LARS units and regional QNH

[bookworm]

On a flight back from Cornwall back to Cambridge yesterday, I got the usual excellent service from a chain of LARS (and non-LARS) units. One minor event made me ponder altimeter setting procedures at Brize, though.

Over Oxford, approaching WCO at FL50, I wanted to descend to get beneath the Luton CTA. I asked specifically for the Brize QNH and said I was descending to altitude 3300 ft. In response I was given the Cotswold QNH and asked to report level at 3300 ft, which I duly did.

At WCO I was asked to freecall Luton, and on this particular occasion had no difficulty establishing contact and getting the Luton QNH, which was a good 8 mbar higher than the Cotswold. That, of course, put me above the 3500 ft base of the approaching Luton CTA. Even if Brize didn't have a London QNH, the Brize QNH would have been much closer than the Cotswold, which, as a forecast lowest QNH for a large region and a whole hour, is often significantly lower than the actual QNH. I have no doubt that the Brize controller was procedurally correct in asking me to fly on the Cotswold QNH, even though my procedure is to set a local QNH whenever possible.

So, can I choose my altimeter setting in class G -- though I appreciate it might make separation under RAS difficult -- and if not, what's the best way to handle such a situation?

[matspart3]

Need some military input here....

Brize do seem to have some funny ideas about altimetry outside CAS. When we prenote inbounds, they normally allocate a height based on their QFE. Transits either get Brize QFE or the Cotswold, but they always ask for our QNH for inbounds. I'm guessing it's Unit specific procedures?

As far as I'm concerned you can use whatever setting you like in Class G, that is appropriate to your flight and I'm not entirely comfortable with the 'climb/descend and report level..xxx ft, on xxx mb' military phraseology under anything other than a RAS, but that's a whole other can of worms!!

In the situation you describe, I would have asked for the London QNH.

Notwithstanding the above, Brize do an excellent job with limited resources

[Chilli Monster]

Originally Posted by bookworm

So, can I choose my altimeter setting in class G -- though I appreciate it might make separation under RAS difficult -- and if not, what's the best way to handle such a situation?

The Brize Controller, although not at fault, should have had a little bit more airspace awareness.

Yes - you can choose what you fly on, especially in that scenario. I would have re-iterated your request, giving in as few words as possible why you wanted it. You could probably have asked for the London, as I believe Brize might hold that for exactly the reason you wanted it - that might have been a bit of a prompt to the ATCO. If you had bust CAS because of this then it would have been an interesting debate as to who would have been at fault.

Yet another reason why Regional Pressure Setting (It hasn't been technically called the Regional QNH for a while) should be shown the door.

[2 sheds]

Even the Brize QNH would have been better than nothing, pending obtaining a LTMA QNH.

It would be a great advance to flight safety if RPS were kicked into touch (apart from certain specific areas, perhaps - the middle of Wales, Scottish highlands, North Sea) - and the military stopped this ludicrous obsession with QFE reference outside the immediate circuit area.

[bookworm]

Originally Posted by matspart3

Notwithstanding the above, Brize do an excellent job with limited resources

I strongly agree. I guess the main thrust of my original post was along the lines of

It would be a great advance to flight safety if RPS were kicked into touch

Actually, kicked out of the stadium, perhaps...

[vapourer]

Hesitate to get involved in the regional pressure setting dog's dinner but the plain fact is that the airspace beneath TMAs and CTAs, except airways and Daventry and Worthing CTAs, does not form part of the Altimeter Setting Region system and pilots should use the QNH of an adjacent aerodrome. Therefore in my opinion the person who would be responsible for an unauthorised excursion into CAS by an aircraft flying on the RPS is the pilot.

In fact back in the late 80s early 90s there were a couple of Airproxes caused by military aircraft aborting low-level missions and climbing into controlled airspace whilst using the RPS because they had no means of obtaining a local QNH. As far as I know this particular issue has never been satisfactorily addressed despite CAA Recommendations to the MOD at the time, although I stand to be corrected.

[CAP493]

vapourer is absolutely correct.

Golden Rule = never never use anything other than a local QNH when operating below any controlled airspace (CAS) the base of which is defined as an altitude. In all cases the altitude concerned will give you a clue as to the local Transition Altitude which could well be above 3000 feet.

Irrespective of whatever pressure setting you are given by any ATC unit, if you're operating below CAS where the base is an altitude, set a local QNH to avoid infringing it because the RPS will always be lower than the actual QNH and so if you use it, you'll actually be higher than your altimeter indicates.

[Spitoon]

The key to all this is understanding what the various pressure settings mean and what they can or should be used for. RPS is for assuring terrain clearance whilst a local QNH is more useful for avoiding airspace or for ATC to separate traffic in the area. The bottom line is that the pilot should use the most appropriate pressure setting(s) unless specifically instructed to use a different setting by ATC. The real problem arises when you're trying to avoid terrain and airspace at the same time!!

