A dispatcher is optimizing a jet transport's cruise speed schedule to maximize fuel mileage (nautical air miles per pound of fuel burned) at a fixed cruise altitude. Which speed condition typically produces the maximum specific range for a turbojet at that altitude?
- AThe minimum speed at which the autopilot can maintain altitude hold
- BThe speed corresponding to best angle-of-climb speed (Vx)
- CThe speed for maximum lift-to-drag ratio, adjusted slightly faster for compressibility effects (long-range cruise speed)
- DThe maximum structural cruising speed (VMO/MMO)
Show answer & explanationAnswer & explanation
Correct answer: C. The speed for maximum lift-to-drag ratio, adjusted slightly faster for compressibility effects (long-range cruise speed)
For jet aircraft, maximum specific range occurs near the speed for maximum L/D, but because fuel flow for jets does not increase as sharply with speed as for props, the practical Long-Range Cruise (LRC) speed is set slightly higher than max L/D speed to gain a small time savings for only about a 1% penalty in specific range.
Why the other options are wrong
- A. Minimum controllable speed for autopilot altitude hold is unrelated to fuel efficiency optimization.
- B. Vx is a climb speed for obstacle clearance, not a cruise fuel-efficiency speed.
- D. VMO/MMO is a structural limit, far above the speed for best fuel mileage.
Specific Range Optimization
Specific range (NAM per lb of fuel) is maximized near the speed for maximum lift-to-drag ratio; jets typically cruise slightly faster than this at 'Long-Range Cruise' speed for a small time benefit at minimal fuel penalty.
- Specific range = nautical air miles flown per pound of fuel burned.
- Maximum specific range occurs near max L/D speed.
- Long-Range Cruise (LRC) speed trades ~1% range loss for faster cruise speed.
- Weight, altitude, and temperature all affect the optimum speed schedule.
Memory trick: Fly near best L/D, then nudge faster for 'Long Range Cruise' bonus speed.