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An aircraft at FL370 is required to commence descent when 100 NM from a DME facility, and to cross the station at FL120. The mean GS during the descent is 396 kt. What is approximately the minimum rate of descent required?
  • A
    1550 ft/min
  • B
    1650 ft/min
  • C
    1000 ft/min
  • D
    2400 ft/min

Even though we are given a slant range and not a ground range, the difference between them, in this case, will be very small (100 NM – 99.81) => therefore, we will disregard and use 100 NM for ground range and show you the fastest method to solve this question.

We are cruising at 37 000 ft and need to descend to 12 000 ft => Therefore, the aircraft has to descend 25 000 ft in 100 NM.

Use the formula:

ROD = (height difference x GS) / (Distance x 60)
ROD = (25 000 x 396) / (100 x 60)
ROD = 1650 ft/min

Alternatively,

Distance = GS x Time
100 NM = 396 kt x Time
Time = 15.15 min

We have 15.15 min to descend 25 000 ft:
25 000 ft /15.15 min = 1650 ft/min

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