Usable range: why the brochure figure is not the one that matters

Every aircraft type has a range in its brochure, and every route page on Overfly asks whether the aircraft can make it. The two are not the same question. The brochure range is measured at long-range cruise, with a reference load, in still air, landing with whatever reserve the manufacturer assumed. A real flight has to land with fuel to spare, may be carrying more, and almost never has still air.

Reserves

An airliner does not plan to arrive with empty tanks. Rules built on ICAO's typically ask for contingency fuel (a few percent of the trip), fuel to fly to an alternate airport, and a final reserve, for a turbine aircraft 30 minutes' holding. Business aviation has its own conventions, commonly expressed as a distance or as a set of these figures for a typical alternate.

Overfly turns this into one distance subtracted from book range: 250 nm for the airliners in its list and 200 nm for the others. That is Overfly's convention, not any regulation's, and the RESERVE figure in the mapper is yours to change. What is left, book range less reserve, is usable range, and it is what every leg is checked against.

So the Gulfstream G650ER, quoted at 7,500 nm, is checked against 7,300 nm, and the Boeing 777-300ER's 7,370 nm becomes 7,120 nm. Sydney to Los Angeles, 6,512 nm, clears the G650ER at 89% of its usable range and the 777-300ER at 91%.

Payload

A brochure range is quoted at a reference payload. Load more, and once the aircraft is at its maximum take-off weight the extra is paid for with fuel left behind, so range falls. How fast depends on the type's payload-range curve, which is in its flight manual and not on any public page.

Overfly does not invent that curve. In the mapper you can enter passengers and a weight each, and a gradient in nautical miles per 1,000 lb from your own manual; with no gradient entered, payload changes nothing. It is the same principle as the reserve: a figure the operator knows, entered by the operator, not a guess presented as the aircraft's physics.

Wind: ground distance against air distance

The subtle one is wind. A range is a distance through the air. Flying into a headwind, the aircraft covers more air than ground: it spends longer aloft, burning fuel the whole time, for the same distance over the earth.

Los Angeles to Sydney is 6,512 nm over the ground. In May's average upper wind at FL410 there is a headwind component of about 34 kt along the whole route, and a business jet at Mach 0.85 flies 7,004 nm through the air to get there, 492 nm more. Against the G650ER's 7,300 nm usable range that is 96%, not the 89% still air suggests; a stronger day than average, or a lower cruise, and the leg stops closing.

This is why the mapper, with seasonal winds switched on, judges every leg and every candidate tech stop on air distance. A stop that clears on a still-air map can fail into a headwind, and Overfly will not offer it. The route pages show the month-by-month effect for each listed route.

What Overfly does not model

Overfly is for reference and planning interest, not operational planning. It does not model step climbs, temperature, the fuel cost of a missed approach at the destination, or each type's real cruise schedule. Its ranges are the manufacturers' published figures where there are any, checked against other references, which the attributions page lists. A real flight plan starts from the aircraft's own performance data.