FLYER Forums

If you have a given flight level and calculating actual altitude you adjust for temperature and pressure.

Say you are adjusting (down) for temperature correction (1% per 2.5degC from ISA) and also for pressure (27ft per Hpa) then which way round to you do it - temp correction or pressure correction first? And why.

What if ground elevation AMSL is > 0

(Deleted for reasons given below!).

Hey. That’s what I bought but some sample questions show it does make a difference.

Eg FL150
6% ISA temp deviation
10 HPa ISA Devaiation (270ft)
1000ft elevation.

1) taking 6% off 15000 feet then taking off 270 feet gives different value than taking off 270 feet from 15000 then taking off 6%
2) the question bank answer states that the %age temp correction is based on the height of the column of air ie 15000-1000 x 6%

Tradition has it you adjust for pressure first, then temp. Wel. That’s what my CBIR books say and they got me through the exam.

Not sure I quite follow your examples. I don’t understand what you are trying to do.

6% temp adjustment taken away is for 15 deg C below ISA. Is that what you have?

(Deleted, as I had a brain fart and was talking bolleaux)

Where’s the “tradition” written. I couldn’t find anything explicit in the study book.

I was getting the wrong answer by correcting temp first then pressure.

marioair wrote:Where’s the “tradition” written. I couldn’t find anything explicit in the study book.

I was getting the wrong answer by correcting temp first then pressure.

It’s written in both the text books I have for my CBIR. That’s the BGS and CAPT books.

How about giving us the whole question? Might be able to work through it.

The two ways are not significantly different.

15,000 ft - 840 (14000*.06) -270 = 13.890

Or

150000 ft- 270 - 823.8 (13,730*.06) = 13,906.2

13,900ft in anyone’s money.

Unless of course you spot my deliberate mistake.

GolfHotel wrote:

marioair wrote:Where’s the “tradition” written. I couldn’t find anything explicit in the study book.

I was getting the wrong answer by correcting temp first then pressure.

It’s written in both the text books I have for my CBIR. That’s the BGS and CAPT books.

How about giving us the whole question? Might be able to work through it.

“An aircraft is flying at FL150 with an OAT of -30 deg C above an airport of elevation 1660 feet. The QNH is 993hPa. What is the true altitude.”

ISA temp @ fl150. = 15- (15*2) = -15. So @-30 it is ISA -15

PL 1013 Is 20mb below MSL that we are told is 993mb. So 27*20=540. So take 15,000 - 540 = 14,460

The column of air is 14,460-1660 = 12,800 ft high (before temp correction) (rock is not affected by temp, as far as we are concerned)

Air shrinks 0.4% for each deg C (or expands, but we are cold so shrinks) we must work with ISA as that’s what FL is based on. So -15*0.04 = -6%

12,800*-.06=768ft of shrinkage.

The question asked for altitude so you have to take the shrinkage off the altitude of 14,460 not the height of 12,800.

14,460 - 768 = 13,692ft Hopefully that or something like 13,700 is one of the options you are given?
Given in good faith, but I’m dyslexic so check for silly errors. Your altimeter is over reading by 1,300ft and that sounds about right to me.

Answer is correct!

What I don’t understand is why you adjust for pressure first then temperature.

marioair wrote:Answer is correct!

What I don’t understand is why you adjust for pressure first then temperature.

The temperature adjustment is based on the actual height of the air. So you want to get as close as you can to the actual height. Before applying the correction. In your example you don’t want to apply a temp correction to the ground from sea level to 1660ft and likewise you don’t want to apply the correction to the 540 ft between the imaginary 1013 level and sea level. So you’ve got 2300ft less air to expand or contract. I’ve never really though it through before. I just accepted the advice in the books.

Glad I got the maths right

marioair wrote:

GolfHotel wrote:

marioair wrote:Where’s the “tradition” written. I couldn’t find anything explicit in the study book.

I was getting the wrong answer by correcting temp first then pressure.

It’s written in both the text books I have for my CBIR. That’s the BGS and CAPT books.

How about giving us the whole question? Might be able to work through it.

“An aircraft is flying at FL150 with an OAT of -30 deg C above an airport of elevation 1660 feet. The QNH is 993hPa. What is the true altitude.”

I may be completely wrong here but the question asks “What is the true altitude”.
Surely altitude is distance above sea level, relative to QNH / FL 1013. The airfield elevation is irrelevant, isn’t it?

Crash one wrote:
I may be completely wrong here but the question asks “What is the true altitude”.
Surely altitude is distance above sea level, relative to QNH / FL 1013. The airfield elevation is irrelevant, isn’t it?

But it’s only the height of the air that is affected by the change in temperature. So you have to work out the true height and add back the height of the airfield to get the altitude.

GolfHotel wrote:

Crash one wrote:
I may be completely wrong here but the question asks “What is the true altitude”.
Surely altitude is distance above sea level, relative to QNH / FL 1013. The airfield elevation is irrelevant, isn’t it?

But it’s only the height of the air that is affected by the change in temperature. So you have to work out the true height and add back the height of the airfield to get the altitude.

If the temperature/pressure of the air above terrain changes with the elevation of the terrain then your altimeter would be jumping up and down as you passed over land.
The temperature changes whether there is granite in it or not.