How to Calculate an Equal Time Point (ETP)

The equal time point (ETP) is the point on a route where it takes the same time to continue to the destination as it does to return to the departure airport. Flight planners use ETPs to decide where a diversion should go, which is why they appear in EDTO planning.

The formula

Let D be the distance between the two airports. Let O be the ground speed when continuing and H the ground speed when returning. The distance from the first airport to the ETP is:

ETP distance = D × H ÷ (O + H)

It comes from setting the two times equal: x ÷ H = (D − x) ÷ O.

Worked example

Two airports are 3,000 NM apart. The speed in the diversion condition is 400 knots. With no wind, O and H are both 400, so the ETP is at 3,000 × 400 ÷ 800 = 1,500 NM, the midpoint.

Now add a 50 knot tailwind toward the destination. Continuing gives O = 450 knots. Returning means flying into that wind, so H = 350 knots. The ETP is at 3,000 × 350 ÷ 800 = 1,312.5 NM from the first airport. Check: the return takes 1,312.5 ÷ 350 = 3.75 hours, and continuing takes (3,000 − 1,312.5) ÷ 450 = 3.75 hours.

Why wind moves the ETP

A tailwind toward the destination makes continuing quick and returning slow, so the ETP moves back toward the departure airport. A headwind toward the destination moves it the other way.

In practice

Planners compute ETPs between pairs of alternates along the route, with forecast winds and the approved diversion speed, and check weather at each pair for the time of use. This guide covers only the arithmetic.

Open the equal time point calculator

FAQ

Is the ETP always at the midpoint?

Only in still air. Any wind component along the route moves it toward the airport that is hardest to reach.

Does the ETP use the cruise speed?

For diversion planning it uses the speed in the diversion condition, such as the approved one-engine-inoperative speed. Check your own procedures.

More guides

• How to Calculate Crosswind and Headwind Components
• How to Read a METAR: A Plain-English Guide
• How to Read a TAF: Airport Forecasts Explained
• What Is in an ICAO Flight Plan? Every Item Explained
• The Wind Triangle Explained: Heading, Ground Speed and Drift
• FAA Part 117 Explained: Flight Time, Duty Period and Rest
• Flight Fuel Planning Explained: Trip, Contingency, Alternate and Reserve
• Density Altitude Explained: How Heat and Height Cut Performance
• How to File a Flight Plan: Steps for Pilots and Dispatchers
• How to Read a NOTAM: ICAO Format Explained
• EASA Flight Time Limitations: The Basics and How They Differ from FAA Part 117
• EDTO Explained: Extended Diversion Time Operations from Scratch
• Adequate vs Suitable Aerodrome: What Is the Difference?
• EDTO Approval and Dispatch Requirements Explained
• The North Atlantic Organised Track System (NAT OTS) Explained
• The NAT Track Message and TMI Number Explained
• NAT High Level Airspace (HLA) Requirements Explained
• Oceanic Clearance on the North Atlantic: How It Works
• NAT Contingency Procedures and SLOP Explained
• Polar Routes Explained: What Is Special About Flying Near the Poles
• Polar Operations Requirements Checklist (FAA)

For training and reference only. Check official sources before any real flight.