Halfway along a long over-water leg, a passenger becomes seriously ill. Do you turn back or press on? With a headwind on the nose, the answer is not “whichever airport is closer”. The point where turning back and continuing take exactly the same time is the Equal Time Point (ETP). A second, separate number tells you the last moment you can still turn back with the fuel you have: the Point of No Return (PNR). Dispatchers and long-range crews work out both before departure. You can run your own numbers with the ETP and PNR Calculator.
ETP and PNR are answers to different questions
| Equal Time Point (ETP) | Point of No Return (PNR) | |
|---|---|---|
| Question it answers | From here, is it quicker to go on or go back? | How far can I go and still get back to where I started? |
| Depends on fuel? | No. Only distance and groundspeeds. | Yes. It is limited by usable endurance. |
| Typical use | Engine failure, depressurisation, medical diversion | Routes to remote or single-runway destinations with no en-route alternate |
| Where it lies with no wind | Exactly halfway | Exactly half the endurance |
The ETP formula
Let D be the total distance, O the groundspeed continuing on to the destination, and H the groundspeed returning home. The distance from the departure point to the ETP is:
Distance to ETP = D × H ÷ (O + H)
The time to reach the ETP is that distance divided by the outbound groundspeed O. The idea is easy to check: from the ETP, the time to go on, (D − ETP) ÷ O, equals the time to go back, ETP ÷ H.
The wind moves the ETP into the wind. With a headwind on the way out you get a tailwind on the way back, so going back is quicker and the ETP moves beyond the halfway point.
Worked example: 1,200 NM over water
- Distance A to B: 1,200 NM
- TAS: 450 kt, with a 50 kt headwind on the way out
- Groundspeed on (O): 400 kt. Groundspeed home (H): 500 kt.
Distance to ETP = 1,200 × 500 ÷ (400 + 500) = 666.7 NM from A, beyond the 600 NM halfway point.
Time to ETP = 666.7 ÷ 400 = 1.667 h = 1 h 40 min after departure.
Check from the ETP: going on takes 533.3 ÷ 400 = 80 min, and going back takes 666.7 ÷ 500 = 80 min. They match.
ETP depends on the scenario
The speeds in the formula must match the emergency you are planning for. An all-engines-operating ETP for a medical diversion uses normal cruise TAS. An engine-out ETP uses the slower one-engine-inoperative TAS at a lower level, where the wind is usually different. A depressurisation ETP uses the speed and level of an emergency descent, often around 10,000 ft. Each gives a different ETP, which is why flight plans for long over-water or remote routes often show several. A route can also have ETPs between en-route alternates rather than between departure and destination. The method is the same: D is the distance between the two alternates being compared.
The PNR formula
Let E be the safe endurance in hours: usable fuel minus the reserve you must keep, divided by the fuel flow. The time to the PNR is:
Time to PNR = E × H ÷ (O + H)
The distance to the PNR is that time multiplied by the outbound groundspeed O.
Worked example: PNR on the same leg
Same aircraft and wind, with 5 h of safe endurance after reserves.
Time to PNR = 5 × 500 ÷ 900 = 2.778 h = 2 h 47 min.
Distance to PNR = 2.778 × 400 = 1,111 NM.
Check: the return from there takes 1,111 ÷ 500 = 2.22 h, and 2.78 + 2.22 = 5.0 h, exactly the endurance. Past 1,111 NM the aircraft no longer has the fuel to get back to A with its reserve intact.
Common mistakes
- Using TAS instead of groundspeed. Without wind the ETP is always halfway, which hides exactly the effect the calculation exists to show.
- Swapping O and H. If your ETP with a headwind comes out before halfway, the groundspeeds are the wrong way round.
- Using total fuel for the PNR. Endurance must exclude the reserve and any unusable fuel.
- One wind for the whole leg. Over long distances the wind changes. Real flight plans compute the ETP from the forecast wind segment by segment.
- Treating the ETP as a decision. It tells you which way is quicker, not which way is better. Weather, runway length, medical facilities and terrain at each option still decide.
FAQ
Is the ETP the same as the critical point?
Yes. Many textbooks, especially in Europe, call the equal time point the critical point (CP). The formula is the same.
Does fuel affect the ETP?
No. The ETP depends only on distance and the two groundspeeds. Fuel matters for the PNR, and for whether the diversion from the ETP is actually possible.
Why does a tailwind outbound move the ETP back towards departure?
With a tailwind out you face a headwind on the way back, so returning is slow. The point where both options take the same time is therefore closer to the departure airport.
Do light aircraft need an ETP?
It is most common in long-range and over-water operations, but the same thinking helps any pilot crossing water, desert or high terrain decide which way to turn if something goes wrong.
Sources and further reading
Method as taught in flight-planning and dispatcher training material, including the FAA Aircraft Dispatcher syllabus and EASA ATPL flight-planning theory. For real operations use your operator’s computerised flight plan and procedures.
Last reviewed: November 2026.