1 — Setting Cruise Power
Once you level off, the climb is over and the aeroplane settles into the cruise — the longest phase of most flights, and the one your fuel planning lives or dies on. You choose a power setting and the aircraft trades that power for speed.
On a typical fixed-pitch SEP you set power with the throttle, reading RPM on the tachometer. With a constant-speed propeller you set manifold pressure (the throttle) and RPM (the prop lever) together. Either way the POH cruise tables express the result as a percentage of maximum power — most light singles are cruised between 55% and 75%.
2 — Why True Airspeed Grows With Height
Your airspeed indicator reads indicated airspeed (IAS) — really a measure of dynamic pressure. Up high the air is thinner, so the same IAS slips through fewer air molecules: the aircraft is actually moving faster through the air than the dial shows. That real speed is true airspeed (TAS).
A useful rule of thumb: TAS is about 2% higher than IAS for every 1 000 ft of altitude. At 10 000 ft, 100 kt IAS is roughly 120 kt TAS — free speed for the same indicated number. This is a big reason cross-countries are often flown higher.
3 — Specific Range: Distance Per Litre
For planning, the number that matters is specific range — how far you travel per unit of fuel (NM per litre). Plot it against TAS and you get a dome: too slow and the wing is inefficient (high induced drag); too fast and parasite drag eats the fuel.
The peak is the best-range speed. Flying faster trades a lot of fuel for a little speed.
Two speeds sit on this curve. Maximum endurance speed is the slowest — it burns the least fuel per hour, so it keeps you airborne the longest (think holding, or stretching time aloft). Maximum range speed is a little faster — it gives the most distance per litre, the peak of the dome. They answer different questions: most time vs most distance.
4 — The Speed-vs-Fuel Trade-off
Pull the sliders: raise the power setting and watch TAS climb a little while fuel flow climbs a lot — endurance and specific range both fall. Climb to altitude and TAS rises for the same fuel flow.
Assumes 120 L usable fuel (no reserve). Illustrative SEP — your POH is the only source that counts.
5 — Wind, and a Worked Example
Specific range is measured in still air. The wind decides your range over the ground. A headwind shortens your ground range; a tailwind stretches it. The counter-intuitive fix: into a strong headwind you should fly a little faster than best-range speed (you spend less time being held back); with a tailwind, slow down and let the wind do the work.
Worked example. Cruising at 8 000 ft, 110 kt IAS, 30 kt headwind, 90 L usable fuel, fuel flow 30 L/hr:
- TAS = 110 × (1 + 0.02 × 8) = 110 × 1.16 = 128 kt.
- Groundspeed = 128 − 30 headwind = 98 kt.
- Endurance = 90 ÷ 30 = 3.0 hours.
- Range over the ground = 98 × 3.0 = 294 NM (still-air range would be 128 × 3.0 = 384 NM — the headwind cost 90 NM).