Pressure Setting Question

[windsock9]

GBBPY DEPARTS AIRFIELD A AND FLYS TO AIRFIELD B. ON ROUTE THE RADIO FAILS SO THAT NO PRESSURE SETTINGS CAN BE OBTAINED. AIRFIELD A IS 600FT AMSL, AND WITH THE SUBSCALE 1020mbs SET (QNH) THE ALTIMETER READS 600FT.
THE QNH AT AIRFIELD B IS 998mbs. AND AIRFIELD B IS 300FT AMSL. THE FLIGHT IS CARRIED OUT VFR. ON ARRIVAL AT AIRFIELD B WHAT WILL THE ALTIMETER HANDS READ?

This has confused lots of people, i have a large variety of answers from different people! What is the answer

[englishal]

1260 feet?

600' = 1020mb there is a drop in pressure of 22mb which equals a height of +660' [1mb change ~ 30'] so 600+660 = 1260....Assuming a lot of assumptions of course, like ISA temps and the like.

EA
:D

Edited becasue I thought I made a major balls up. But in hind sight the 300' AMSL of airfield B is a red herring, all you need is the pressure settings as far as I can see....

[Shaggy Sheep Driver]

I'll jump in with both feet.......

Assuming 30 feet per millibar.

QNH at 'A' is 1020
QNH at 'B' 998, so that's 22 Millibars less which would translate into an overread of 660 feet if both airfields were at same height. Altimeter would therefore read 1260 feet.

But 'B' is 300 feet lower than 'A', so the sltimeter would actually read 960 feet on arrival at 'B'.

SSD

[Dufwer]

Presuming 30ft/mb, I get 960ft. At airfield A with 1020 set, sea level would be displayed as 0ft. At B if you had 1020 set then the altimeter would read 0ft at 660ft below sea level (1020-998=22 *30 = 660, high to low, down you go). the airfield is 300 ft further up giving a reading of 960ft.

What's the prize?

D

SSD obviously types faster than me and works top down, not bottom up!

[englishal]

Just been reading the other replies, I think the 300' thing is irrelevant, its the local pressure setting which dictates what the altimeter reads....so I'll stick with the 1260' !

Cyer
EA:D

[Monocock]

Assuming the rule of 30 feet = 1 mb and no pressure change over the distance, the altimeter would read 300 feet.

If, as you say, the QNH at B is 998 there has therefore been a pressure drop over the flight of 12 mb (equivalent to 360 feet) allowing for the difference in height AMSL of both airfields.

Therefore, on arrival at B (having not twiddled with the altimeter knob on route) the altimeter would read 60 feet.

The pressure drop over the flight is 12 mb (equivalent to 360 feet). Airfield B is 300 feet lower than A (360-300 = 60).

I stand to be corrected and have opened myself up for some serious flak on this one:ugh:

[Dufwer]

englishal, the datumn you have used for the starting point of your calculation is 600'. If you ignore the 300' of airfield B then you appear to be assuming your new postion is still 600' actual. It would be a long drop when you step out of the aircraft ;)

All this assumes that when the question says 'arrives' it means lands.

D

[FlyingForFun]

I don't understand why there's so much confusion. All you need to do is understand the question, and get rid of the red herring information. Who cares that the flight is VFR, or what the elevation was at an airfield you left some time ago?

What it boils down to is this:You are at an eleveation of 300'. The QNH is 998mb. The altimeter sub-scale is set to 1020mb. What does the altimeter readAs others have said, (1020-998) * 30 = 660', so you will over-read by 660'. Answer: 960'. The prize goes to Dufwer.

FFF
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[Monocock]

I can see this one getting seiously out of hand!!

Best new thread I've seen on Private Flying for months, nice one windsock9!:ok: :ok:

[High Wing Drifter]

Twas a good 'un. However I can understand Fun's frustration :uhoh:

[englishal]

Oops...yep, suddenly everything is clear :O ok, amend my answer to read 960'

[Knew I should have laid off the red wine last night :D]

EA

[Shaggy Sheep Driver]

FFF said:

As others have said, (1020-998) * 30 = 660', so you will over-read by 660'. Answer: 960'. The prize goes to Dufwer.

