Point of No Return & Equal Time Point Calculator
Where you can still turn back, and where going on or turning back takes the same time: with wind, fuel, reserves and failure cases.
Use true directions for both. The wind is where it blows from, as in a forecast. Crosswind slows you a little both ways.
Fuel you must still have when you get back. Use your own authority's and operator's rules.
Lighter on the way back, so often a little lower.
Adds clock times to the PNR and each ETP.
The aircraft slows down and often descends, so the ETP moves and the fuel flow changes. The figures filled in are examples only: use your aircraft's own data.
Add airports you could divert to. You get an ETP between each pair, starting from departure and ending at the destination.
How your PNR and ETP are worked out
Both come from the ground speeds out and home. The PNR also needs the fuel you can burn.
- 1
Ground speeds
Out 400 − 50 = 350Home 400 + 50 = 450Sum 800 kt
- 2
Safe endurance
10,000 − 1,000 kg÷ 2,000 kg/h= 4.50 h
- 3
Point of no return
4.50 h × 450 ÷ 800= 2 h 32 min out= 886 nm
- 4
Equal time point
1,000 × 450 ÷ 8001 h 15 min each way= 563 nm
Read more: the formula, a worked example, engine-out ETPs and more
The point of no return (PNR) is the furthest point along your route from which you can still fly back to where you started and land with your reserve intact. The equal time point (ETP), also called the critical point, is where carrying on to the destination takes the same time as turning back. Both depend on the ground speeds out and home, so the wind matters a lot. Only the PNR depends on fuel.
The formula
ground speed out = TAS − headwind · ground speed home = TAS + headwind
safe endurance = (fuel on board − taxi − reserve) ÷ fuel flow
PNR time = safe endurance × GS home ÷ (GS out + GS home)
PNR distance = PNR time × GS out
with a different fuel flow home: PNR time = usable fuel ÷ (flow out + flow home × GS out ÷ GS home)
ETP distance from departure = total distance × GS home ÷ (GS on + GS home)
time to the ETP = ETP distance ÷ normal GS out
time from the ETP to either airport = ETP distance ÷ GS home
For an engine-out or depressurised ETP, the "on" and "home" ground speeds come from the slower airspeed after the event, but you still reach the ETP at your normal cruise speed. When you give the wind as a direction and speed, the ground speeds come from the wind triangle, so a crosswind lowers both.
Worked example
A 1,000 nm route, true airspeed 400 kt, with a 50 kt headwind on the way out. The ground speed out is 400 − 50 = 350 kt and home is 400 + 50 = 450 kt; together 800 kt.
You have 10,000 kg of fuel, burn 2,000 kg an hour and want 30 minutes in reserve: 0.5 × 2,000 = 1,000 kg. That leaves 9,000 kg to fly on, a safe endurance of 9,000 ÷ 2,000 = 4.5 hours.
PNR time = 4.5 × 450 ÷ 800 = 2.53125 hours, which is 2 h 32 min. The PNR is 2.53125 × 350 = 886 nm from departure (885.9 nm). Flying back takes 885.9 ÷ 450 = 1.97 hours (1 h 58 min). Check: 5,062.5 kg out plus 3,937.5 kg back is the 9,000 kg you had to spare.
ETP = 1,000 × 450 ÷ 800 = 562.5 nm from departure, reached after 562.5 ÷ 350 = 1.61 hours (1 h 36 min). From there it is 562.5 ÷ 450 = 1.25 hours back and 437.5 ÷ 350 = 1.25 hours on: 1 h 15 min either way. You would have 10,000 − 1.607 × 2,000 = 6,786 kg at the ETP, and the diversion burns 1.25 × 2,000 = 2,500 kg, leaving 4,286 kg.
Advanced examples: with one engine out at 340 kt and 1,800 kg/h, the ground speeds become 290 kt on and 390 kt home, so the ETP moves to 1,000 × 390 ÷ 680 = 573.5 nm (1 h 38 min after take-off, then 1 h 28 min to either airport, burning 2,647 kg and leaving 4,076 kg). Depressurised at 300 kt and 2,600 kg/h gives 250 kt on and 350 kt home, an ETP at 1,000 × 350 ÷ 600 = 583.3 nm, 1 h 40 min to either airport, burning 4,333 kg and leaving 2,333 kg: still above the 1,000 kg reserve. A diversion airport on the track 500 nm out splits the route into two pairs, with ETPs at 500 × 450 ÷ 800 = 281.25 nm and 500 + 281.25 = 781.25 nm.
Which way does the wind push them?
- Headwind out: the ETP moves towards the destination (turning back is faster), and the PNR moves towards home (you cover less ground out).
- Tailwind out: the ETP moves towards home, and the PNR moves further out, but you'll face a headwind coming back.
- Still air: the ETP is exactly halfway along the route and the PNR is halfway through your safe endurance.
ETPs in real operations
Long-range and extended diversion time (EDTO / ETOPS) planning works out ETPs between pairs of suitable en-route airports, not just departure and destination, for several cases: all engines (for a medical emergency), one engine out, and depressurisation. The depressurised case is often the worst for fuel because the aircraft has to fly low, where fuel flow is high. Operators then check that the fuel on board at each ETP covers the diversion with the required allowances. This calculator gives you the geometry and a simple fuel check. It does not model descent profiles, icing, APU burn, oxygen limits or the operator's critical fuel allowances.
Off-track airports
For a diversion airport off the track, the calculator finds the point on the track where the flying time to each airport is the same, flying straight to each one at the ground speed for that direction. On track this gives the same answer as the formula above.
Units
1 nm = 1.852 km. 1 kt = 1.852 km/h. 1 US gallon = 3.785 litres; 1 imperial gallon = 4.546 litres; 1 lb = 0.4536 kg. Weight and volume are joined by the fuel density: Jet A-1 is about 0.80 kg per litre and avgas about 0.72 kg per litre, changing with temperature.
A planning and training aid only. Use your aircraft's approved performance data, your operator's procedures and your authority's fuel rules. Checked 11 October 2026.
Common questions
How do you calculate the point of no return?
Work out the safe endurance: fuel on board less the reserve, divided by the fuel flow. Then PNR time = safe endurance × ground speed home ÷ (ground speed out + ground speed home). Multiply by the ground speed out for the distance. With 4.5 hours of safe endurance, 350 kt out and 450 kt home, the PNR is 2 h 32 min and 886 nm from departure.
How do you calculate the equal time point?
Equal time point distance from departure = total distance × ground speed home ÷ (ground speed on + ground speed home). On a 1,000 nm route with 350 kt on and 450 kt home, the ETP is 1,000 × 450 ÷ 800 = 562.5 nm from departure. From there, going on or turning back both take 1 h 15 min.
What is the difference between PNR and ETP?
The ETP is about time: where going on and turning back take the same time, which matters after an engine failure, a depressurisation or a medical emergency. The PNR is about fuel: the furthest point from which you can still get back to departure with your reserve. The ETP does not depend on fuel at all; the PNR does.
Which way does wind move the PNR and ETP?
A headwind on the way out moves the ETP towards the destination, because turning back gives you a tailwind. The same headwind moves the PNR closer to home, because you cover less ground on the way out. In still air the ETP is exactly halfway along the route and the PNR is halfway through the safe endurance.
Why work out an engine-out or depressurisation ETP?
After an engine failure or a loss of pressurisation the aircraft flies slower and usually lower, so the ground speeds and fuel flow change. The ETP moves, and the fuel needed to reach an airport can rise sharply at low level. Planners check that the fuel at the ETP covers the diversion in the worst case.
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