Why is flight distance different from a straight line on a map?

Great Circle Distance

Great Circle Distance is the shortest spherical arc between two points. The tool uses it for every adjacent airport segment, the route total and the direct distance from the first airport to the last airport, showing nautical miles and kilometres.

Actual flown distance

Aircraft operate along airway networks, air-traffic-control clearances and airport departure and arrival procedures. A flight may be longer than the great circle, or choose a different direction because of winds and traffic. A tracked flight is an observation of one operation, not a fixed geometric baseline.

ETOPS and diversion coverage

ETOPS (Extended Operations) concerns operational approval and planning conditions for applicable aircraft when a route moves far from suitable diversion airports. It involves alternates, weather, aircraft performance, operating specifications and approval; it is not a fixed circle on a map. FlightRouteMap's ETOPS overlay is geographic illustration only.

Winds, weather and rerouting

Headwinds and tailwinds change the time- or fuel-efficient choice. Thunderstorms, volcanic ash, icing, military areas, temporary restrictions and congestion can also move an actual route away from the great circle. The same airport pair can therefore have different flown distances on different days.

Three comparison examples

Long-haul routes make the distinction easier to see. These examples illustrate route structure rather than promising a particular day's flight mileage:

• LHR–HKG: high-altitude winds and airspace structure affect the operational route.
• JFK–NRT: North Pacific winds and routing arrangements can change the track.
• AKL–SCL: diversion planning, weather and Southern Hemisphere airspace matter on the route.

Keep three layers of distance separate

The first layer is the direct great-circle distance between airport coordinates. The second is the sum of the route list's great-circle segments. The third is the track flown by a particular flight on a particular day. The first two are stable and repeatable here; the third needs flight, weather and operational data and cannot be represented by one static percentage.

Why show nautical miles and kilometres

Nautical miles make the result easier to compare with aviation references and charts, while kilometres are familiar in everyday and cross-industry contexts. They are two units for the same geometric distance, not two different route assumptions.

How to read the detour ratio

Detour ratio = (sum of adjacent great-circle segments ÷ direct origin-to-destination great-circle distance − 1) × 100%. It answers how much longer the route list is than its direct geometric baseline, not how many miles a particular flight actually flew.