# Are there more refined rules-of-thumb for Top of Descent calculation, than the simple FL x 3?

Forgetting the excellent capabilities of VNAV for a moment and reverting to simpler days, is there a rule of thumb to further refine the FL x 3 rule for Top of Descent calculation, when there are excessive head or tailwinds? Like so many miles to be added/subtracted to the distance for every x number of headwind or tailwind knots?

This is assuming you want to stay on your standard descent speed profile and not alter it.

• In my experience, prior to VNAV the 3/1 rule of thumb was just a general baseline for the TOD (top of descent) point. We always adjusted the actual descent point based on an educated guess based on the tail/head wind as estimated from our existing ground speed. Of course the wind velocity often would change substantially during the descent making a more accurate rule of thumb difficult. Since adjusting our descent rate to meet any ATC restrictions was fairly simple, the 3/1 profile (adjusted for grnd speed) worked reasonably well. VNAV changed everything and saves lots of fuel.
– user22445
Jun 21, 2021 at 0:25

Yes, there were refinements to it. In the 737-200, 3:1 was a 250 knot descent, while 280 knots was 2.5:1, and 320 knots was 2:1. The book also said add/subtract 1NM per 10 knots of tail/head wind, although that's a pretty rough wag.

The newer 737's are cleaner, so 3:1 is a better bet at the typical 280 kt descent, and slowing to 250 will flatten things out from there.

The old rule of thumb that 1NM = 10 knots slowed at idle & level flight still seems to hold well for the newer jets.

pre-VNAV/early VNAV days we used FL x 3 plus/minus 10% of avg wind component and added 15nm for decel if the vector/STAR/approach and landing was more or less 'straight in', or 10nm for a vector to base turn or 5nm for vectors equivalent to a downwind.

alternately,

FL divided by 3. +/- avg wind comp and add 5 nm for a straight in.

Keep recalculating and looking out for significant wind changes - worked pretty well.

For the latest generation jets I would add another 5 to 10 nm without hesitation, though the issue has become meaningless most of the time due STAR and traffic constraints. current day usage would be confined to figuring altitude loss over a distance to meet such constraints or because of such constraints.