Finding Clearance above a ridge
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Gear Jerker
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Re: Finding Clearance above a ridge
The idea behind the formula is line of sight. Between the antenna and the horizon, then between the horizon and the aircraft.
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Re: Finding Clearance above a ridge
Do yourself a favour - buy a @*lhanes for the test bank. The author of the Sharper Edge is a nice guy, and the study portion of the book was well done, but the test questions did not prep me for the written.
Re: Finding Clearance above a ridge
Hello all I am stuck on this part of the formula - Difference between ICAO and actual temp? I know standard temp is 15 however how do they get an answer of 42 when finding the difference between Indicated OAT -40 and ISA temp. ?
how exactly do i calculate this difference with accuracy? my particular question is:
ground elevation of airport: 3000 ft
altimeter setting: 30.43
indicated OAT at 6,500 ft: -40C
indiated alt: 6,500 ft
height of ridge: 5,850 ft
ans: 62 feet
any help would be greatly appreciated.
how exactly do i calculate this difference with accuracy? my particular question is:
ground elevation of airport: 3000 ft
altimeter setting: 30.43
indicated OAT at 6,500 ft: -40C
indiated alt: 6,500 ft
height of ridge: 5,850 ft
ans: 62 feet
any help would be greatly appreciated.
AP
Re: Finding Clearance above a ridge
Easy!
So you need to find the difference between the actual temperature and the ICAO temp.
ICAO temp = 15 C at MSL. You deduct 2 degrees per thousand from there as you go up. So at 6,500 feet you take 6.5 X 2 = 13, then deduct 13 from 15 = -2 at 6,500.
So if the ICAO temp at 6,500 is -2, and the indicated is -40, we have a differential of 42. That is to say, from -40 to 0 = 40, then from 0 to -2 is another 2, for a total difference of 42. Then you plug this into the true altitude formula. Piece of cake.
So you need to find the difference between the actual temperature and the ICAO temp.
ICAO temp = 15 C at MSL. You deduct 2 degrees per thousand from there as you go up. So at 6,500 feet you take 6.5 X 2 = 13, then deduct 13 from 15 = -2 at 6,500.
So if the ICAO temp at 6,500 is -2, and the indicated is -40, we have a differential of 42. That is to say, from -40 to 0 = 40, then from 0 to -2 is another 2, for a total difference of 42. Then you plug this into the true altitude formula. Piece of cake.
Re: Finding Clearance above a ridge
Wow 455tt, I don't know what to say about your math ...
15 - 13 = 2, not -2 and (-40) - (-2) = -38, not -42.
But (-40) - (2) = (-42) so I'm at a loss.
15 - 13 = 2, not -2 and (-40) - (-2) = -38, not -42.
But (-40) - (2) = (-42) so I'm at a loss.
Re: Finding Clearance above a ridge
Right you are! Thanks for posting the correction.
Re: Finding Clearance above a ridge
So
The formula requires that you must input the DIFFERENCE between the actual and the ICAO temperature. The ICAO temperature as you state quite correctly is +2 C at 6,500 feet. The actual temperature is -40 degrees C. Thus the difference between these is 42 and this is what you input into the formula. So, 42 X 3.5 X 4 = 588 feet which you deduct from your indicated altitude of 6,500 feet giving you a true altitude of 5,912 and a clearance about the ridge at 5,850 of 62 feet.
Are you OK with this - the true altitude formula can be found in the Air Command weather manual.
I see this example is taken from MC's book where the full explanation and math has been provided.
Here is the math:
The formula requires that you must input the DIFFERENCE between the actual and the ICAO temperature. The ICAO temperature as you state quite correctly is +2 C at 6,500 feet. The actual temperature is -40 degrees C. Thus the difference between these is 42 and this is what you input into the formula. So, 42 X 3.5 X 4 = 588 feet which you deduct from your indicated altitude of 6,500 feet giving you a true altitude of 5,912 and a clearance about the ridge at 5,850 of 62 feet.
Are you OK with this - the true altitude formula can be found in the Air Command weather manual.
I see this example is taken from MC's book where the full explanation and math has been provided.
Re: Finding Clearance above a ridge
If ICAO is -2 and ambient is -40 then the differentialis -38.....
Going for the deck at corner
Re: Finding Clearance above a ridge
ICAO temp is +2.
That is to say, 15 at sea level, minus 13 (6.5 X 2) = +2. And if the actual is minus 40, then the differential is 42. From -40 to 0 is 40, plus another 2 to get to +2 = 42.
Now you need to plug in to the true altitude formula, which says you take the altitude of the air column in thousands of feet, you multiply this by the difference between the ICAO and the actual temperature, and then you multiply by 4 feet, and this is the error you will deduct from the indicated altitude.
Let me try this to explain: the altimeter is calibrated to accurately measure true altitude only in standard conditions. Once the temperature gets really cold, it over-indicates the altitude, and your true altitude will be lower. So, you need to factor in how large this difference between the actual temperature and the ICAO temperature, since the greater the differential, the greater the amount of error and the lower will be the true altitude.
That is to say, 15 at sea level, minus 13 (6.5 X 2) = +2. And if the actual is minus 40, then the differential is 42. From -40 to 0 is 40, plus another 2 to get to +2 = 42.
Now you need to plug in to the true altitude formula, which says you take the altitude of the air column in thousands of feet, you multiply this by the difference between the ICAO and the actual temperature, and then you multiply by 4 feet, and this is the error you will deduct from the indicated altitude.
Let me try this to explain: the altimeter is calibrated to accurately measure true altitude only in standard conditions. Once the temperature gets really cold, it over-indicates the altitude, and your true altitude will be lower. So, you need to factor in how large this difference between the actual temperature and the ICAO temperature, since the greater the differential, the greater the amount of error and the lower will be the true altitude.
Re: Finding Clearance above a ridge
455tt, we understand the formula. It's just that your first post completely fucked up the math, which could be confusing to some, especially since you arrived at the correct answer. Glad we got it sorted out 
.
.Re: Finding Clearance above a ridge
That's good to hear ahramin that it all makes sense to you now but what I'd love to know is whether the person that originally asked is all clear now on this subject - so sagabwoy please let us know if we have helped or whether any of this is still confusing.
