FAA Commercial Pilot Airplane (CAX)Navigation and Flight PlanningMedium
A pilot is planning a descent from a cruising altitude of 9,500 feet MSL to a pattern altitude of 1,500 feet MSL, using a planned descent rate of 500 feet per minute and an expected groundspeed of 150 knots during the descent. Approximately how many nautical miles from the airport should the descent begin?
- A60 NM
- B40 NM
- C30 NM
- D50 NM
Show answer & explanationAnswer & explanation
Correct answer: B. 40 NM
The altitude to lose is 9,500 − 1,500 = 8,000 feet. At 500 fpm, the time required is 8,000/500 = 16 minutes. At a groundspeed of 150 knots, distance covered is 150 × (16/60) = 40 NM, so descent should begin 40 NM from the airport.
Why the other options are wrong
- A. This distance would require a much slower descent rate than planned.
- C. This underestimates the required descent distance.
- D. This overestimates the distance needed for this descent rate.
Top of Descent (TOD) Calculation
TOD distance is found by determining the time needed to descend at a planned rate and converting that time to distance using groundspeed.
- Time to descend = altitude to lose ÷ descent rate (fpm)
- Distance = groundspeed × (time in minutes/60)
- A common rule of thumb is 3 NM per 1,000 ft at a 3° descent path, but actual rate/groundspeed should be used for precision
Memory trick: Lose the feet, find the minutes, multiply for miles