Pressure Setting Question 2

[iainpoll]

This is a hard one, give it a go...

An aircraft takes off from airfield A, ELEVATION 270ft, with the QFE of 994mb correctly set on the altimeter.

WITHOUT resetting the altimeter the pilot lands at airfield B, 405ft HIGHER than A, with a QNH of 1000mb.

The lapse rate is 27ft/mb.

On landing at B what will the altimeter read?

[vintage ATCO]

Do you want the answer on here? :D

[iainpoll]

There will be a prize at the private flying bash:ok:

[Monocock]

Using 1 mb= 27 feet my guess is that the altimeter would read 513 feet ?????????:confused:

[FlyingForFun]

This isn't a hard one at all. It's exactly the same as the one from yesterday. Just ignore the red herring information, and realise that you're at 675', QNH=1000mb, and altimeter sub-scale is set to 994mb. Everything else is irrelevant.

The altimeter is 6mb out, 6mbx27ft/mb = 162'. The sub-scale is set low this time, so the altimeter will under-read by 162'. Answer: 675'-162' = 513'.

Do I get the prize this time? I promise that if it's champagne I will spray it liberally over everyone!

FFF
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Edit - Monocock beat me to it, by 1 minute. But since I described how I got my answer, and he didn't, I still think I should claim the prize!

[Monocock]

FFF - While you were getting p****d up on yesterdays winnings I have been honing my altimeter setting skills overnight!!!

Glad I got that one 'cos my effort yesterday was particularly embarrassing:O :O

[englishal]

After my previous poor attempt at an answer I'm not even going to try....Instead I think FFF should get the shampoo, and I'm going to copy his answer and say "Its easy, 513' "

:D

[iainpoll]

OK OK OK, I was a little kind with that one, but I was aiming at PPL's, rather than ATPL'S:ok:

FFF and Monocock win a Sm!rn0ff Ice each...

Now if you fancy winning a bottle of my fave vodka at the next bash (5 July) have a crack at this....



An aircraft must clear a mast, elevation 610m agl, by a minimum of 1000 ft.

The regional QNH is 998mb, and this has been correctly set in the altimeter subscale.

The indicated OAT at 3000ft is 14 degrees F.

At what indicated height will the 1000ft clearance be achieved?


Come and have a go if you think your hard enough....

[FlyingForFun]

Ah yes, I was wondering how long it was before someone introduced temperature into the problem!

But, unless I'm mistaken, we need more information to answer the question - the elevation of the ground on which the mast is situated.

FFF
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PS - If your favourite vodka is Odessa, I'll let someone else have a go :yuk:

[In Altissimus]

Why not make a fool of myself (hardly the first time :} :} )

As a naive PPL, I'm gonna say that the temperature is a red herring.

So, 610m = 2001' + 1000' = 3001'

I suspect this is wrong...

[FlyingForFun]

Ok, in that case:

The mast is 610m agl, or 610m msl, or 2000' msl. To clear it by 1000', we need to be at 3000'.

Since we have QNH correctly set, if it were an ISA day, the indicated altitude would be 3000'.

But it's not an ISA day. The temperature at 3000' on an ISA day would be 11 degrees C. The temperature we're given is 14 degrees F, or -10 degrees C - 21 degrees below ISA.

Because it's cold, we will be lower than indicated. The approximate correction is 4% of your height above ground for every 10 degrees below ISA. So for 21 degrees below ISA, we need to correct by 8.4% of our height above ground. Height above ground is 3000', so the altimeter will indicate 3252'.

FFF
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[iainpoll]

FFF,

I believe your last post to be a delaying tactic as you have forgotten the formula;)

As for vodkas, me faves at the moment are Bison, Ursus, and Smirnoff black.....

As for the pressure problem, you may assume the datum height to be msl...'regional' QNH should have been the clue for that;)

[Monocock]

I would say that it should be 3253 as 610 metres is not 2000 feet, it's 2001!!!!

Shall I bring some vol au vents to eat with the Cliquot, as it's my birthday on the 5th July.

:)

[FlyingForFun]

Is it just me, or has the server gone and mixed up the order of the posts, and given my answer before Iain had finished answering the question? :D

Monocock, you are quite right in pointing out that 610m is 2001'. However, you failed to notice that the ISA temperature at 3000' is actually 9 degrees, not 11 degrees!

So - second attempt. The actual temperature is ISA minus 19 degrees, so the correction to the altitude of 3001' is 7.6%. 3001' plus 7.6% is 3229'.

