What is optimum cruise speed (Mopt) and what factors affect fuel burn?

• A. Mopt is the maximum endurance speed
• B. Mopt increases with altitude
• C. Mopt is the speed that minimizes fuel burn under given conditions; fuel burn varies with weight, ambient temperature, and winds
• D. Mopt is unrelated to weather

Answer: (C)

Optimum cruise speed is the airspeed (often expressed as Mach) that yields the lowest fuel burn per unit distance under the current flight conditions. This isn’t a fixed number; it depends on weight, ambient temperature, and winds because these factors change drag, engine efficiency, and ground speed.

Weight influences the lift required and the overall drag the airplane must overcome. Heavier aircraft burn more fuel to maintain altitude, so the speed that minimizes fuel per mile can shift as weight changes (and it also shifts during a flight as fuel is burned).

Ambient temperature affects air density and engine performance. Warmer air reduces density, which changes lift and drag characteristics and can alter where the fuel-per-distance minimum lies. Cooler air increases density and can improve engine efficiency, again affecting the optimum speed for fuel savings.

Winds change ground speed for a given airspeed. A headwind increases the fuel used per mile flown, while a tailwind decreases it. Because fuel burn per distance depends on the combination of airspeed and ground speed, the presence and direction of winds move the speed that minimizes fuel per mile.

Thus, the statement that matches the concept is that optimum cruise speed minimizes fuel burn under the current conditions, and fuel burn varies with weight, ambient temperature, and winds.