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Guest Ray51

Partial Throttle Descent (long post)

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This is what Bill Bulfer says about a partial throttle descent:Using the thunbwheel (MPC), one can select a rate of descent that will maintain a constant angle. The throttles are kept slightly off the idle stops. The top-of-descent will occur earlier than the idle throttle descent technique because some power is used throughout the descent. The airplane is not in a power-off glide configuation. There is no significatant fuel savings between idle throttle and partial throttle decents especially if speed brakes are used during an idle throttle descent. The partial throttle descent works best in situations where ATC is likely to interfere with speed and/or heading commands, or a high speed is desired. Other advantages are a more comfortable and constant deck angle, better control of energy or "mass management", resulting in a position to meet speed and route changes without using the speed brakes because your profile does not change. There is also less thermal shock to the engines and it offers a more gradual pressurization change for anyone onboard with a head cold. The disadvantage to this technic is that it demands a higher level of monitoring and pilot skill. The partial throttle-constant angle descent technique uses 3:1 method to calculate distance to descend. It is set up this way. Lets assume we're crusing westbound at FL350. There is no significant headwind or tailwind. The restriction down route is 250 kts and 10,000 ft at a waypoint named DAISETTA. It is found on the DAISETTE arrival into Houston International from the northeast. If the airplane is crusing at FL350, we have to lose 25,000 ft and then decelerate to 250 kts.Divide the altitude that must be lost by 1,000 then multiply by 3 .25,000/1,000= 25 x 3 = 75nm. Note: if you have a tailwind greater than 75 multiply by 4.Lets use a descent crossover speed of 300 kts.In this example a deceleration segment is required because the clearance includes a 250 kts speed restriction. You will need about a mile per ten kots of airspeed to decelerate in no wind. so add 5 nm to slow from 300 to 250 kts. Now the top of descent point (T/D) is 80 nm from DAISETTA.Just prior to reaching T/D, wee'll roll the thunbwheel over and smothly start our descent. But what rate to select? Take the ground speed, divide by 2, add a zero and 10%.Example: ground speed of 440 divided by 2 is 220 add a zero and 10% (2200 + 220) is 2400 fpm. This is your inital V/S selection. Continually re-check your postion during the descent. The rate of descent will decrease slightly in the lower atmosphere because TAS is slowing as the CAS remains constant.Adjust the thumbwheel to keep the airplane on 3:1 ratio plus the deceleration segment. If a correction is needed, change the V/S by a third in the window until you're back on your desired ratio.As the airplane levels at 10,000, dial the speed back to 250. you'll get a great deal of satisfaction from feeling like you're in chanrge instead of feeling like the computer is flying. I usually will put an along track way point, in this case 5 nm before DAISETTA, and place my altitude restriction there. Ray

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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

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>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%?

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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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