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mirkoni
14th Jul 2015, 11:10
Hi chaps,

Just wandering if anyone knows why we use 2/3 of the cruising altitude for calculating average TAS during climbing (and why we use 1/2 for descent). Why this value? Where is it derived from?

Mirkoni.

darkroomsource
14th Jul 2015, 11:28
I would guess, and it's just a guess, that its due to the rate of climb and rate of descent.

If we descend at a constant rate, then the average altitude will be 1/2 of the initial altitude if we descend to zero.

But, if we climb as a rate that is not constant, but rather slows as we climb, then we spend more time in the higher altitudes, so the "average" altitude will be higher than 1/2. I would guess that 2/3 is "close enough" for most aircraft.

To expand on this, if you climb at 1000 fpm for the first 5000 feet, then at 500 fpm for the next 5000 feet, you will spend 2/3 of your time between 5001 and 10000 feet.

A Squared
16th Jul 2015, 09:30
Hi chaps,

Just wandering if anyone knows why we use 2/3 of the cruising altitude for calculating average TAS during climbing (and why we use 1/2 for descent). Why this value? Where is it derived from?