I was first, FFF. Assuming 960 is correct, I claim the bottle of bubbly - but I'll spray some all over Dufwer if you like, F1 style ;~))

SSD

[FlyingForFun]

You got the right answer, SSD, but you found a bl00dy long-winded way of solving a very simple problem!

FFF
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[Dufwer]

FFF, still got a hangover from the bash? Perhaps SSD should shower some bubbly your way too. Best cure IMHO.

D

[rustle]

Dufwer's answer was also the first correctly spelled one...

Not sure what a "sltimeter" is SSD - something to do with beer goggles perhaps? :eek:

[fu 24 950]

Left A in a light aircraft to fly to B . Flight time at a guess of about an hour. Pressure drop of 12 mb, very very nasty storm, (that is why the radio failed),, don't care about the height of B just pleasd to be on the ground

[Monocock]

Hmm........I take my hat off to Triple S and Triple F. Whilst you liberally spray your champagne I shall be twiddling with my sub-scale knob in the hangar!!

Can someone explain the "high to low-down you go" theory to me, it sounds like I need to learn it.

On the subject of sub-scale settings is it a REQUIREMENT to apply 1013 above 3000ft in Class G or only advisory? I recently called up a well known military frequency on my return from Cornwall and told them I was at FL50 and they asked me to fly on the Portland pressure instead. Why would they rather I did that? Is it because lots of people were VFR at that level that day on the Portland?

[Lucifer]

High to Low: Down you Go

means that with a pressure fall over the flight, while you maintain the same altitude on a given pressure setting, will lead to a gradual descent as the flight progresses. So if you are approaching hills and a storm is coming, you could be a lot lower than you think if not flying VFR.

[Shaggy Sheep Driver]

I'd heard it as "high to low, look out below". If you fly into a lower pressure area, your altimeter will 'overread' if you don't reset the subscale.

In the example that started this thread, that could make you beleive you are 660 feet higher than you actually are.

SSD

[High Wing Drifter]

Anybody know why Blackbushe, at 327' AMSL, always give me a QFE 13mb less than the QNH? I always seem to be 60' underground upon landing!

[rustle]

Blackbushe, in common with Farnborough, will give you the London QNH - which is not necessarily the same as 11mb higher than Blackbushe QFE...

Under the TMA you see :D

[rupetime]

If your radio has failed how did you know the QNH at the arrival airfield ?

rt

[Geardownandlocked]

just make a drawing, and things will be clearer. I draw all the time. I can do an ILS approach and draw a castle at the same time...

[Monocock]

"High to low, down you go"......

I wonder if that's something that Mrs Monocok should be learning too.:} :E :} :E :}





















:eek:

[bluskis]

Ruptime

Its on the metar that you got before takeoff. That will be pretty close.

[High Wing Drifter]

Blackbushe, in common with Farnborough, will give you the London QNH - which is not necessarily the same as 11mb higher than Blackbushe QFE...
But the is the QNH, don't explain non why my alty shows me 60' underground on the QFE tho :)

Not just on one aircraft either, but several.

[FormationFlyer]

12mb Pressure drop - who counted that?!??

1020 - 998 = 22mb....not just a nasty storm - a *really* nasty one...i think in pilot terms it comes under the 'lively' and 'interesting' categories.....i.e. Shouldnt be flying to start with - how silly can you be?!

If your altimeter reads correctly on QNH and then -60 on QFE - even though there is the correct difference between QNH & QFE then you should check your altimeter operation... +50/-75 is typical accpeted tolerences - so you are still inside - however - check the altimeter by winding it through 300' if the difference isnt precisely 10mb it probably needs recalibrating...

[FlyingForFun]

It's quite common for an altimeter to read less than zero on the ground with QFE set. QFE is the pressure at the highest point on the airfield's manoevering area. So unles you happen to be parked on the highest point, you will read less than zero.

Even so, 13mb difference between QFE and QNH, for an elevation of 327', sounds like a lot to me. 27ft/mb is, I think, the generally accepted approximation close to sea level, 13mb difference would suggest that they are using 26ft/mb. It is true that the TMA QNH may be higher than the local QNH, so that might be the reason. Why not pop into the control tower the next time you're there, and ask them? Then you can post the answer here, too!

FFF
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