Range - ATPL 032 — Performance | Pilot Assessment Training

Specific fuel consumption

The engine's appetite is measured by its specific fuel consumption (SFC), and the definition splits by powerplant exactly as fuel flow did in the endurance lesson. A turbojet's SFC is the fuel flow per unit of thrust; a propeller powerplant's SFC is the fuel flow per unit of power. In both cases a lower SFC means less fuel burned for the same work, so the fuel flow falls and the specific range rises in direct proportion. SFC is the quality figure of an engine generation: it is why a modern turbofan crosses an ocean on fuel a 1960s jet would have burned halfway.

The best-range speeds

Maximum specific range means the best ratio of TAS to fuel flow, and the geometric trick for finding it is the tangent from the origin: on any curve of fuel flow against speed, the touch point of that tangent is the speed with the highest speed-per-fuel-flow ratio. What stands in for fuel flow differs by powerplant. For the jet, fuel flow follows thrust, which is drag, so the tangent is drawn to the drag curve and touches above the minimum-drag speed: for the classic drag polar, at about 1.32 times Vmd. For the propeller aeroplane, fuel flow follows power, so the tangent is drawn to the power-required curve, and a line from the origin touching that curve touches exactly at Vmd: power is drag times TAS, so maximising TAS over power is the same as minimising drag.

Weight and CG

More weight means more lift, more induced drag, and the whole drag and power picture lifted upward and shifted toward higher speeds. The consequences follow the fraction: fuel consumption rises, range falls, and the optimum speed for maximum SR moves higher. The centre of gravity plays the same tune more quietly: a forward CG demands more tail download, so the wing must lift more than the weight and carries extra induced and trim drag; fuel consumption rises and range falls. An aft CG within limits is the cheaper place to load.

What moves a jet's SFC

The syllabus asks two specific behaviours of the turbojet's SFC. It rises with temperature: a hot day makes every newton of thrust more expensive. And it varies with thrust setting: the engine is most efficient near the high thrust settings it was designed around, and its SFC deteriorates at low settings, which is one reason idling around the sky at low thrust is so unrewarding per kilogram.

Altitude and the weather

For the jet, altitude is the great range multiplier. Climb at a constant IAS and the TAS grows while the colder air trims the SFC, so the specific range improves with altitude up to an optimum in the region of the tropopause; push above it and compressibility drag and engine limits turn the trend around. Colder-than-ISA air improves a jet's SR for the same reason the climb did. The propeller aeroplane reads differently: its SR changes comparatively little with altitude (a piston engine does best near its full-throttle height), so the level for a prop cruise is chosen mostly for the wind: the meteorological conditions decide the SRG, and the SRG is what pays for the trip.

The optimum altitude and weight

The optimum altitude is not a property of the aeroplane alone but of the aeroplane at its current weight: the heavier the aeroplane, the lower the altitude at which its specific range peaks. As fuel burns off, the optimum drifts slowly upward, which is why a long jet cruise wants either a continuous gentle cruise-climb or the step climbs that approximate it under ATC. The class A cruise lesson returns to this with numbers.

Flying for ground miles: wind and SRG

Range over the ground is what the flight plan buys, and wind drives a wedge between SR and SRG. A headwind makes SRG smaller than SR; a tailwind makes it larger. The subtler exam point is what wind does to the optimum speed. Ground distance only accumulates at the ground speed, so the tangent construction must start not from zero but from the wind speed on the speed axis. Shift the origin into a headwind and the tangent touches the curve at a higher speed: fly somewhat faster into a headwind to spend less time losing ground to it. A tailwind shifts the origin the other way, and the optimum speed for SRG falls slightly below the still-air best-SR speed.