altimeter setting
 

Have just seen tonight's TV weather forecast. Storm force winds and heavy rain over the Christmas period. But what caught my eye, from an aviation point of view, was the pressure in the "low" just to the north of the UK - it was 930 mB !

It got me thinking, can an altimeter scale be wound down that far ? And, if not, what setting would one use (as if anyone would want to be flying in that kind of stuff anyway ! ) ?

Anyone any thoughts on this ? :ok:

I think most alts go down to 950mb (ooh sorry EASA, hectopascals). 930, bloody hell that is low. One of the reasons we should fly on QNH IMO and dump QFE altogether.

My reaction exactly, which is why I thought I'd ask !

One of the Met men said that the lowest pressure ever recorded over UK land was 925 mB ( the 930 was over the ocean) , and they were expecting it to drop later tonight to 927 mB !:=

Dust off the QNEs!

I would be expecting a slight breeze over your way then...:)

If you think about it, 930 is 83 hecto thingies below standard atmosphere, that by my rough maths is around 2,300' difference. You would have to get the performance charts out if you were thinking of flying in that.

altimeter setting
 

Dave Wilson.
There are a few good reasons why we could ditch QFE but with most of our airfields close to sea level it won't make much difference what you call it. QFE or QNH you'll still have to wind the knob down to 930.

Indeed Bob (well you couldn't wind it down to 930 I think) I agree. What would you actually do then in that circumstance, maybe wind it down to the nearest thousand and subtract it? I think I would just add on another 35 hp's and call 1,000' sea level.

As an aside the highest airfield I visit in UK is around 825' ASL (Crosland Moor at Huddersfield) so you would have to wind down another 30 hps to get QFE for that field. IE have an altimeter that spun down to around 900 hps or less.

altimeter setting
 

In my current jet the scale goes down to 850. I guess when they designed that particular altimeter they researched the lowest recorded altimeter setting and subtracted a healthy margin to be on the safe side. I'm surprised that anyone would design one that only went to 950. Lazy or foolish. You decide.

I'm intrigued now. I'm off to the club tomorrow and will wind one down and report. Maybe you're right they go down to 850 and not 950.

I had one that would go down to 850 but most, in my experience, won't.

That's our third big storm this year isn't it. We could start to feel a bit got at.

G

The whole question is totally academic since no one will fly with a pressure setting lower than 950. Your sole worry will be how to stop the hangar doors taking off.

Funny you say that, I was overseas when we had the gales a few weeks ago left the plane in the hangar which has no doors with a west facing opening of 40ft wide by 15 hight, the east side has a door thats 10 ft by 12 feet which was destroyed by the wind blowing thru the shed, with the runners all bent and sheet blown 500ft across the field, funny (lucky) thing is the plane was parked without brakes or chalks facing the opening and never moved an inch, how'd that happen???

Flat plate of the door effect? I used to be able to do those sort of calcs, probably still could but I would put it down to that. You would be surprised how much pressure hangar doors have to put up with as opposed to an a/c which is designed to have blowy stuff passing over it.

out of curiosity I wound back a smiths altimeter I have here as a spare.
800mb was that instrument's lowest full setting.
I think you'll survive.

Here at Strathaven, we are 847ft above sea level.

It is not unheard of to be able to fly microlights when the pressure is low enough that you cannot set QFE. I have seen some fantastic low pressure flying days (maybe we were in the eye of the storm!)

I understand that, in the past at least, Loganair were once stuck in the Orkneys because they were unable to set QNH on their altimeters.

After a 24 hour delay, they got permission to set them 1,000ft out, and then correct once in the cruise.

For those who don't fly commercial ops, its an easy fix.

Xrayalpha
IMHO that's wot QNE is all about, strange Loganair didn't use it.

Definition of QNE - Landing altimeter reading when subscale set 1013.2 hPa.

See CAP413:
The QNE tables are in MATS Part 1 Appendix A Page 2, and were available long before Loganair was ever thought of.

For example, if the Strathaven QFE is 945 hPa then the Strathaven QNE is 1917 ft.

Easy… ;)

Checked our 182 this afternoon it goes down to 955, however you can still keep winding it down after that although there's not much point as you don't know what it's set to.

So, essentially, QNE is the Flight Level at which your wheels touch the tarmac, right?

(Obviously it won't be a *usable* Flight Level, as it'll be below the Transition Layer, but I just want to check I've got the concept right.)

Sort of. But try not to think of it as such. QNE is the reading on the altimeter on landing with the subscale set 1013.2 hPa. In the Strathaven example you would be rounding out at 'FL19'. But best not to think of flight levels existing below the Transition Altitude. All you are looking for is, say, flying a circuit at approx 2900ft, final at approx 2400ft, landing at 1917ft etc. Needs special care in IMC! Use a RadAlt if you've got one…

Try it out in low pressure in VMC. The tables start at 979 hPa falling, so you should have at least 29 hPa or so to play with to set a real QFE on a second altimeter for comparison until you run out of subscale.