FAA Aircraft Dispatcher (ADX)Flight Planning and PerformanceHard
A dispatcher is flight planning an overwater flight of 1,400 NM. The aircraft's true ground speed proceeding to destination is 350 knots, and the ground speed returning to the departure airport (in the event of a turn-back) is 280 knots. What is the distance from the departure airport to the Equal Time Point (ETP)?
- A500 NM
- B778 NM
- C700 NM
- D622 NM
Show answer & explanationAnswer & explanation
Correct answer: D. 622 NM
The ETP is the point where time to continue equals time to return. ETP distance from departure = (Total Distance x GS-back) / (GS-out + GS-back) = (1,400 x 280) / (350+280) = 392,000/630 = 622.2 NM.
Why the other options are wrong
- A. Too low; does not match the ratio of ground speeds.
- B. Results from reversing the ground speeds in the formula.
- C. This is simply the midpoint of the route, not the ETP, since ground speeds are unequal.
Equal Time Point (ETP)
The point along a route where the time required to continue to destination equals the time required to return to the departure point, used for critical fuel/route decisions.
- ETP distance (from departure) = D x GS-back / (GS-out + GS-back)
- Also called Critical Point (CP) in some contexts
- Used heavily in ETOPS and overwater dispatch planning
- Depends on both distance and the two direction-dependent ground speeds
Memory trick: Equal Time = Equal Cost either way you fly