# Partial Throttle Descent (long post)

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Not bad at all. I did the math then flew a test flight to see how the FMC would handle the arithmetic. Stacks up very well. The test flight was from FL370 at a cruise IAS of 275. --- The calculation: Distance: 1) From FL370, So FL370/1,000 x 3 = 111 miles required for descent. 2) Cruise speed was 275 (IAS) and need to reduce to 250, so 275-250 = 25 << ~ 3 extra miles 3) Step 1 + Step 2 (111 + 3) 114 miles distance required for descent.Rate: 1) Ground speed was 512, So 512/2 = 256(0) + 10% = ~ 2800 FPM rate. So, 114 miles to 10,000 ft at a ROD of 2800 fpm, without a deceleration segment. This is the descent distance and ROD to get to 10000 ft. The description suggests that the deceleration occurs after the level off at 10,000, so I assume this does not include the deceleration segment/maneuver. (In either case, it's only + or - 2.5 miles). --- FMC flown descent: 91 miles to 10,000 ft at a ROD of 2500 fpm -- that included a deceleration segment/maneuver --.With the calculation's 114 miles, a much kinder, gentler rate of descent could have been used. The FMC just looked at the path (i.e., rate of descent) that would occur from a speed on pitch descent at idle, then said

>Rate: >1) Ground speed was 512, So 512/2 = 256(0) + 10% = ~ 2800 FPM>rate. >>So, 114 miles to 10,000 ft at a ROD of 2800 fpm, without a>deceleration segment. Just to clarify here Sam..is the 10% referring to 10000ft? so if we had to descend to 12000ft would it be 12%?

He was using the formula from Ray's post: Ground Speed/2, add a zero, add 10% to that to get your V/S.

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