Equal Time Point (ETP) Calculator
The mid-route line where continuing and turning back take the same time — wind-shifted toward the upwind end, with the time-to-either-option readout.
With a tailwind toward B, the ETP sits BEFORE halfway — returning is slow, so the 'turn back' option dies early. For engine-failure planning, compute with one-engine-inoperative TAS: the real diversion happens at the degraded speed.
Formula
⚠️ For flight planning and education only — verify with your POH/AFM and official sources. Not for primary navigation or in-flight emergency decision-making without POH data.
The mid-route line where continuing and turning back take the same time — wind-shifted toward the upwind end, with the time-to-either-option readout.
About Equal Time Point (ETP) Calculator
When something breaks mid-leg, the first question is binary: back or on? The equal time point answers it before takeoff — the spot where both options cost identical time, displaced from the geographic midpoint into the wind (because the upwind option is the slow one). This calculator finds it for any leg, with the percentage position and the time-to-either-end readout, and notes the professional wrinkle: for engine-out scenarios, the TAS that belongs in the formula is the degraded one.
How to use Equal Time Point (ETP) Calculator
- 1Enter — sensible defaults are pre-filled so you see a worked result immediately.
- 2Read the live results: .
- 3Check the "With your numbers" line to see the formula ETP from A = D × GS_back / (GS_on + GS_back) — the point slides INTO the wind substituted step by step.
- 4Adjust inputs (or flip the unit toggle) until the scenario matches yours, then copy or share the result.
Why use Equal Time Point (ETP) Calculator?
- ✓Instant, free and private — every calculation runs in your browser, nothing is uploaded
- ✓Built on the published formula ETP from A = D × GS_back / (GS_on + GS_back) — the point slides INTO the wind with sources cited on the page
- ✓With a tailwind toward B, the ETP sits BEFORE halfway — returning is slow, so the 'turn back' option dies early. For engine-failure planning, compute with one-engine-inoperative TAS: the real diversion happens at the degraded speed.
- ✓Switch units, tweak any input and watch every result update live
Frequently asked questions
Why does the ETP move into the wind rather than downwind?+
Think of the two options' speeds: with a tailwind toward B, continuing is fast and returning is slow — so the 'equal time' balance point arrives early, before halfway, while returning is still short enough to compete. The formula D×GS_back/(GS_on+GS_back) encodes it: the slower the return, the smaller the fraction. At 20 kt on a 110-kt aircraft the shift is ~9% of the leg.
What's the medical-diversion version of the ETP?+
The same arithmetic with normal cruise TAS: a passenger emergency at 40% across the leg → the ETP says which runway arrives sooner. Crews flying long overwater legs compute several ETPs — normal-speed, single-engine, and depressurized (lower altitude = different TAS and wind) — because each emergency travels at its own speed and may have its own optimal divert.
How does ETOPS use equal time points?+
ETOPS certification draws the entire route to stay within a rule-time (60/120/180 min at one-engine-inoperative speed) of adequate airports, and ETPs between alternate pairs define which airport 'owns' each route segment. Your single-engine Cessna version of the same logic: between two airfields along a remote leg, the engine-out ETP (computed at glide-augmented or degraded speed) tells you which way to point before you've thought about it.
ETP or PNR — which should I brief for a leg?+
Different organs: ETP is the time-optimal switch line (brief it for medical/mechanical 'which way now?'); PNR is the fuel wall (brief it when return-to-departure is the contingency, as on island or weather-uncertain destinations). Long remote legs brief both: 'ETP at 172 nm, PNR at 290 with today's fuel.' Two numbers, two distinct emergencies, zero improvisation.
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