Cron

This altimetry is getting to me. I'm ok with definitions of TA and TL but applying them to problems throws me. I would sure appreciate a really simplistic explanation to the answer to the following.

Transition altitude is 3000 feet and QNH is 990. Transition level is:
a) FL 20
b) FL 25
c) FL 35
d) FL 40

Thanks in advance for your assistance.

powdermonkey

Me thinks d)

If you use 30ft per hpa or 27ft in this case it doesn't matter

1013-990 = 23Hpa
23hpa x 30ft = 690 ft

Trans alt is 3000ft based on QNH, so above msl,
but your flight level is measured off 1013Hpa which is 690ft below the 990 level- so at 3000ft on the QNH, you will be at 3690ft above the 1013 level, so nearest FL above that will be at 4000ft.
I stand to be corrected, I haven't done these in a little while and it's early!!!

FFP

Don't you just love Air Law . . .. . . .!

Answer (e) for the real world . .

(e) Is calculated by ATC and given on the ATIS.

CPL_Ace

Easiest way for me to think about it:

If you wind your Altimeter UP to 1013 from the QNH (eg 990) then you wind your altitude reading UP by "27ft per mb" and are therefore HIGHER on the altimeter - then you just round (ALWAYS UP) to the nearest FL

And vice versa. If you wind it DOWN from (eg) 1023 to 1013 then your altimeter will read a lower reading which you round (ALWAYS UP) to the nearest FL

So first work out your altimeter reading after setting 1013 and then round up.

(This is very much clearer with the altimeter in front of you - I think I've hurt my brain again!)

AlexL

just remember 'wind on millibars, wind on altitude'
So with a QNH of 990 when you get to 3000 ft and twiddle the knob from 990 to 1013 your altimiter will wind up from 3000 to 3621 (23 mb * 27ft). Therefore the first available FL above 3621 ft is FL40.
I'm doing Met at the moment and it has a lot of questions like that, but in more detail. Draw a little diagram for each question and it all becomes clear.