30 miles). The map gives you the airfield elevation and if you really must, you can convert to QFE, another UK obsession!

Overall, its much simpler and safer to just fly QNH!

[englishal]

Why are 'Regional QNH' and 'aerodrome QNH' often quite different? Leaving the zone I transfer to regional, and often its a ~200'+ difference....I assume its something to do with the update time..........

About time we got ASOS / AWOS in the UK (automated weather / SIGMETS transmitted from VOR's etc).

Cheers
EA ;)

[FlyingForFun]

englishal,

First of all, aerodrome QNH and regional QNH are not the same. Aerodrome QNH is based on the actual pressure at one specific place - the aerodrome. Regional QNH is the lowest pressure recorded over the whole region. A high pressure gradient across the region (probably indicated by high winds) could therefore cause a big difference.

The time at which the reading is taken could make a difference, as you say. There are other time factors apart from when the actual reading is taken, too. At White Waltham, we use the QNH from Heathrow. When Heathrow update their QNH, we don't pick up the new value immediately, so Waltham Radio sometimes give slightly out-of-date settings to pilots in the short time before we get the new setting.

There are other minor things which could cause a difference. For example, QNH is not the sea-level pressure. QNH is a pressure setting which will cause the altimeter to show elevation when the aircraft is parked on the ground. In ISA conditions, the two are the same. But if temperature is significantly different to ISA, and you're taking two or more pressure readings from places with very different altitudes, you may find that the QNH you get is slightly different. Regional QNH is the lowest of these, which maybe different to where your airfield is located, even though the actual sea level pressure (QFF) is the same. I don't have time to get a calculator out and work out how significant this would be, but my gut feeling is that it would be very small, and that most of the difference would come from a genuine sea-level pressure gradient.

FFF
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Edit - I'm bored, so I got my calculator out. Imagine the following scenario. Sea level pressure (QFF) = 1013mb. Temperature = ISA. An airfield at sea level will show a QFE of 1013mb, and a QNH of 1013mb. An airfield at 5000' will show a QFE of around 834mb, and a QNH of 1013mb.

Now, assume that the temperature drops to ISA-30 degrees, but that the QFF doesn't change. At sea level, the QFE and QNH are still both 1013mb. However, the lower temperature will cause the 834mb level to drop by about 12%, or 600'. The QFE at the 5000' airfield will therefore be lower - about 813mb in fact. Because the surface level pressure is lower than it was in ISA temperatures, the QNH will be lower as well - it will be 992mb.

The regional QNH will be given as 992mb - significantly different to the airfield QNH you'll be given by the sea-level airfield. So yes, this can make a quite a big difference. Although I can't think of too many places where there are airfields with a 5000' difference in elevation in close proximity to each other!

I hope I've got the maths right! :eek:

FFF
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[PPRuNe Radar]

Why are 'Regional QNH' and 'aerodrome QNH' often quite different? Leaving the zone I transfer to regional, and often its a ~200'+ difference....I assume its something to do with the update time..........

Regional QNH is the lowest pressure recorded over the whole region. A high pressure gradient across the region (probably indicated by high winds) could therefore cause a big difference.

Neither of those. Regional QNHs are FORECASTS made in advance. It is lowest forecast pressure which might be experienced in the region over an hourly period. Normally the Regional for the current hour and the next hour are available.

This is another example of how the UK antiquated system of Regional Pressure is no use to man nor beast.

It is useful when flying in the Highlands of Scotland and preventing flight through Cumulo Granite. There are very very few aerodromes up there with facilities to get the QNH. And those that do are usually a great distance away. For more populated parts of the UK, then using a local aerodrome QNH does make more sense.

[matspart3]

'Regional Pressure Setting' as it is now called is the lowest FORECAST value of QNH for the particular Altimeter Setting Region. It's available up to 2 hours in advance from any Met. Office or, in most cases, airfields with an ATC Unit as they are received via the AFTN system (Aeronautical Fixed Telecommunications Network....big 'teleprinter' system linking major airfields worldwide).

Blue Line
What's wrong with QFE? I like my altimeter to read zero when I hit the runway!!

[FlyingForFun]

Ok, so regional QNH is a forecaset. But how does this explain why regional and aerodrome QNH are sometimes different? Or is the aerodrome QNH an actual and not a forecast? If that's the case, I didn't realise it before!

FFF
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[Another_CFI]

As stated by PPrune Radar a Regional QNH is a forecast of the lowest QNH for an entire Altimeter Setting Region and is normally available up to two hours in advance. An airfield QNH is an actual QNH and at many airfields the QNH is updated every thirty minutes.

