Calculating descent rate?

Question

Okay, i know how to calculate the v/s and when to start, but what speed should i descend at? I'm in a 738 at 25,000ft v/s = -1250 fpm beginning at 80 nm from airport. (assuming it's at sea-level) what speed should i descend?

Answer 1

Your descent path should be 3 degrees most of the time. Rounded off that would be 300 feet per mile.

Five times ground speed will give you the desired rate.

At 25,000 feet in the descent you might be 10 knots under your maximum speed.

Reaching 10,000 feet and below you'll be at 250 knots until nearing the airport, then slow to 210 knots. The beginning of the approach will be between 160 and 180 knots. From there you will extend the gear for landing and set final flaps.

Examples:

300 knots ground speed use 1500 fpm.

240 knots ground speed use 1200 fpm.

200 knots ground speed use 1000 fpm.

120 knots ground speed use 600 fpm.

Answer 2

You keep asking questions that depend on two many variables. In this case what do  you want to accomplish. Save fuel, make up some time, make a controllers restriction, what? The most efficient decent in a large jet is to stay up as long as possible then when its time to go down, idle the throttles and keep it on the barber pole. That means Mmo to Vmo until about twelve thousand feet then begin to slow to 250 knots. If you ever have to add power or speed breaks you blew it. A neet trick is to hit 250 and ten at exactly the same time without leveling and the passengers will feel nothing. This decent is not normal but it sure is the best if you can work it out. Learning tricks like this is what they mean when you hear people talk about "the right stuff". If you keep focusing on numbers like you keep doing, you'll never have that stuff.

Answer 3

Well at my airline we usually descend at 290 or .74m above 10k and 250 below 10k

Answer 4

Purely theoretical, since you will not maintain a perfect 1250 fpm descent all the way to the ground, etc., but it will take 20 minutes to descend.  You have 80 miles to travel, therefore 80 miles/20 minutes = 4 miles per minute, or 240 knots (assuming you're planning in knots, but the numbers would be the same for statute miles).

EDIT:  OK,1300 FPM.  Same math, speed is now 249.6 - well let's just call it 250 Kts.  (FYI - In the real world that is on the slow side for most air carrier type aircraft, although with rising fuel prices it may become closer to the norm.)

Answer 5

Try 300 knots.

Decelerate to 250 knots before descending below 10,000' MSL.