How to calculate an accurate flight descent planning
profile for any aircraft.
Flight Descent Planning
Also known as Vertical Navigation, flight descent planning allows you to determine when to start a descent, or what rate-of-descent to use, so you can descend to a predetermined altitude at a specified distance from a navaid or airport. You can also use flight descent planning procedures to monitor and back up ATC altitude guidance and control, and to confidently comply with complex altitude clearances.
The procedures on this page are also applicable for climb planning (i.e., when to initiate a climb in response to an ATC crossing altitude restriction clearance) – just change Descent/Descend to Climb, and change Altitude to be Lost to Altitude to be Gained. (See Practical Example #3 below).
Flight Descent Planning Calculations
Rate-of-Descent x Distance to Descend =
Descent Groundspeed x Altitude to Be Lost
Rate-of-Descent = (Descent Groundspeed x Altitude to be Lost) / Distance to Descend. (See Practical Example #1 below).
Distance to Descend = (Descent Groundspeed x Altitude to be Lost) / Rate-of-Descent. (See Practical Example #2 below).
Unless otherwise stated, the illustrative descent planning examples given below are based on the following values:
• Descent Groundspeed: 180 Knots (3 Miles per Minute)
• Altitude to be Lost: 10,000 Feet
• Rate-of-Descent: 600 Feet per Minute
• Distance to Descend: 50 NM
Method #1: Determine Rate-of-Descent (Classical Method)
To determine the rate-of-descent required for a descent,
first think –
Miles / Minute (Miles per Minute) which is your Descent Groundspeed divided by 60; i.e., 180 knots is 3 miles per minute.
next think –
Feet / Mile (Feet per Mile) which is the Altitude to be Lost divided by the Distance to Descend; i.e., 10,000 feet divided by 50 miles is 200 feet per mile.
multiply (Feet per Mile) by (Miles per Minute) to get your Rate-of-Descent –
Rate-of-Descent = (Feet / Mile) x (Miles / Minute) ; i.e., 200 x 3 = 600 Feet per Minute.
Method #2: Determine Rate-of-Descent (Preferred Method)
Here, first multiply the Altitude to be Lost by the Descent Groundspeed in miles per minute –
Let A = Feet x (Miles per Minute) = 10,000 x 3 = 30,000 Miles x (Feet per Minute)
then, to get your answer, divide the result (A ) by the Distance to Descend you want to use –
Rate-of-Descent = (A) / Miles = 30,000 / 50 = 600 Feet per Minute.
This answer equates to the Rate-of-Descent required to descend 10,000 feet over a slant distance of 50 miles at 180 knots groundspeed.
Practical Example #1
You are in a Piper Cheyenne cruising at 11,500′ MSL and 75 miles from your destination, an airport located at Sea Level. You need to delay your descent until you are 35 miles out for terrain clearance. What rate-of-descent will allow you to reach an altitude of 1,500′ MSL when 5 miles from the airport? (Assume a descent groundspeed of 180 knots.)
Descent groundspeed = 180/60 = 3 miles per minute.
Altitude to be lost = 11,500′ – 1,500′ = 10,000′
3 times 10,000 = 30,000
Distance to descend = 35 – 5 = 30 NM
30,000 divided by 30 = 1,000 feet per minute (Answer)
Method #3: Determine When to Begin the Descent
Here again, multiply the Altitude to be Lost by the Descent Groundspeed in miles per minute –
Let A = Feet x (Miles / Minute) = 10,000 x 3 = 30,000 Miles x (Feet / Minute)
then, to get your answer, divide the result (A ) by the Rate-of-Descent you want to use –
Distance-to-Descend = (A) / (Feet per Minute) = 30,000 / 600 = 50 Miles.
This answer equates to the slant distance required to descend 10,000 feet at 600 feet per minute and 180 knots groundspeed.
Practical Example #2
You are in a Piper Chieftain cruising at 13,500′ MSL and 85 miles from your destination, an airport located at 2,500′ MSL. You would like to descend at a smooth, constant rate of 500 feet per minute to reach 1,000′ AGL at a position five miles from the airport. How far from the airport should you begin your descent? (Assume a descent groundspeed of 150 knots.)
Descent groundspeed = 150/60 = 2.5 miles per minute.
Altitude to be lost = 13,500′ -2,500′ – 1,000′ =10,000′
2.5 times 10,000 = 25,000
Rate-of-descent = 500 feet per minute
25,000 divided by 500 = 50 NM
50 miles + 5 miles = 55 NM (Answer)
Practical Example #3
You are in a Piper Chieftain cruising IFR at 7,000′ MSL on an eastbound route and 25 miles from the ABC VORTAC. You receive an ATC clearance to climb to and maintain 11,000′, cross the ABC VORTAC at 9,000′. You would like to climb at a smooth, constant rate of 500 feet per minute to reach 9,000′ at a position one mile from the ABC VORTAC. How far from the ABC VORTAC should you begin your climb to 9,000′? (Assume a climb groundspeed of 150 knots.)
Climb groundspeed = 150/60 = 2.5 miles per minute.
Altitude to be gained = 2,000′ (7,000′ to 9,000′)
2.5 times 2,000 = 5,000
Rate-of-climb = 500 feet per minute
5,000 divided by 500 = 10 NM
10 miles + 1 mile = 11 NM (Answer)
Method #4: (Deleted.)
For optimum planning accuracy, use descent and climb gradients at or less than 1:10 (5.7º). For example, descending 12,000 feet over a horizontal distance of 20 NM is a descent gradient of approximately 1:10, or 600 feet per nautical mile.
Since the earth’s curvature becomes a factor for very long distance and/or very low-gradient descents, in these cases check the projected distance to descend to make sure it is less than the distance to the horizon. The geometric distance to the horizon in nautical miles is approximately equal to the square root of the altitude in feet, SQRT(altitude); i.e., the distance to the horizon from an altitude of 10,000 feet is approximately 100 NM.