On a day with a shallow pressure gradient the two may be fairly close but on a day with a steep pressure gradient there can be a significant difference given the size of the ASRs. Also given the vagaries of forecasting the Regional QNH may be forecast incorrectly with a tendency to produce a lower value than the QNH anywhere within the ASR.

[FlyingForFun]

Thanks - very interesting that regional setting is a forecast whereas aerodrome setting is an actual.

Another_CFI, are you saying that englishal, PPRuNe Radar and I are all correct in our reasons? But that the biggest difference is caused by the forecast/actual issue? Seems to make sense to me.

FFF
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[Keef]

I've often left home base with a QNH that worked (showed airfield elevation when I lined up), then called Lon Info and got an RPS that's 6 mb less. Seems not unusual round here.

That's only 180 feet (ish). The good news is the RPS will have me that much higher so the risk of inadvertant landing is less.

To answer the original question: if I'm taking off from an airfield where I don't have access to an official QNH, I just set the airfield elevation on the altimeter. That's a pretty good QNH - it even corrects for any errors in my altimeter!

[India Four Two]

Keef,

"if I'm taking off from an airfield where I don't have access to an official QNH, I just set the airfield elevation on the altimeter."

What a good idea. Oh, wait a minute, that's what we do all the time in Canada and the US. ;)

Matspart,

You might have difficulty using QFE where I fly from - airfield elevation is 3700'. Once in a fit of boredom, I tried winding the altimeter back to zero, but I ran out of subscale!

One of the other benefits of North American flying - apart from cheap prices and no landing fees of course :) - is that the Transition Altitude is 18,000', so unless I'm doing a wave flight, I never have to bother about Flight Levels.

An interesting topic though and reminds me of my training days at White Waltham and Shawbury.

[Whipping Boy's SATCO]

India Four Two, if you couldn't set QFE, obviously you should have immeditely set QNE!!!!!!!!!!!!!!!!!

[alphaalpha]

Just slightly off thread, but relavant, I think:

You should use the actual QNH of a nearby airfield, not regional QNH, when flying under certain TMA, such as the London TMA. If you don't then you could easily bust the controlled airspace above, the base of which is an altitude (determined using actual QNH).

Even worse, you could have a seriously bad close encounter if a) you used refional QNH which was 200 ft different from actual (quite common as others have said), and b) you planned to fly 100 ft below the base of the TMA, but actually flew 100 ft above according to your own altimeter and c) your altimeter was wrong in that it read low by 50 ft and d) the heavy traffic in the TMA was assigned an altitude 500 ft above the base, which is also common and d) the heavy flew 50 ft low (perhaps error in setting QNH) then your separation is reduced from 600 ft to 200 ft. Close!!

Regards

[Hairyplane]

I have a simple answer to No.2's original question.

If he has a radio - all he needs do is dial up Volmet and listen for his nearest airport data.

I operate from a Midlands farm strip and always load 128.6 London Volmet South. I can't quite get it on the ground but it kicks in immediately after takeoff.

Non radio - you just need to do the calcs.

Happy landings!

[BlueLine]

Why do we continually refer to Regional"QNH"? A QNH is the pressure measured at a point on the surface, reduced to sea level.

The Regional Pressure is neither measured, nor referenced to a point. It is the best guess for an area and is NOT a QNH.

Historically, it was used when non radio aircraft flew from one end of the country to another. Its importance and usefulness can be summed up by the number of Countries that have adopted it!

It contributes little to safety and may well be the cause of numerous infringments of controlled airspace.

To expect inexperienced pilots to deal with so many different pressure settings when they are not needed is not in the best interest of safety.

The logic of Regional Pressure is on a par with setting pub clocks 15 minutes fast!

[matspart3]

Example:

Last weekend I flew from Gloucester to Caernarfon on a direct track. Took off using the Gloster QNH, encountered lowish cloud and went 'on top' at FL65. Shawbury, Llanbedr and Valley were all closed, so no QNH available but we were able to set the RPS (obtained from London Info) and start our descent safely over an area of known high ground. Same applied on the return leg, when the cloudbase was lower still.
I agree that the use of multiple pressure settings can confuse students but altimetry isn't rocket science. If the tricks of the trade were taught better, there'd be fewer CFIT accidents!!

(BTW Caernarfon is lovely, beautiful mountains to fly (safely!) over and 2 minutes from the 5 mile stretch of golden sandy beach. Nice little museum and first class all day breakfast)

[GoneWest]

If anybody reading this thread is training for (or refreshing) their Radio Telephony phraseology and procedures - would they please note the comments of BlueLine and Matspart3.

There is no such creature as "Regional QNH" - it is now known as a "Regional Pressure Setting".

<<edit to say that I didn't realise there were two pages to this. BlueLine has just said it all - Regional Pressure Setting>>