Equal Time Point & Point of No Return Calculator
Flight-planning calculator for the equal time point (critical point) and point of safe return: distance and time from departure using true airspeed and wind components out and back, with the endurance available and the effect of a headwind or tailwind on both points.
How it works
ETP distance = D × GS_home ÷ (GS_on + GS_home); PNR distance = E × GS_out × GS_home ÷ (GS_out + GS_home); time = distance ÷ GS_out
Two planning points protect a flight over water, desert or high terrain. The equal time point is where continuing to the destination takes exactly as long as turning back — the decision point for an engine failure, depressurisation or medical emergency, computed with the ground speeds the aircraft will actually have after the failure. The point of no return (or point of safe return) is set by fuel: beyond it there is not enough endurance to get home with reserves. Wind moves both: a headwind outbound means a fast trip home, so the ETP shifts toward the destination while the PNR, which must budget for the slow outbound leg, creeps closer to departure. Airlines compute these for ETOPS routes with several alternates; light aircraft pilots use the same arithmetic for island hops.
Worked example
A 600 NM leg at 150 kt TAS into a 20 kt headwind gives 130 kt out and 170 kt back. The ETP is 600 × 170 ÷ 300 = 340 NM out (57% of the route, after 157 min); with 5 hours of safe endurance the PNR is 5 × 130 × 170 ÷ 300 = 368 NM, after 170 min, while the trip itself takes 277 min.
Frequently asked questions
Why is the ETP not the midpoint?
Because the wind is different in each direction. The ETP is closer to whichever airfield is slower to reach — into a headwind outbound the return is quick, so you should continue only once you are past the halfway point.
How is the PNR with an alternate different?
Replace the homebound ground speed with the ground speed to the alternate and the departure with the alternate; the last point from which you can reach any suitable airfield with reserves is the operational limit.
Related calculators
Planning aid using standard flight-training rules of thumb and ISA atmosphere approximations — not a substitute for the aircraft flight manual (AFM/POH) performance charts, ATC instructions or an approved flight computer. Always apply the manufacturer's data and required safety margins.