Rationale of the altitude*3 theory
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.The rate varies a lot when you make idle descents. But, for 3 deg slope, If you want to calculate if you are making 3 deg or not, try this, using GS:
300 kt aprox 1700 fpm
250 kt aprox 1500 fpm
200 kt aprox 1000 fpm
300 kt aprox 1700 fpm
250 kt aprox 1500 fpm
200 kt aprox 1000 fpm
300kt. 1500fpm
250kt. 1250fpm
200kt. 1000fpm
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wrong
3º is not 5%
we use 5% for the glideslope, though, because it is much simpler and is always approximate, and glideslopes are not always exactly 3º. For a descent from thousands of feet, there is a difference between a 5% and a 5.24%
3º is not 5%
we use 5% for the glideslope, though, because it is much simpler and is always approximate, and glideslopes are not always exactly 3º. For a descent from thousands of feet, there is a difference between a 5% and a 5.24%
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ok fair enough, so if you like maths, lets have a close look:
200kt*tan3°=A(rate of descent in NM/h)
A/60*1852/0,3048=1061ft/min
300kt=>1592ft/min
250kt=>1326ft/min
so if you like rocket science you use that, personnally i divide GS by 2
300kt=>1500ft/min
250kt=>1250ft/min
200kt=>1000ft/min
simple and useful rule of thumb, but anyway you and me are not so far from the truth
like a 3° path is exactly 318ft/nm 300 ft for me 330ft for you
Do you use mach number * 3= V/S for 3° path without too much wind and high mach number
mach number ruffely *10= nm/ min
200kt*tan3°=A(rate of descent in NM/h)
A/60*1852/0,3048=1061ft/min
300kt=>1592ft/min
250kt=>1326ft/min
so if you like rocket science you use that, personnally i divide GS by 2
300kt=>1500ft/min
250kt=>1250ft/min
200kt=>1000ft/min
simple and useful rule of thumb, but anyway you and me are not so far from the truth
like a 3° path is exactly 318ft/nm 300 ft for me 330ft for you
Do you use mach number * 3= V/S for 3° path without too much wind and high mach number
mach number ruffely *10= nm/ min
i found it really easy to check, don t you?
BTW do you really live near/in Toulouse? - 'cos if you do we're probably neighbours.
Last edited by wiggy; 11th Jul 2012 at 00:06.