Cruise performance

Best range vs best endurance

Engine power and speed/range trade-off

Increasing engine power in cruise causes:

• Speed rises (more thrust → higher TAS).
• Range decreases — fuel burn rises disproportionately (drag curve is quadratic in IAS).

→ More power = faster, but shorter range. The pilot chooses power per mission: fast to destination (75 %) or maximum range (55–65 %).

Specific Fuel Consumption (SFC)

SFC = fuel per power per time (e.g. kg/kW·h). Piston engines have an optimum SFC at a specific RPM and mixture — see Lycoming/Continental Operator's Manuals.

Practical optimisation:

• Lean mixture in cruise above AFM threshold (typically 3 000–5 000 ft DA) — improves SFC substantially.
• Power setting per AFM cruise table (typically 55–75 %).
• Propeller at optimum RPM on CSU aircraft.

Mixture at altitude — fuel reduction

When flying at higher altitudes the mixture is leaned, to reduce required fuel:

• At altitude air density is lower → the standard rich mixture is too rich → fuel waste and poorer SFC.
• Leaning brings the air-fuel ratio back to optimum → minimum required fuel, best efficiency.

Source: Lycoming Operator's Manual; AFM/POH mixture procedure; FAA-H-8083-25 §7.

Carburettor engine — power loss with altitude

A carburetted engine (standard in most PPL trainers) loses power with increasing altitude, because:

• Air density falls → less oxygen mass per volume of intake air.
• At the same RPM, less air mass flows through the carburettor per second.
• Since combustion is mass-based, full-throttle torque and thus maximum power drops.

Rule of thumb: a normally-aspirated piston engine loses about 3 % maximum power per 1000 ft pressure altitude. A 180-HP engine at 8000 ft produces only about 140 HP at full power.

Consequence: climb performance decreases with altitude, cruise speed drops, service ceiling is reached (see climb-performance lesson).

Turbocharging offsets this effect up to a critical altitude (typically 10 000–18 000 ft, depending on turbo).

Variable-pitch propeller (variable pitch / constant speed) — advantage

A variable-pitch propeller (Variable Pitch / Constant Speed Unit, CSU) has, compared to a fixed-pitch propeller, the main advantage of higher efficiency in multiple flight phases — especially:

• On take-off: blade automatically at fine pitch → optimum thrust at low TAS.
• In cruise: blade automatically at coarse pitch → optimum angle for high TAS, lower engine RPM at same thrust → better SFC.

→ A fixed-pitch propeller is only optimal in a narrow speed range (either take-off or cruise), while a CSU can work near optimum in both phases. This is the main reason for CSU in high-performance GA aircraft.

Wind and cruise speed

In practice, most PPL students use the AFM cruise table at 65–75 % power — slightly faster than best range, more practical.