FAQ & Aviation Glossary

Great Circle Pro is a free, browser-based aviation mapping tool for plotting great circle routes, drawing wind-adjusted aircraft range circles driven by live NOAA GFS wind data, and running ETOPS/EDTO diversion analysis. No login, no download, no paywall — runs entirely in your browser.

Yes — completely free, no account required. Every feature including live wind data, ETOPS analysis, and the flight plan calculator is available without restriction.

12,174 airports worldwide, searchable by IATA or ICAO code. Source: OpenFlights dataset, covering commercial, regional, and general aviation airports globally.

The shortest path between two points on a sphere, equivalent to a straight line when looking at the Earth from space. On a Mercator map great circles appear as curves; on an orthographic globe projection they appear perfectly straight. Airlines fly great circle routes (or close approximations adjusted for wind) to minimise distance and fuel burn.

A great circle is the geodesically shortest path between two points. A rhumb line (loxodrome) crosses all meridians at a constant compass angle, making it easy to fly without constant heading changes, but it is always longer than the great circle except on east-west equatorial routes. Great Circle Pro shows the extra rhumb distance on every route card so you can see the cost of constant-heading navigation.

Using the Haversine formula with Earth radius 3,440.065 nm. Distance is shown simultaneously in nautical miles, statute miles, and kilometres on every route card. Initial bearing uses the forward azimuth formula computed from the origin and destination coordinates.

A range circle shows every destination reachable from a given airport within a specified number of nautical miles, computed along the Earth's curved surface. Great Circle Pro calculates a separate geodesic reach for each of 720 bearing directions, so the final shape reflects real atmospheric conditions rather than a simple circle.

Because the jet stream gives the aircraft very different ground speeds depending on direction of travel. On a typical departure from a mid-latitude hub like New York or London, an eastbound flight gets a substantial tailwind while a westbound flight fights the same jet stream as a headwind. Great Circle Pro applies live 250 hPa wind data across all 720 radial bearings — tailwinds stretch the ring outward, headwinds compress it inward. The result is an asymmetric envelope that accurately reflects where you can and cannot reach from that airport today.

Live NOAA GFS (Global Forecast System) — real-time 250 hPa wind data updated every 6 hours. Great Circle Pro samples 18 latitude bands from equator to pole, extracts the zonal (east-west) wind component for each, and builds a full latitude profile. This is the primary and default data source. The 250 hPa level corresponds to roughly 34,000 ft, the typical cruise altitude for commercial jets.

Wind strength is further modulated by your chosen percentile setting (50th, 85th, or 95th), which scales the live profile to represent different intensity levels. See the next question for details.

Percentile models express how often the wind speed at a given latitude is exceeded. They allow you to plan for different levels of wind intensity using the same live GFS data as a base profile:

50th percentile (Mean) — the median wind speed; conditions you would expect on an average flight. Useful for typical block time estimates and baseline route comparisons.

85th percentile (Boeing ETOPS Standard) — the wind speed exceeded only 15% of the time. This is the planning standard mandated by Boeing and referenced in FAA Advisory Circular AC 120-42B for ETOPS diversion analysis. An aircraft that can reach its diversion airport under 85th-percentile headwinds can do so on 85 out of 100 actual flights — the accepted threshold for oceanic route certification.

95th percentile (Worst-Case) — the wind speed exceeded only 5% of the time. This represents near-worst-case conditions and is used for stress-testing range margins, evaluating minimum fuel reserves, and understanding how thin an ETOPS buffer really is when conditions are adversarial.

When an engine fails over a remote oceanic area, the aircraft must divert at reduced One Engine Inoperative (OEI) speed — typically 330–390 kt depending on aircraft type — which can mean flying into a headwind for an extended period. ETOPS certification requires that the diversion remain feasible under statistically unfavourable wind conditions. The 85th percentile standard ensures the OEI radius shown on screen holds up on 85 out of 100 actual flights, giving operators and regulators confidence the margin is real and not just a calm-day estimate.