FFF
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PS - Yes, I'm bored.

PPS - Yes, I did have to look up the temperature correction formula! :O But that only shows just how bored I am! :D

PPPS - Happy Birthday, Monocock!

[iainpoll]

OK guys, close enough, I actually worked out 3240ft, but you were being more precise than me (I just used 8%)!

As the server has mixed the posts up, I cant say who won - so you will all have to come to the bash to share the vodka!

ps I said vodka - cliquot is champagne innit:}

[In Altissimus]

Why is there no 'temperature subscale' on the Altimeter like the one on the ASI?

Or is there one on the posh machines you IFR-types mash about in?

[FlyingForFun]

I seem to recall a particularly nasty question coming up at some point in the ATPL studies. It was something along the lines of this:

The elevation of the ground is (x), and the aircraft is flying over a mast, heigh (y) above the ground. The local QNH is (q), and the temperature is (t). The aircraft is flying IFR, on a magnetic track of (m), with a variation of (v), and (d) degrees of left drift. What is the lowest flight level the aircraft can use?

Answering the question requires you to first of all realise that you must be 1000' above the mast if you are IFR. Then, you have to take the temperature to find the indicated altitude at which you must be. After that, you use the difference between the QNH and the standard pressure setting to establish the flight level equivalent to the lowest possible altitude. You then need to know the quadrant rule (you can guarantee that including/excluding the drift and variation will put you into different quadrants, so you need to know which to include) to establish the lowest flight level in the correct quadrant. Unfortunately, I can't find the question, and I can't be bothered to work out the numbers to use to make the question appropriately difficult! :D

FFF
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Edit to answer In Altisimus's question: there is no need. It only affects you at altitude, it makes no difference on the ground. And the further away from the ground you get, the less relevant your exact physical height becomes.

The only people who actually care about this are some people who operate IFR in arctic conditions. In these cases, instrument approach minima need to be adjusted (before flight) to account for the difference. There was a thread about this a while ago, but bu66ered if I can remember where it was or what it was called!

[iainpoll]

Formerley Bloke in a formerly Archer type Altissimus, known hereafter purely as :}

As I understand it, they do not include a temperature subscale (although I'm not even sure it is possible, thinking about the internal workings of an altimeter) because the temperature deviation affects all aircraft equally, and is only large enough to be an issue at height. 4% of 100 ft is not a lot, but 4% of 30,000 ft is....

Large aircraft on instrument approaches do take this correction into account when, although I think they just add 50ft to the decision height. Perhaps FFF could elaborate 'cause I have not got to that bit yet!

Are you coming on the 5th by the way?

[FormationFlyer]

Answering the question requires you to first of all realise that you must be 1000' above the mast if you are IFR.

Um...well not strcitly true - the problem with IFR is that everyone uses IMC and IFR interchangeably. If however you are IFR but flying in VMC clear of cloud in sight of surface below 3000' then you dont need to be >1000' above the highest obstacle. The thing you must realise here is that you do not always have to fly quadrantel rule - ONLY above 3000'.

This is a commonly promoted error.

Simiarly - being IMC does not mean you aare actually flying the a/c by sole reference to instruments - if you fail to have the required separation from cloud etc it is EASY to be IMC yet flying visually - even though at this point you must be IFR.....

So technically the answer is not necessarily as asked - because the assumption now is that you can only fly at the first available flight level above 3000 transition altitude....but that then assumes you arent within the LTMA with a TA of 6000' - Where obviously the lowest available flight level is a lot higher and the mast is irrelevent....

...to be truely correct the answer is mast height + 500' (unless in a congested area + 1500).. turn that into a flight level and whatever that is *is* correct (unless it puts you above 3000' - then you must fly quadrantel) because a flight level is merely a repsentation of a height from the 1013 pressure level. I can fly at FL20 if I so choose.....

Which all highlights the point that the question is severely defficient in the appropriate amount of information and is reliant on assumptions....a very very poorly posed question.

Large a/c on approaches do not take PEC into account - they use radar altimeters below 2500'agl....

Smaller aircraft apply a PEC - typically 50' but on some a/c the CAA will now accept lower values - such as 30' on a PA34.

[FlyingForFun]

FF - I was tempted to say that your reply demonstrates why I don't write exam questions - I'm sure the actual question was better-worded than my memory of it. But, on reflection, there are many questions in the ATPL question bank which are much worse than this, so maybe I do have a possible future as a question-writer! :eek:

FFF
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