Here's something I noticed that I find very useful. It seems to agree with dvt's figures.
Approaching the top of a 300 kt descent, we were instructed by atc to descend at 250 kts. This then meant we were suddenly about 4000 ft high on profile.
I noticed that when speedbrakes were fully deployed at 250 kts, the descent angle increased by almost exactly 1 degree (from say 3.1 deg to 4.1) (Comparing 250 kts with speedbrakes down vs 250 kt with speedbrakes at flight detent).
That's roughly a 30 percent increase in descent angle which seems to agree with the rate of descent increasing from 1700 fpm to 2300 fpm.
Thinking of the one in sixty rule, 1 degree equates to:
1nm in 60 nm, or
6000 ft in 60 nm, or
1000 ft in 10 nm.
So if I am 3000 ft high on profile, I know it will take approximately 30 nm to get back on profile.
If I'm 1800 ft high, it will take another 18 nm.
I've checked this rule of thumb on other flights and it seems to be a reliable one.
Of course we're talking about air nm, not ground nm. An allowance would have to be made for the effect of wind.
dvt's figures suggest that the 30 percent difference applies at 210 kts as well (27 percent actually) but I haven't had the opportunity to try this one out on line.
I think if you keep this 30 percent figure in your mind, you can then use your speedbrakes more thoughtfully.
You can choose to:
increase the descent path by 30 percent, or
increase the deceleration in level flight by 30 percent, or
increase the descent path by 15 percent and increase the deceleration by 15 percent, or
use any other combination that suites you.
