Critical point and point of no return

Equal time point (critical point)

The point from which it takes the same time to continue to the destination as to return (or to go to two chosen aerodromes). With a headwind or tailwind component it is not halfway: it moves into the wind. An engine-failure ETP is worked with one-engine-out ground speeds.

Distance from departure to the ETP = D × H / (O + H), where D is the total distance, O the ground speed onward and H the ground speed home (back).

Example: D = 1,000 NM, O = 400 kt (headwind), H = 500 kt (tailwind back): 1,000 × 500 / 900 ≈ 556 NM from departure.