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 ERA5-derived seasonal 250 hPa wind grids, and running ETOPS/EDTO diversion analysis. No login, no download, no paywall, runs entirely in your browser.

Yes, completely free, and nothing to pay ever. Routes, range circles, ETOPS analysis, and the flight plan calculator work without an account; a free email sign-in (no password) unlocks the full aircraft fleet, the wind modeling toggles, and the Tailwind credit style.

More than 12,000 airport records are bundled in the browser database, searchable by IATA or ICAO code. The dataset covers commercial, regional, and general aviation airports globally.

The labelled "Style" tab on the right edge of the map opens a visual customisation drawer. Inside: nine theme presets including Cupertino, the familiar phone-map look with real shaded relief, a Topography underlay that works on any theme, custom colour pickers for ocean / land / borders / graticule, label size and glow controls, country highlighting (auto-paints countries containing route endpoints), gradient strokes (origin colour → destination colour along each route), distance and duration labels at route midpoints, aircraft glyphs at midpoints, hub auto-treatment (airports touching three or more routes render larger with an outer ring), parallel-route fan-out (so reciprocal pairs like LAX-NRT and NRT-LAX don't stack on top of each other), smart label collision avoidance, and a numbered route legend overlay. All settings persist in your browser.

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 samples 250 hPa wind data across all 720 radial bearings. Tailwinds can stretch the ring outward, while headwinds compress it inward. The result is an asymmetric planning envelope that reflects where you can and cannot reach under the selected season and percentile.

The Airspace toggle on the Flight Plan tool decides whether routes draw straight through restricted overflight regions or route around them.

Off (the default): the tool draws the pure great circle, the geometric shortest path. This is the right view when you want the geographic ideal, for studying network design, comparing routes airline-to-airline at the planning level, or modelling a future scenario where current restrictions might be lifted.

On: any route whose great circle would cross a closed region (Russia, Belarus, Ukraine FIR, Libya, Yemen, Sudan, North Korea, Lebanon, Kuwait, and western Iran) detours around that region along its boundary. The tool then recalculates block time, fuel, range check, and emissions against the longer flown arc, not the great-circle haversine. The detour is highlighted on the route card with an amber chip listing the regions avoided and the extra distance added. Some zones carry an altitude floor, so eastern Iran, open only above FL285, is avoided for a turboprop but overflown by a jet; and caution zones such as Iraq, Israel, Syria, and Afghanistan are drawn and flagged but not automatically detoured.

The polygons are curated against two authoritative public sources: the EASA Conflict Zone Information Bulletins (CZIBs) issued by the European Union Aviation Safety Agency, and the FAA Special Federal Aviation Regulations (SFARs) issued by the United States Federal Aviation Administration. Each polygon entry carries an effective-from date and the originating bulletin or regulation reference, so you can trace any region back to its source.

The list is reviewed quarterly. Airspace closures move slowly: most current entries (Russia, Belarus, Ukraine FIR) trace back to 2021 or 2022 and have not changed status since. When new regions are added or existing regions are lifted, the data file is updated, the page cache buster is bumped, and the change propagates to every user on next load.

The polygons are simplified country and FIR outlines, sufficient for route-planning fidelity at the long-haul scale this tool models. They are not exact flight-management-system airway boundaries. For operational dispatch use, refer to current NOTAMs and your operator's published procedures.

When a route's great circle would cross a closed region, the tool routes around it the most efficient way. It weighs both the polar (northern) and the southern way around and draws whichever gives the shorter clean path, while keeping a safe operational margin, roughly 45 nm, clear of the boundary rather than hugging the edge.

Where a real-world airline routing is known for that city pair, the tool uses that filed routing as the default. For pairs without a published routing it computes an efficient estimate, clearly labelled as an estimate so you always know which you are looking at.

It also handles the compound cases on its own, such as a European route that must clear both Russia and Belarus, so you do not have to reason through which regions a leg happens to skirt.

