Learning Objective 061.01.05.06.02 Calculate rate of descent (ROD) on a given glide-path angle or gradient using the following rule of thumb formulae: ROD (ft/min) = GP degrees  × GS (NM/min) × 100, ROD (ft/min) = GP per cent × GS (kt)

The formula to be used in order to determine the Rate of Descent depends on the unit of measurement of Glide path (degrees or percentage):

• Rate of descent (ft/min) = Glide path (degrees) x Ground speed (NM/min) x 100
• Rate of descent (ft/min) = Glide path (%) x Ground speed (kt)

1. Calculate Groundspeed in NM/min
The True Airspeed (TAS) is 100 kt and the tailwind is 15 kt. The Groundspeed can be found by the formula:

• Groundspeed = TAS + tailwind = 100 kt  + 15 kt = 115 kt

The Groundspeed of 115 kt (NM/hr) is equal to:

• Groundspeed (NM/min) = Groundspeed (NM/hr) / 60 = 115 / 60 = 1.92 NM/min

2. Calculate the Rate of descent
The Groundspeed is 1.92 NM/min. The rate of descent can be found by the formula:

• Rate of descent (ft/min) = Glide path (degrees) x Ground speed (NM/min) x 100 = 5.5 x 1.92 x 100 = 1056 ft/min

The closest option is 1000 ft/min.