FAA Aircraft Dispatcher (ADX)Navigation and ATCHard
A dispatcher is calculating the Equal Time Point (critical point) for an oceanic flight. The total distance between departure and destination is 1,400 NM. The groundspeed continuing to destination is 440 knots, and the groundspeed if the aircraft turned back to the departure airport is 500 knots. What is the approximate distance from the departure airport to the ETP?
- A700 NM
- B800 NM
- C745 NM
- D655 NM
Show answer & explanationAnswer & explanation
Correct answer: C. 745 NM
ETP distance from departure = (Total Distance × GS home) / (GS out + GS home) = (1,400 × 500) / (440 + 500) = 700,000 / 940 ≈ 744.7 NM, which rounds to approximately 745 NM.
Why the other options are wrong
- A. Close but results from a rounding/arithmetic error in the division.
- B. This overestimates the distance; recompute using the correct groundspeed values.
- D. This underestimates the ETP distance; check the formula's numerator and denominator.
Equal Time Point (ETP) / Critical Point
The point along a route where the time to continue to destination equals the time to return to departure, calculated using groundspeeds in each direction.
- Formula: ETP distance from departure = D × GS_home / (GS_out + GS_home)
- Used for oceanic and ETOPS planning to decide continue-vs-return decisions
- Time to ETP = ETP distance / GS_out
Memory trick: 'Distance times the OTHER way's speed, over the sum of both speeds'