Block time, fuel, range check, and emissions all consume the detoured distance, not the great-circle haversine, so a route that picks up several hundred nautical miles of detour also picks up the proportional extra fuel burn and CO2. The route card lets you compare the detour against the plain great circle, so you can see exactly what the closure costs.

Great Circle Pro supports three season-based wind models, selected from the wind buttons on the Flight Plan and Range Circle tabs. Each model is an ERA5-derived 250 hPa wind grid, so the calculations use both longitude and latitude rather than a simple latitude profile.

Winter and Summer apply distinct upper-air wind models calibrated to represent the typical conditions at long-haul cruise altitude in each season. Winter applies a stronger, equatorward-displaced jet-stream pattern; Summer applies the weakened, poleward-shifted pattern. The season you click is the season that drives the math.

Average blends the two into a single year-round baseline useful for annual comparisons and route studies that should not favour either hemisphere's peak season.

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

Percentile models express how often the wind speed at a given point on the grid is exceeded. They allow you to plan for different levels of wind intensity using the same seasonal wind grid as a base:

50th percentile (Mean), the default on the Flight Plan and Range Circle tabs. The seasonal average wind; conditions you would expect on a typical flight. This is the dispatcher's normal-ops view and the right baseline for block-time estimates, network analysis, and day-to-day route planning.

85th percentile, the wind speed exceeded only 15% of the time. This is the conservative adverse-wind view used when you want margin for range or diversion analysis. It is especially useful for ETOPS-style oceanic planning because it tests the route against stronger-than-average headwinds rather than median conditions.

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. Great Circle Pro uses the 85th percentile on the ETOPS sensitivity panel so the displayed OEI radius reflects an adverse-wind case rather than a calm-day estimate.

Payload is passengers, bags, and cargo, anything you are paid to carry. Fuel is not payload; it is loaded separately and tracked through the aircraft's fuel system. The Payload-Range diagram's Y-axis shows revenue payload only, never fuel.

One practical wrinkle worth knowing, Boeing and Airbus publish their Payload-Range curves at passengers plus baggage only. Neither manufacturer assumes belly cargo in the headline "up to X nm" range figure. Great Circle Pro mirrors that convention so the on-screen ring matches the brochure number out of the box:

• Boeing and Airbus passenger aircraft default with cargo OFF. Both manufacturers anchor the brochure typical-payload point at passengers + bags only. Toggling cargo ON adds an aircraft-specific belly load (about 2 t on a narrowbody, 10-15 t on a widebody) above that baseline.
• Freighters force cargo ON and lock the toggle. On a freighter, cargo IS the payload axis; there is no separate pax slider.

Both Boeing and Airbus widebodies have substantial belly cargo capacity, the brochure ranges just don't include it. If you want an apples-to-apples comparison (e.g. a 777-300ER carrying the same belly cargo as an A350-1000), toggle cargo ON for both and the model adds each airframe's typical belly load. The Payload-Range curve, range estimate, and CO₂ figures update live.

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 an aircraft maker quotes in marketing materials and spec sheets. Great Circle Pro stores a per-aircraft typical configuration so the default ring matches that brochure point rather than forcing one manufacturer-wide load factor rule. Passenger aircraft default to passengers plus bags with cargo OFF; turning cargo ON adds the aircraft-specific belly payload and shortens the ring. Ultra-long-range, corporate, and freighter variants use their own per-aircraft overrides so the default still matches the published mission. The slider stays adjustable: drag it up to load more payload and shorten range, or down to fly farther toward ferry. Point C is zero payload with maximum fuel, the absolute theoretical maximum.

Payload-Range curves are modelled from published OEM Airport Planning documents (Boeing, Airbus) and cross-referenced with publicly available performance data. Passenger weight uses each manufacturer's own convention: Airbus aircraft use 95 kg per passenger including baggage (ACAP standard); Boeing aircraft use 102 kg / 225 lbs (APD standard). 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.

