View Full Version : Why 2/3 of the cruising altitude for TAS

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?

Thanks in advance.

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?

Thanks in advance.

Mirkoni.

It's a or heuristic, or rule of thumb. Rules of thumb, by nature often contain numbers which have no deeper meaning other than it's a convenient, easily remembered number which when used will give you results which are "close enough" to explicitly calculating the number you want using rigorous equations and physically significant constants.

Example: The rule of thumb for figuring your rate of descent to maintain a 3 degree glide-slope is Rate of descent in ft/min = one half your groundspeed in Knots. That "one half" has no deeper meaning other than some clever guy noticed that half your groundspeed in knots is pretty close to the rate of descent you need to maintain a 3 degree glideslope.