A Q about ISA that baffles me

[urdaddy]

In one of the usual study books there is a question that states:

"Given a temperature of ISA 36 deg. C, the pressure altimeter will overread, underread, read correctly"

I think I need more information. Such as, temperature at altitude, to make a comparison to ISA after a standard lapse rate calculation.

answer states it will over read because temp deviation is colder than international standard ...... How is that immediately obvious? No altitude! and 15c is the baseline from sea level.

I understand the high P to low P or warm air to cold air versus the altimeter but the original question baffles me.

Please shed some light guys! Thanks in advance.

Cheers,
UD

[fumes]

My 2 cents...the question is poorly, or incorrectly, worded. Should say temp is ISA + /- 36, or temp is 36 C at xxxx feet, as you suggest. Don't get too concerned, if you know the theory (over / under read by 2% for every 5 degree ISA variation), you'll work out any question you're given.

[urdaddy]

How are you fumes? Thanks for the response!

Glad you see it as being poorly worded as well. However, I haven't used or heard of the "over / under read by 2% for every 5 degree ISA variation" Are you refering to the altimeter over-under reads by that Rule of thumb?

Is that an accepted rot? If so, nice one, I didn't know that.

UD

[ashcroft79]

My understanding is that your altimeter will over read by 2% for every 5 degrees colder than standard, and that it is an accepted thought

[urdaddy]

Hello Ashcroft, you think it's only for colder?

cheers!

[ashcroft79]

No, I was not trying to imply that it was only for colder but that was most likely how the question that he posted was meant to read so that is how I answered it. It would work just the same if it was warmer:cool:

[fumes]

All true. Works the same for warmer air. You may get a question like "you're coming down the ILS on an ISA + 20 day and the OM check height is 1500ft. What would you expect your altimeter to read at the OM?". Good luck!!!

[urdaddy]

Ok crystal clear now! Thanks again Mr. Fumes, appreciate your time!
UD

[turnandburn]

A small clue to questions and answers. Have a look at the aircraft types and destinations we operate to and that may explain where the questions come from.
i.e. Bejing in winter -20c, Dubai in summer +40c, you can operate a mixture of hot and high (jo'burg summer then up to anchorage Alaska deep winter 5 days later) non precision approach into cebu short haul then transpac to New York 2 days later.