FAA Aircraft Dispatcher (ADX)Flight Planning and PerformanceMedium
Using the standard 3-to-1 descent planning rule, a dispatcher is estimating the top of descent (TOD) point for a flight cruising at FL350 en route to a sea-level destination airport. An additional 10 nautical miles is added for deceleration and level-off. At approximately what distance (in nautical miles) from the destination should descent begin?
- A95 NM
- B105 NM
- C125 NM
- D115 NM
Show answer & explanationAnswer & explanation
Correct answer: D. 115 NM
The 3:1 rule estimates 3 NM of distance for every 1,000 feet of altitude to lose. 35,000 ft / 1,000 = 35; 35 x 3 = 105 NM, plus the 10 NM buffer for deceleration/level-off = 115 NM from the destination.
Why the other options are wrong
- A. Omits the altitude-to-distance conversion correctly, too short.
- B. This is the descent distance alone without the 10 NM buffer.
- C. Overestimates by adding too much buffer distance.
3:1 Descent Rule (Top of Descent)
A rule of thumb used to estimate top-of-descent distance: approximately 3 NM of ground distance for each 1,000 feet of altitude to be lost.
- Distance (NM) = Altitude to lose (in thousands) x 3
- Add extra distance for deceleration/level-off segments
- Used for quick in-flight or planning estimates, not precise FMS calculations
- Wind and speed changes affect actual required distance
Memory trick: 3 miles down for every thousand feet — then add a cushion