Rationale of the altitude*3 theory

[tony montana]

.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

wrong, 3°is 5%, so divide your GS by 2
300kt. 1500fpm
250kt. 1250fpm
200kt. 1000fpm

[Microburst2002]

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%

[tony montana]

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

[wiggy]

Do you do this when you're hand flying?

[tony montana]

actually yes, it comes in my scan to check GS and my V/S, i found it really easy to check, don t you?

[wiggy]

i found it really easy to check, don t you?

Yes, I do, and I've done over 10 miles a minute and agree your Mach approximation is fine, , and after over 35 years of flying I reckon using the 5 and 3 times table works fine as well.......FWIW I'm a great believer in KISS, especially when teaching newbies, so I'm not sure I'd bring the tan function into it.

BTW do you really live near/in Toulouse? - 'cos if you do we're probably neighbours.

[tony montana]

Toulouse, is homebase. I agree with out with KISS, the tan function is useless is just to proove that a simple rule iof thumb is good enough

so you fly military A/C