Yes. An aircraft is heaviest at takeoff and burns off weight every mile it flies, so it burns more fuel per mile early in a flight and less per mile late. This is the classic fuel carries fuel effect, described by the Breguet range equation, the most accurate way to tie fuel to weight and distance. Great Circle Pro uses it, so a shorter, lighter leg correctly shows less fuel and CO₂ than a single flat burn rate would. To be precise, the airplane always burns its real amount; it is the flat rate that over-counts the fuel on anything short of a full-range flight. Every aircraft's published maximum range is unchanged. See the interactive chart in Fuel Carries Fuel: The Breguet Effect.

Both figures are passenger plus baggage, they just use different reference standards. Airbus's ACAP (Aircraft Characteristics for Airport Planning) documents quote 95 kg per passenger including bags; Boeing's Airport Planning documents quote 102 kg (225 lb) per passenger including bags. That ~7 kg per-passenger weight gap is the systematic difference between the two brochure conventions. Both manufacturers publish their "up to X nm" range figures at passengers + bags only. Neither includes belly cargo in the headline number. Once the per-passenger weight is normalised, the brochure ranges between the two manufacturers are directly comparable. Great Circle Pro defaults cargo OFF on every passenger aircraft so the on-screen ring matches the brochure number out of the box, regardless of manufacturer.

It controls whether revenue belly cargo is added to the passenger-and-baggage payload before the range is read off the Payload-Range curve. With cargo OFF (the default for all passenger aircraft), only passengers + bags. With cargo ON, the curve uses passengers + bags + an aircraft-specific belly cargo number (about 2 t on a narrowbody, 10-15 t on a widebody).

The default is now OFF for every passenger aircraft, Boeing and Airbus alike:

• Boeing and Airbus passenger aircraft → OFF by default. Both manufacturers publish their brochure "up to X nm" ranges at passengers + bags only, with no belly cargo assumed. Leaving cargo off matches the brochure number out of the box.
• Regional / GA aircraft → OFF by default (no meaningful belly cargo in standard ops).
• Freighters → ON and locked (cargo IS the payload axis on a freighter; there is nothing to toggle).

Turn cargo ON to add the aircraft's typical belly-cargo allowance and see how the ring shortens, or leave OFF to match the brochure baseline. The toggle stays in sync between the Flight Plan and Range tabs and your choice persists across aircraft selections within a session.

Because the brochure ranges published by Boeing and Airbus are passengers + bags only, with no belly cargo assumed. When you toggle cargo ON you add ~2 t (narrowbody) or ~10-15 t (widebody) of belly cargo to the payload, which moves the operating point right along the Payload-Range diagram toward Max Structural Payload, and that means less range. The aircraft is heavier, so it burns more fuel for the same distance, so the published curve gives it fewer nautical miles.

This is not a bug; it is the diagram working correctly. Leave the toggle OFF (the default) to match the published brochure number, or turn ON to model a fully-loaded mission with revenue cargo. The same direction applies to both Boeing and Airbus airframes. Cargo ON always shortens the ring, since neither manufacturer's brochure assumed belly freight.

The per-ring derating breakdown line on each ring card spells this out explicitly: it shows the brochure number, the LF/cargo curve delta, any reserves credit, the hot-high penalty, and the resulting still-air nm, so you can see exactly where the range went.

Ultra-Long-Range variants are physically designed for sparse, premium-only cabins. The Passenger & Cargo Payload slider represents passengers as a fraction of the airframe's high-density single-class certification max (440 seats on an A350). The A350-900ULR's real Singapore Airlines SQ22 (SIN-EWR) 2-class configuration carries 161 premium seats, that's 161 / 440 ≈ 37%. Loading the ULR closer to its high-density max would mean filling it with 374 passengers (mathematically possible on the airframe but operationally absurd for a ULR mission).

