A helicopter with a cruise groundspeed of 98 kt is en-route to its destination when the Pilot is instructed by the ATC to descend from 4800 ft to 2100 ft before reaching the next reporting point, located 4.9 NM ahead. What is the minimum rate of descent required to comply with the ATC instructions?
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 4800 ft to 2100 ft, thus the height difference is: 4800 ft - 2100 ft = 2700 ft.
2. The flight time during the descent is: Horizontal distance / GS = 4.9 NM / 98 kt = 0.05 hr or 3 min.
4. Therefore, its minimum rate of descent must be: ROD = Height difference / Flight time = 2700 ft / 3 min = 900 ft/min.
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