[aluminium persuader]

We (military ATC, not a LARS unit but near as dammit!) have recently changed our procedures & no longer issue a regional unless specifically asked for. We're near lots of CAS & the local QNH is so much more accurate.

ap.

[2 sheds]

Well done, sir. One down, one to go (QFE)!

2s

[Standard Noise]

If a pilot wants the RPS so be it. Doesn't make a blind bit of difference to me as long as I know which setting he's on.

[chevvron]

I seem to remember that when the military abandoned their 'QNH only' procedures, they also abolished local QNH; departures were supposed to change from QFE to SPS on climbout and vice versa on descent; if staying below transition altitude, only RPS was to be used. Of course this meant that Northolt, Benson and Odiham, being in or below the LTMA, had to operate non-standard, which met a lot of resistance especially with new controllers straight from Shawbury!!
Never did get a reasonable explanation why the military chose to revert to using QFE, especially as the change was very poorly notified, whereas the change to QNH procedures was well planned and executed, with the excellent Dave Harrison visiting most units to brief them. I WAS told it was because an 'elderly gentleman' (senior officer to those unfamiliar) cocked it up one day on a check ride thinking he was on QFE and trying to descend to zero on his altimeter, but that was only a rumour.

[Pierre Argh]

Never did get a reasonable explanation why the military chose to revert to using QFE, especially as the change was very poorly notified, whereas the change to QNH procedures was well planned and executed, with the excellent Dave Harrison visiting most units to brief them. I WAS told it was because an 'elderly gentleman' (senior officer to those unfamiliar) cocked it up one day on a check ride thinking he was on QFE and trying to descend to zero on his altimeter, but that was only a rumour.

IIRC it was a large lobby of Transport pilots who pressed for the introduction of QNH, that was ignored by a silent majority... until it was introduced and then a vociferous bunch of FJ pilots stood up (probably inc the senior gent mentioned) and demanded a reversal on the grounds of flight safety.

The RAF introduced the change without reference to the RN, pushing ahead with an ammendment to Military Flying Regulations... which as you can imagine didn't go down well.

IMHO the snag was a making an attempt to switch from one pressure setting with the total exclusion of the other that casued many snags. The effect of this Cock-up has had a lingering effect, even mention QNH anywhere near STC these days and they'll cry foul citing this badly managed/considered event

back to the subject of the original post... one of the problems of RPS is that its a forecast lowest pressure acroos the region... some of these are large, so for example when there's a massive Low in one corner the difference between RPS and QNH in the opposite corner (and indeed between adjacent RPSs) can be significant.

[2 sheds]

Originally Posted by Standard Noise

If a pilot wants the RPS so be it. Doesn't make a blind bit of difference to me as long as I know which setting he's on.

There is, however, a small matter of verifying the Mode C !

[tmmorris]

Benson do the same thing - blindly give RPS regardless of the circumstances. I've often departed from Benson for CPT and been given the RPS, and virtually had to go to 'words twice' to explain that actually I would be climbing to and maintaining my chosen altitude on the LONDON QNH to avoid busting CAS at Compton; though at least Benson do quote the Lon QNH on their ATIS.

T

[dolphinops]

Transiting in the local area you'd get the RPS (and for the next area as you leave cos I'm good like that)
Transiting the MATZ 3000' or less and you should be politely requested or politely instructed to set the QFE.
This is primarily to make coordinating departing or Radar pattern traffic a damn sight quicker and safer for all concerned.
If you want to stay on RPS fine by me. Just keep a good look out
That's notwithstanding CAS concerns. I don't have that snag.

[Standard Noise]

Originally Posted by 2 sheds

There is, however, a small matter of verifying the Mode C !

Which doesn't matter too much unless you're using it for separation purposes.

[2 sheds]

WHAT....?!!

[Il Duce]

"...unless you're using it for separation purposes"!!!????
As opposed to, perhaps, using it for cooking purposes or medical purposes?

[neilmac]

RPS

I am guessing the Brize controller wasnt busy so by giving you the Cotswold and not his QNH/QFE to transit his Zone, thus saving you another pressure change on leaving. In that area I believe you are in the Cotswold RPS? So at WCO change to Luton and get either their QNH or London RPS. Its not really an awareness problem whey should a BZN controller worry about your altitude on crossing the Luton CTZ? In a GA plane WCO to edge of Luton zone is plenty of time to adjust if you give Luton an early call.
Benson usually give the Cotswold when tracking towards CPT because 8/10 times in previous experience pilots GH to the west of CPT in the Cotswold RPS area! To the east of CPT they will give you the London RPS or at least I would expect them to. QFE when flying next to mil aerodromes helps co-ordinate you against IFR traffic inbound. Our Mode C readout is always based on 1013 so at least with a/c on QFE its easy to work out internal co-odrination rather then a long winded call right so your on the Cotswold im on QFE .....waiting to work out differerences do I have enough separation when your planes are getting closer! Most of the time its pretty easy but when your busy it helps. Im off for a ly down pressure questions do that to me!
Happy Flying

NM