Great Circle Pro queries the Open-Meteo API, a free CORS-enabled weather service that proxies NOAA GFS data. The tool requests 250 hPa wind speed and direction at 18 latitude sample points, extracts a 6-hour mean zonal component for each, and assembles a latitude-indexed wind profile. This happens automatically when the tool loads or when you toggle wind on. No identifying information is sent in the request. If the live fetch fails, the tool falls back to a built-in climatological profile.

Payload is passengers, bags, and cargo only. Fuel is not payload — it is loaded separately and tracked through the aircraft's fuel system. This is the same convention Boeing and Airbus use in their Airport Planning documents. Great Circle Pro follows that standard: the Payload-Range diagram's Y-axis shows revenue payload only, never fuel.

Load factor is the percentage of maximum structural payload that is actually carried. At 85% load factor on a 737 MAX 8 (max payload 20,730 kg), the aircraft carries ~17,600 kg of passengers and bags. A lighter aircraft burns less fuel, which is why load factor affects range: lower load factor → less weight → less fuel burn → slightly more range. Great Circle Pro models this by interpolating along the published Payload-Range curve at the selected load factor point.

Maximum Zero Fuel Weight (MZFW) is the maximum allowable weight of the aircraft plus everything in it except fuel. It is a structural limit — exceeding it would overstress the wing root. On the Payload-Range diagram, MZFW defines the top of the payload axis (the green dashed horizontal line). Above this line is not physically possible regardless of fuel load.

The Fuel Tank Limit (the amber dashed vertical line on the Payload-Range chart) marks the range at which the aircraft's tanks are completely full. To the left of this line the aircraft is weight-limited — it could carry more fuel but is already at MZFW. To the right the tanks are full and the only way to fly further is to carry less payload, freeing up weight for that full tank of fuel. This is why the curve bends sharply downward at that point.

Point B is the OEM published range — the range figure Boeing or Airbus quotes in their marketing materials and spec sheets. It is typically flown at approximately 85% load factor with full fuel tanks. This is why 85% is the default load factor in Great Circle Pro and the Boeing standard for range planning. Point C (ferry range) represents zero payload with maximum fuel — the absolute maximum theoretical range.

Payload-Range curves are modelled from published OEM Airport Planning documents (Boeing, Airbus) and cross-referenced with publicly available performance data. Passenger weight uses the Boeing standard of 102 kg / 225 lbs per person including bags. Fuel burn uses published cruise fuel flow scaled by load factor and block time. These are planning estimates — not certified performance data and not a substitute for actual dispatch calculations.

Extended-range Twin-engine Operational Performance Standards (ICAO Annex 6 / FAA AC 120-42B). ETOPS governs how far a twin-engine airliner may operate from an adequate diversion airport at any point along its route. The equivalent ICAO term is EDTO (Extended Diversion Time Operations). Historically applied only to twins, modern ETOPS standards also increasingly apply to multi-engine aircraft on remote oceanic routes.

All standard ratings: 60, 90, 120, 138, 180, 207, 240, 330, and 370 minutes. The ETOPS sensitivity panel lets you compare all ratings simultaneously to see exactly which rating a route requires and by how much margin.

EEP (ETOPS Entry Point) — the first point along the route that lies beyond the OEI diversion range of any adequate alternate airport. This is where the aircraft officially enters the ETOPS segment of the flight. EXP (ETOPS Exit Point) — the point where the route re-enters coverage of at least one adequate alternate. The gap between EEP and EXP is the critical ETOPS exposure window.

One Engine Inoperative — the performance condition used to calculate the ETOPS diversion radius. At OEI, the aircraft flies at reduced thrust and typically a lower airspeed (330–390 kt depending on type), reducing its effective range per unit time compared to normal cruise. OEI speed is applied in the diversion time calculation: t = distance / V_OEI.