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Given the following information, calculate the rate of descent (ROD) for an aircraft that is approaching the destination.

Ground distance during the descent: 25 NM
Average TAS: 130 kt
Average tailwind component: 30 kt
Descent: from FL180 to pressure altitude 3000 ft
  • A
    1600 ft/min
  • B
    1920 ft/min
  • C
    1000 ft/min
  • D
    1300 ft/min

Refer to figure.
There are multiple methods available to address this question, and the choice among them depends on personal preference.

Method 1:

The Height difference is: 18 000 ft - 3 000 ft = 15 000 ft
The Groundspeed is: TAS + Tailwind component = 130 kt + 30 kt = 160 kt
25 NM / 160 kt = 0.15625 hours
0.15625 hours × 60 minutes/hour ≈ 9.4 minutes
15 000 ft / 9.4 minutes ≈ 1 600 ft/min

Method 2:
The following formula can be used: Height difference (ft) = Glidepath Angle(o) x Covered Distance (ft) / 60.

The Height difference is: 18 000 ft - 3 000 ft = 15 000 ft.

So, solving for Glidepath Angle, we get: Glidepath Angle(o) = (Height difference (ft) x 60) / Covered Distance (ft) = 15 000 ft x 60 / (25 x 6080 ) ft = 5.9o.

The Groundspeed is: TAS + Tailwind component = 130 kt + 30 kt = 160 kt.

Thus, ROD = (Glide Angle/60) x (Groundspeed/60) x 6080 = (5.9/60) x (160/60) x 6080 = 1 594 ft/min, which is very close to 1 600 ft/min.

Method 3:

  1. For a 3o glideslope angle only, the following formula states: ROD = 5 x GS = 5 x 160 = 800 fpm.
  2. Thus, for a 5.9o glideslope angle, the ROD will be: 800 fpm x 5.9o / 3o = 1 573 fpm, which is very close to 1 600 ft/min.

Method 4 (using your Flight/Nav computer):

  1. Set the glide path angle 5.9º(59) over 60.
  2. Read out the ROD 1 600 ft/min (16) approximately over the GS 160 kt (16).

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