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The ATC requests from a helicopter pilot to descend from a cruise altitude at 6 500 ft to 4 000 ft in still air conditions, where an approach procedure commences. If the helicopter flies with a TAS of 154 kt and must cover the descent in 8.3 NM, what is the required rate of descent?
  • A
    618 ft/min
  • B
    642 ft/min
  • C
    802 ft/min
  • D
    773 ft/min

Rate of descent (ROD) is a vertical speed and represents the rate of decrease of the vertical height per unit of time, usually expressed in feet per minute (ft/min):

1. The helicopter descends from the Top of Descent at 6 500 ft to 4 000 ft, thus the height difference is: 6 500 ft - 4 000 ft = 2 500 ft.

2. The helicopter descends in still air conditions, thus its Groundspeed will be equal to its TAS: 154 kt.

3. The flight time during the descent is: Horizontal distance / GS = 8.3 NM / 154 kt = 0.0538 hr or 3.23 min.

4. Therefore, its Rate of Descent must be: ROD = Height difference / Flight time = 2 500 ft / 3.23 min = 773 ft/min.

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