Great Circle Pro snaps the slider to each aircraft's brochure typical config on selection, which differs significantly across the fleet:

• 737 MAX 8 → 77% (162 passengers 2-class)
• A320neo → 85% (165 passengers 2-class)
• 787-9 → 73% (296 passengers 2-class)
• A330-900neo → 62% (287 passengers 3-class)
• A350-900 → 74% (325 passengers 3-class)
• A350-900ULR → 37% (161 passengers 2-class, SQ22 config)
• A350-1000 → 76% (366 passengers 3-class)
• 777-9 → 82% (349 passengers 2-class)
• ACJ350-900 / ACJ350-1000 → 6% / 5% (25-passenger corporate fit-out)

The green OEM tick marks the brochure typical config on the slider track, and the cell shows whether you are at, above, or below it (the delta chip on the right of the headline reads "= OEM", "+9 OEM", "-12 OEM", etc.). The ↻ OEM button snaps the slider back to typical in one click. The footnote under the cell names the cabin convention each aircraft was brochured against (2-class typical, 3-class typical, corporate cabin, freighter, charter, GA).

It is the single payload control for both the Flight Plan and Range Circle tabs. The slider position is a percentage of the airframe's high-density single-class certification max (e.g. 210 on the 737 MAX 8, 440 on the A350-900). The headline pax count and the kg total update live as you drag, so the equation row reads "162 passengers + 0 t cargo = 16.5 t total" at the brochure typical config and grows from there.

Combined with the Cargo toggle, this gives you one knob for the full payload picture: slide for passengers, click for belly cargo. Both tabs share the same slider value so you can switch between Range and FP without the two views getting out of sync.

Internally the slider sets two globals (loadFactor for the route fuel/time math and ringPayload for the range-ring curve scaling) to the same number. Freighters disable the slider entirely since cargo is the payload axis on a freighter; the cell collapses to a "Cargo IS the payload. Slider disabled." footnote.

It is the one-line audit trail that shows how the headline brochure range became the on-screen ring. The card under each ring lists each adjustment in sequence: brochure number (e.g. 3500 nm for the 737 MAX 8) → curve scaling for LF / cargo (the curve gives more range under brochure typical, less above) → reserves credit (if Reserves is OFF, the brochure-baked reserves are credited back) → hot-high penalty (if the airport is hot or high) → resulting still-air nm. Wind effects are added geometrically by windRing3D after this still-air number is computed, so they show as the egg-shape on the map rather than as a breakdown line.

The point of the breakdown is so you can always answer "where did my range go?" without guessing. If the on-screen ring looks smaller than the dropdown headline, the breakdown shows whether it was cargo, hot-high, LF above brochure typical, or something else.

Reserves (default ON): Boeing and Airbus brochure ranges already include ICAO dispatch reserves (taxi + 5% contingency + 200 nm alternate + 30 min final reserve). With Reserves ON, the ring is the brochure number as-is, no math change. With Reserves OFF, those baked-in reserves are credited back to show the gross still-air ferry capability hidden inside the brochure number, so the ring grows. The card shows "+X nm reserves credit" in green when Reserves is OFF.

Step-climb (default OFF): When OFF, the curve uses single-altitude cruise burn. Turning it ON models the 1.5-4% fuel savings of a stepped cruise profile (FL340 → FL360 → FL380 as the aircraft burns down its takeoff weight), which extends the ring for long-haul flights. Short hops (under 2 hours) see no change; 8 hour flights see about 3%, and flights of 12 hours or more see the full 4%. Step-climb is a global aircraft assumption shared between FP and Range tabs.

Both toggles default to the brochure baseline (Reserves ON, Step-climb OFF) so the on-screen ring matches the published headline number out of the box. Turning either changes a downstream multiplier; the breakdown line shows the resulting delta.

