Equal time point calculator
Somewhere along a leg there is a point where continuing to the destination and turning back to where you came from take exactly the same time. With any wind at all, that point is not halfway.
Calculator
This is an educational and planning aid. Use current official weather, your aircraft flight manual, and the rules that apply to your flight before you act on any figure here. See the aviation disclaimer.
What the result means
The answer is a distance from departure. Before it, turning back is quicker. After it, pressing on is quicker.
If the return would be faster than continuing, because the wind reverses in your favour, the point sits beyond halfway. If the return would be slower, it sits before halfway.
This is a time calculation for a situation where time matters most, such as a medical problem on board or a decision that needs a runway quickly.
When this is useful
- Planning any leg with a long stretch where no other airfield is available, over water, mountains or wilderness.
- Before a flight where the wind is strong enough that the two directions have very different groundspeeds.
- Considering what to do if something goes wrong at the worst moment in a leg.
How it is calculated
The distance to the point is the total distance multiplied by the return groundspeed, divided by the sum of the two groundspeeds. That expression falls out of setting the time onward equal to the time back and solving.
The calculator also gives the time to reach the point and the time from there to either airfield, which are equal by definition and act as a check on the answer.
ETP distance = total distance x groundspeed back / (groundspeed on + groundspeed back)
Worked example
- Total leg distance
- 1,000 NM
- Groundspeed continuing on
- 400 kt
- Groundspeed turning back
- 500 kt
The equal time point is 556 NM from departure, reached after 1 hour 23 minutes. From there it is 1 hour 7 minutes to either airfield.
Check: From the point, continuing covers the remaining 444 NM at 400 kt, taking 1.111 hours. Turning back covers 556 NM at 500 kt, also taking 1.111 hours. The two match exactly, which is the definition of the equal time point.
Limitations
- It is about time only. It does not tell you whether you have the fuel to reach either airfield, which is a separate question answered by the point of no return.
- It assumes both groundspeeds hold steady. A real wind changes along the route and with altitude.
- It only considers two airfields, the departure and the destination. A real diversion plan usually has better options in between.
Common mistakes
- Using the same groundspeed both ways. With no wind the point is simply halfway and the calculation adds nothing.
- Entering true airspeed instead of groundspeed, which removes the wind that makes this calculation worth doing.
- Confusing it with the point of no return. They answer different questions and usually sit in different places.
Sources
- Air Force Manual 11-2C-145A, Volume 3: Equal Time Point Formula , US Department of the Air Force
- Pilot's Handbook of Aeronautical Knowledge, navigation chapter , US Federal Aviation Administration