Altimeter QNH exam Question

[stevoneil]

hiya. studying met and having real trouble getting my head around working out answers to this question.

an aircraft flies from aerodrome A 800ft 1020mb QNH to aerodrome B which is 500ft 999mb

whats the altimeter reading on the scale when at point b.

a)1430ft
b)130ft
c)1130ft
d)-130ft

i dont understand how to get to the answer. any help would be apreciated.

[dublinpilot]

When the aircraft is on the ground at aerodrome B, if it had the correct setting of 999mb then the altimeter would read 500ft.

However it actually has an incorrect setting of 1020mb set.

Imagine you are set in the aircraft at aerodrome B, with 999mb set. It reads 500ft. Now you wind on 21mb extra to bring you to 1020mb. (Wind on mb's then you wind on altitude. Wind off mb's, then you wind off altitude).

So you wind on 21mb, so you wind on altitiude of 21mb*30ft per mb. You wind on 630ft.

So you started with 500ft, and you've wound on 630ft, so your altitmeter now reads 1130ft.

I hope that helps.

dp

[alvin-sfc]

QNH

Hi Steve, its been a long time since I learnt to fly but if I remember right one millibar of pressure roughly equals 30 ft so you take the difference between the given altitudes and equate it out that way.Hope this helps.If I'm wrong I'm sure someone will correct me.

[sir.pratt]

the question doesn't appear complete. what is the 999mb figure? QNH? did the pilot adjust the subscale to this (knowing the new airfield elevation - in which case the answer would be E: 500')
if we 'presume' that the QNH is now 999, however the overworked student pilot failed to change the subscale from its setting of 1020 (a huge drop - that's a pretty nasty low!), that's 21hpa difference. with a pressure lapse rate of 30'/hpa should be 21hpa x 30' = 630' difference. the actual qnh px is lower than the set qnh, so the altimeter will think it's higher, so 500' + 630' = 1130'

[sir.pratt]

and no dublindude - i didn't read your post first! but i see we both picked the missing info in the question it's nice to know it works the same at both ends of the world!

[stevoneil]

thanks for your help with that guys. it seems so mind boggling until you know exactly how to work it out and so easy when you know how. not much of a profound statement but to the trainee pilot all these DALRs and ELRS seem hard until you realise they are related to fairly simple processes and effects.

[east_sider]

Take care with how they word the question and possible answers in the exam. I've just taken Met and got this Q wrong, the possible answers you gave in your first post were worded something like this in my exam Q:

Upon arrival overhead point b is the altimeter:

a) Over reading by 630ft
b) Under reading by 630ft
c) Over reading by 130ft
d) Under reading by 130ft

You can discount c) and d) fairly quickly but be careful with exam pressure whether you choose a) or b)... still managed to pass though

[tiggermoth]

An just to remember which way the pressure works
"From high to low, beware below"

So, in other words, if the air pressure drops then your altimeter will think that it is higher than it is (by 30ft per millibar)

So, yes, if the QNH was not set to 999 after takeoff, then the indicated altitude would be higher:

1020 - 999 = 21 mbar difference

21 x 30 = 630 (it would overindicate by 630ft - ouch!!)

630 + 500 = 1130 ft

T.