reviewer questions

[1515Blue]

...i saw these two questions in one of the reviewers.

-At a temp of ISA -25C how much altitude should be added to MEA?
5,10 or 20 %?



-On a 3degree glide path, OAT -25, OM crossing height of 1760'. the aircraft altitude will indicate...
1820', 1760' or 1720'

i don't know how to answer them... can someone help me out?
thanks

[quid]

ISA -25 would be -10C at sea level. According to the CA-19 chart in the Jepps, about 10% would be the closest answer.

Assuming -25C is being reported at the airport, I don't see a correct answer for the second question. The altimeter correction should be 280 feet,.... that would give an indicated altitude of 2040'.

[1515Blue]

CA 19 charts? where in the jepps can i find that chart? misc?
thanks

[quid]

CA-19 is the cold weather altimeter correction chart, usually in the Terminal section.

The same chart (almost) now appears in the AIM. (US)

[1515Blue]

thanks

[Tonic Please]

So what exactly are the formulae to answering the questions that were posted? :confused:

Smooth skies

[1515Blue]

i'm still trying to find the CA 19 chart

but i wish i knew how to answer them without one

[Checkboard]

have a look at Altimeter Correction Cold WX OPS (http://www.pprune.org/forums/showthread.php?s=&threadid=48014&highlight=temperature) for the formula to correct for non-standard tmeperatures - ignore the duff gen on the first page, and look at my post on the second.

[FlyingForFun]

My ATPL notes suggest adding 4% for every 10 degrees below ISA. This gives an answer of 10% for ISA-25 - the same answer as quid got from his chart.

I also agree on the second one. The temperature is ISA-40, so you need to add 16% to the actual altitude to get the indicated altitude - 2042'. That's assuming that the temperature remains at an average of ISA-40 all the way up to your altitude, of course - the question doesn't specify this.

[Edit - actually, re-reading the question, the OAT is -25. This implies that -25 is the temperature at 1760', where ISA is 11.5 degrees, not 15 degrees. But even with that correction, I still can't get any of the suggested answers.]

FFF
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[quid]

FFF-

re: your edit. Actually, I think you had it right the first time. He says the OAT is -25C. The ISA deviation would be approx. -40, but the OAT is -25 and that's what you enter the chart with, not the ISA deviation.

Splitting hairs, but assuming a standard lapse rate, it makes little difference if the reported temp from the airport is -25, or if the OAT guage in the a/c is reading -25

[FlyingForFun]

I've come back to this one again, and I'm sure the question is incomplete.

Quid, I'm going to respectfully disagree with you on this one - you get the OAT from the thermometer in the aircraft, which means it will be the temperature at the aircraft's altitude. So I think the intention is that the temperature at height 1760' is -25 degrees. But there is certainly ambiguity in the question, and your interpretation definitely could be considered correct.

But one thing which is missing is the airfield elevation. Without knowing this, we can't even guess what the ISA temperature is (we've both assumed sea level). If the airfield elevation is reasonably high, then the ISA temperature will be much lower than we've been assuming. The ISA deviation will therefore be smaller, and the indicated altitude will not differ from the actual altitude by so much. I think this is why we can't get to any of 1515's answers.

1515, where did the question come from? Is there any chance you could go back and check that you haven't missed some details?

FFF
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[1515Blue]

FFF,

its from the cx reviewer
and the reviewer gives 1820' as the answer.