I've been reading the FMC User Guide recently and I was wondering if any of you is multiplying the altitude to lose down to 10.000 ft by 2,5 rather than 3. This should take into account a steeper ROD at high altitude due to less dense air.
For example, from FL300 the FMC calculated TOD would be around 87 miles.
At a typical 280 kt descent (still wind) 30.000-10.000=20.000 ft so 20x2,5=50 nm. Then add 30nm for the last 10.000 ft and it would be 80. Plus a few miles to account for deceleration.
The classic rule of multiplying by 3 would give us at least 10 miles extra in this case. And to that, we still add miles for a deceleration segment.
Geeky stuff, I know, but I have got plenty of free time at the moment
