Pressure and Warm/Cold Atmosphere
 

The altitude indicated on board an aircraft flying in an atmosphere
where all the atmosphere layers below the aircraft are cold is?:

Why is the answer to the above question

"lower than the real altitude"

This one has me bamboozled?

If someone could explain in v simple terms I'd really appreciate it. I do not understand why it doesnt follow the "High to Low -Look out Below' adage

Many thanks

'Search' will find lots of answers for you.

R1Tamer,

The atmosphere is not vertically constrained, and will expand and contract in accordance with temperature variation. The International Standard Atmosphere (ISA) assigns altitudes for the various pressure levels assuming standatd ISA temperatures throughout. The Altimeter is calibrated for this.

On a day hotter than ISA, the vertical distance between pressure levels increases, and conversely, on a day colder than ISA, the vertical distance between pressure levels decreases. These pressure levels we read as Indicated Altitude. The degree of expansion or contraction is directly proportional to the ratio of the Mean Temperature of the column of air beneath the aircraft to the ISA Mean Temperature of the column of air (All temperatures are, of course, in degrees Absolute / Kelvin).

An example for 10,000 feet Indicated Altitude. On an ISA day, 10,000 feet Indicated is indeed 10,000 feet of True Altitude. On an ISA-15°C day, 10,000 feet indicated is only 9,461 feet of True Altitude - UNSAFE!. On an ISA+15°C day, 10,000 feet indicated is 10,539 feet of True Altitude - SAFE!

Regards,

Old Smokey

Old Smokey,

Thanks for taking time out to look at my query. The normal theory of temp effect on altitude i'm perfectly familiar with.

However if you read the question again, you'll note that according to the JAA Question and their provided answer, - 10000' Indicated Altitude is in fact 10539' True Altitude if all the atmospheric levels below the aircraft are colder.

This is precisely the reverse of what you and everyone else who ever taught me have ever described. I know there are at least two of these questions have cropped up in the question bank and that the alleged correct answers are as above or reversed in the warm version of the question.

My personal feeling is that the answers in both these questions are wrong but I wanted to check here first as well as several other sources before challenging.

Thanks for your time

R1Tamer,

I promise you, their answers are wrong (It does happen you know!).

Unless I've missed some spurious twist in the wording of the question, I stand by my answer.

Best Regards,

Old Smokey

RItamer

Where does the question come from? I can't find it on the JAA Sample Questions, and any other data base is not JAA authorised and could be wrong. I have checked the BGS feedback data base and find a similar question, but with the correct answer given. It might be worth pointing out the error to the question source.

Regards,

Dick W

Dick W

It's safe to say the source of the two questions is France.

Beyond that its probably more gracious to accept the question bank is errant and will remain so while its veracity continues to prove largely unchallenged.

Thanks for your time guys!

R1Tamer

removed

Pack2

Afraid I can't resolve or agree with your logic. This question is predominantly talking about temperature variations and their effect on true altitude - not pressure i.e lower datum variations and there effect on pressure levels.

For example:- imagine you could sit on top of a 10000' metal rule calibrated and graduated to be accurate at all parts only in ISA temperatures and the lower end was set at sea level. If the ruler is allowed to cool down below the calibration temp it will no longer truly be 10000' long since contraction will have caused it to shrink. It will still display ten thousand feet written at the upper end but it won't be ten thousand feet long. i.e. it is over-reading its true length.

Never in these circumstances would it under-read its length?

Hope this helps!

Just a further point that made me chuckle having re-read your earlier response.

What kind of system would it be if the candidate had to read into the question \'what the examiner is getting at\'. Either it\'s a correct fact or it isn\'t - surely it isn\'t an excercise in telepathy.

The answer given is wrong.

Old Smokey has it perfectly right. In cold air pressure decreases faster with increasing altitude. Therefore at a given altitude over cold air (relative to ISA), there will be lower pressure (again relative to ISA), which will cause the altimeter to read out a higher altitude than real altitude.

I had to read question and answer several times, before I figured out the mistake :O

Going home to mind my own buisiness and revise altimetry:(

What the examiner is trying to test makes no difference to the fact that the "Answer" is wrong. The altitude indicated on board an aircraft flying in an atmosphere where all the atmosphere layers below the aircraft are cold is higher than the real altitude.