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An aircraft at FL390 is required to descend to cross a DME facility at FL70. Maximum rate of descent is 2500 FT/MIN, mean GS during descent is 248 kt. What is the minimum range from the DME at which descent should commence?
  • A
    68 NM
  • B
    58 NM
  • C
    53 NM
  • D
    63 NM

Required Rate of Climb/Descent (ft/m) = G/S (kt) x Gradient of Climb/Descent

Gradient of Descent = 2500 / 248 = 10%

Climb/Descent gradient = ( Change in Height / Horizontal distance ) / 100%

10 % = (32 000 / (Horizontal distance x 6000)) x 100 %
Horizontal distance = 53 nm

NOTE: Altitude and horizontal distance must be in the same unit of measurement, 1NM = 6000 ft approximately.

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