Aviation ETP & Point of No Return Calculator
Enter route distance, outbound and return groundspeeds, usable fuel, reserve fuel, and fuel burn rates. The calculator returns the equal time point, the reserve-aware point of no return, and whether time or fuel is the earlier constraint.
Equal time point from departure (NM)218.18
- ETP position along route
- 54.55%
- Time to reach ETP
- 2 hr 11 min
- Diversion time from ETP
- 1 hr 49 min
- Point of no return from departure (NM)
- 218.18
- PNR position along route
- 54.55%
- Return fuel margin at ETP (US gal)
- 0
- Limiting point
- Coincident within display rounding.
ETP compares equal flying time only; it does not evaluate runway, weather, approach, maintenance, or regulatory suitability.
PNR uses fuel above the protected reserve and shows the raw distance even when that distance is beyond the destination.
Fuel quantities and burn rates must use the same selected unit.
How to use this calculator
- Enter the along-track route distance in nautical miles.
- Enter the expected groundspeed while continuing and the groundspeed after turning back.
- Choose a fuel unit, then enter usable fuel, protected reserve, and both hourly burn rates in that same unit.
- Read the ETP, PNR, return fuel margin at the ETP, and limiting-point interpretation.
How the ETP and PNR are calculated
The equal time point is the point on a route where the time to turn back equals the time to continue to the destination or another selected aerodrome. Because wind changes groundspeed differently in each direction, the point is not usually halfway by distance. If the return groundspeed is higher than the outbound groundspeed, the ETP moves farther from departure because the aircraft can cover the return leg more quickly.
Let D be route distance in nautical miles, Go be groundspeed continuing, and Gr be groundspeed returning. The equal time point distance from departure is:
xE = D x Gr / (Go + Gr)
Time to reach the ETP is xE / Go. The diversion time back from the ETP is xE / Gr, which equals the time to continue from the ETP to the destination when the ETP equation is satisfied.
The point of no return uses fuel rather than equal time. Let F be usable fuel available at the start of the analyzed route, R be protected reserve fuel, Bo be outbound fuel burn per hour, and Br be return fuel burn per hour. Fuel available above reserve is A = F - R. Outbound fuel per nautical mile is Bo / Go, and return fuel per nautical mile is Br / Gr. The raw point of no return is:
xP = A / ((Bo / Go) + (Br / Gr))
The return fuel margin at the ETP is:
M = A - xE x (Bo / Go) - xE x (Br / Gr)
What moves the points most
ETP is moved by the two groundspeeds. A slower return groundspeed pulls the ETP closer to departure, while a faster return groundspeed pushes it farther along the route. PNR is moved by fuel above reserve, burn rate, and speed. Higher fuel burn on the return leg or a lower return groundspeed brings the PNR closer because the aircraft must protect enough fuel to come back.
How to read the limiting point
If PNR is before ETP, fuel is the earlier constraint: passing that point removes the ability to return with the protected reserve before the equal-time decision point is reached. If ETP is before PNR, time is earlier: the equal-time decision arrives before fuel becomes the limiting return factor. If the raw PNR lies beyond the destination, the route has no on-route PNR under the entered assumptions.
What this calculator leaves out
This is a constant-average model. It does not calculate climb, descent, holding, approach, route curvature, weather avoidance, changing winds, fuel-temperature effects, abnormal checklists, or aerodrome suitability. Include those effects in the entered averages only when doing so is appropriate for the planning question.
Worked example
Suppose the route is 400 NM, continuing groundspeed is 100 kt, return groundspeed is 120 kt, usable fuel at the route start is 50 US gal, protected reserve is 10 US gal, and both outbound and return burn are 10 US gal/hr.
The ETP is 400 x 120 / (100 + 120) = 218.18 NM from departure, or 54.55% of the route. Time to reach it is 218.18 / 100 = 2.1818 hours, about 2 hr 11 min. Time to return from it is 218.18 / 120 = 1.8182 hours, about 1 hr 49 min.
Fuel above reserve is 50 - 10 = 40 US gal. The combined outbound-and-return fuel per nautical mile is 10 / 100 + 10 / 120 = 0.18333 US gal/NM, so PNR is 40 / 0.18333 = 218.18 NM. The return fuel margin at the ETP is 0 US gal, so the ETP and PNR are coincident within display rounding.
Common questions
What is an equal time point in aviation?
An equal time point is the point on a route where flying time back to one aerodrome equals flying time onward to another. It is a time comparison, not a statement that either aerodrome is suitable or that the aircraft has enough fuel for every operational requirement.
Is ETP the same as the critical point?
In many training and planning contexts, critical point and equal time point refer to the same time-based idea. Some operators use more specific scenario names, such as engine-out or depressurization ETP, when the speeds and fuel flows differ from normal cruise.
What is the difference between ETP and point of no return?
ETP compares time to two options. Point of no return compares fuel required to go out and return while preserving a reserve. They can occur at different distances because fuel burn and groundspeed do not affect the two calculations in the same way.
Why does the return groundspeed appear in the ETP numerator?
A faster return makes it possible to travel farther from departure before the time back equals the time onward. That is why return groundspeed pushes the ETP distance outward in the formula xE = D x Gr / (Go + Gr).
Should PNR use total endurance or safe endurance?
Operationally, PNR should protect whatever reserve, contingency, and procedural fuel the flight requires. This calculator uses fuel above the reserve you enter, so the reserve input is how you convert total usable fuel into safe fuel for the simplified PNR equation.
What does it mean when PNR is before ETP?
It means fuel becomes limiting before the equal-time decision point. Past the PNR, the entered fuel and burn rates no longer support returning to departure with the protected reserve, even though the ETP may not have been reached yet.