Yes, but only up to the aircraft's structural ferry max. The Range tab's nm input is a starting headline; the actual ring is clamped to maxRange from PR_DATA (e.g. 3700 nm on the 737 MAX 8, 11,100 nm on the A350-900ULR). Anything beyond that would be unphysical regardless of how the slider, cargo, and reserves toggles are set.

The clamp is re-applied after the reserves credit-back and hot-high penalty pass, so even with Reserves OFF you cannot manually drive a ring past the structural ferry limit. The card flags the ring with a "beyond range" indicator in the Ring list when this happens, so it is visually obvious that the airframe envelope is the binding constraint, not the input number.

Yes. The Tailwind credit toggle controls whether favorable wind is credited to the range envelope. With Tailwind credit Off, the conservative default, headwinds still dent the upwind side but tailwinds do not extend the downwind side beyond the still-air reach. With Tailwind credit On, favorable winds can stretch the downwind side into a more directional egg. This is a planning display choice, not a claim that one airframer has a universal official convention. Only the wind shaping changes; the aircraft, payload, reserves, hot-day allowance, and airways allowance are identical either way.

Either setting also folds in two further real-world allowances, matching how the airframers plan. A hot-day temperature allowance shortens range on warm days (thinner air means more climb fuel and a lower efficient cruise altitude), with a larger effect in the tropics than near the poles. A routing allowance (the airframers' airways and traffic allowance) accounts for filed routes never being the exact great circle. Each shows as its own line in the ring card's "where did my range go?" breakdown.

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.

The sensitivity panel supports the ratings used by the tool's aircraft database: 60, 120, 180, 207, 240, 285, 330, and 370 minutes. The panel lets you compare the available ratings simultaneously to see exactly which rating a route requires and by how much margin. Buttons above the selected aircraft's certified ceiling are greyed out.

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.

No. The ETOPS sensitivity table is hardcoded to the 85th percentile regardless of what the Flight Plan / Range Circle tab is set to. This is intentional: ETOPS is a conservative diversion-coverage calculation, not a display view. The table keeps the adverse-wind assumption even when you have switched the FP tab to the 50th percentile normal-ops view.

The wind note above the sensitivity table is always shown so it is unambiguous which percentile drove the radii. If the FP tab were the source of truth here, switching the FP tab to 50th would silently downgrade ETOPS to no-wind-adjustment, which is the opposite of conservative diversion planning. The hardcode prevents that quietly-wrong answer.

With the Wind sensitivity on, the diversion radius applies a conservative adverse-wind margin derived from the wind statistics along the route (the default verdict basis is regulatory still-air ISA). The optional Tailwind credit mode goes further: it route-integrates the seasonal 250 hPa ERA5 grid per alternate, so coverage becomes directional instead of a single scalar.

The greyed-out buttons are ratings the selected aircraft is not certified for. Each airframe in Great Circle Pro carries an etopsMax ceiling (e.g. the 737 MAX 8 is currently capped at ETOPS-180, while widebodies like the 787-9, 777-9, A350-900, and A350-1000 are certified to ETOPS-330 or ETOPS-370). The sensitivity table shows all standard ratings, but only the ones the type can legally fly are clickable.

If you select a higher rating than the aircraft is certified for, the row is marked "Not certified" and excluded from the "Minimum Required Rating" callout. The greyed buttons therefore both communicate the certification ceiling AND prevent you from selecting an unattainable rating by mistake.

Certification ceilings change as manufacturers and operators win new approvals; the per-aircraft etopsMax values in the tool are updated as those approvals publish.

Yes. Coverage is two separate tests. First, is the alternate within one engine inoperative diversion range for the selected ETOPS rating and aircraft. Second, does it have a suitable paved runway long enough for that specific aircraft to land. An alternate only counts as covered when it passes both. A field that is in range but whose longest runway is too short for your aircraft, or that is unpaved, is shown as reachable but not usable rather than good coverage, and every alternate displays its longest runway against the length your aircraft needs. These are planning estimates, not a dispatch release.