When to start my descent?

Question

I'm flying in a 738 flying at 250 mph and my v/s is -1250 fpm. How many nm away from the airport do I start to descend?

Answer 1

About 3 nautical miles per thousand feet.  It is a convenient way of approximating a 3º (318'/nm) descent.  

So TRIPLE the number of thousands of feet above airport elevation.  25 times 3 nautical miles => 75 nautical miles.  

Add a few miles to slow to flap speed.  75 + 5 = 80.  

Add a few miles if you expect much tailwind.  80 + 5 = 90.  (Subtract a few if you expect much headwind).

Subtract a few miles if you expect to have to maneuver much--like to maneuver beyond  the airport.  90 - 6 =84.

Answer 2

No answer is available.  You need to state your altitude at the beginning of the descent and the field elevation of the airport.

Answer 3

25000/1250 = 20

In 20 mins you will be at sea level.

If you are flying at 250KT then in 1 hour you will have travelled 250nm. In 20 mins you will have 1/3 of that.

So start descent 83.3nm out.

Answer 4

Another good way is to take the altitude you have to lose, drop the zeros, and multiply by 3 to get the distance away from the airport that you should start your descent.

Example - Cruising at FL350, and need to go down to 1,000 feet.  That's 34,000 feet to lose.  Drop the zeros, and you have 34.  Multiply by 3, and you get 102.  Start your descent 102 NM from the airport.

In combination with this distance calculation, take your groundspeed, divide it by 2, and add a zero to get the appropriate descent rate.

Example - Your average groundspeed is 400 knots.  Divide it by 2, which gives you 200.  Add a zero.  2000 fpm.

In this example, descending 102 NM from the airport at 400 knots average groundspeed, with 34,000 feet to lose.  2,000 fpm descent will get you there pretty close.

Answer 5

The fun part is too get her down as soon as possible.

Answer 6

Think of it as miles per minute because you will be descending in feet per minute.  250 MPH is just over 4 miles per minute.  If you descend at 1250 feet per minute, you're talking about 20 minutes for 25,000 feet … there you go.  20 * 4 is 80 miles.  Then add a couple of minutes (miles) for margin…..You don’t need a calculator for this -- no matter the airplane.