Originally Posted by
Chadflies
So I’ve had differing opinions on how to calculate this, personally I prefer the time method vs. distance but wanted to post and see what other opinions are as to which method works best.. currently I’m of the opinion either is fine.
For time: ETA / altitude needed to be lost.
(IE: if you at 30,000 ft and need to meet a crossing restriction at 10,000ft with a 1000fpm decent - you need to start your decent 20mins prior)
For distance: change in altitude x 3
(IE: you are at 14,000 and need to get to 6000ft, giving you 8000, start your decent 24nm’s back)
Both can factor a 3 degree decent profile if that is something you want to consider. 1/2 ground speed, add a zero.
Both in my opinion work, tho I constantly hear the argument that distance is better, without any reason as to why.
Does anyone have some deeper knowledge base that can provide a rationale? Personally I find time an easier variable to adjust and calculate, but that’s just my preference.
blue skies and tailwinds,
chad
What i do i also add to the calculated distance, the difference between my current IAS and and average speed of 200kts, (i.e. if before the descent i am flying at an indicated speed of 270kts, i add to the calculated distance additional 7 NM), and to conclude I also add or subtract the wind factor (10kts/1NM) depending whether i have tail or headwind.
Doing this i am usually really precise when compared with the TOD on the MCDU.
Example:
FL330, IAS:270Kts, Wind negligible
My calculated TOD will be : (33*3) + (27-20) = roughly106 NM, to be compared with the distance to treshold in the PROG page (Airbus world)