A turbojet transport is in cruise flight and steadily burning fuel, reducing its gross weight over time. Assuming a constant Mach number, how does the airplane's optimum cruise altitude typically change as weight decreases, and why?
- AOptimum altitude increases, because at lower weight the airplane can fly at a higher altitude while still operating near its best lift-to-drag ratio for maximum specific range
- BOptimum altitude remains constant regardless of weight, since it depends only on outside air temperature
- COptimum altitude decreases, because thinner air at higher altitude increases fuel consumption as weight is reduced
- DOptimum altitude decreases, because lower weight requires less lift and thus lower altitude for efficiency
Show answer & explanationAnswer & explanation
Correct answer: A. Optimum altitude increases, because at lower weight the airplane can fly at a higher altitude while still operating near its best lift-to-drag ratio for maximum specific range
As fuel burns off and weight decreases, less lift is required, so the airplane can climb to a higher altitude while still operating at its optimum angle of attack for best L/D and specific range. This is the basis for step climbs during long-range cruise, allowing the airplane to trade decreasing weight for increasing efficient altitude.
Why the other options are wrong
- B. Optimum altitude is a function of weight (and speed/temperature), not temperature alone.
- C. Higher altitude at lower weight typically improves, not worsens, fuel efficiency near optimum L/D.
- D. This reverses the actual relationship; lower weight allows higher, not lower, optimum altitude.
Optimum Cruise Altitude
The altitude at a given weight and Mach number that yields the best specific range (maximum distance per pound of fuel), which increases as weight decreases during cruise.
- Lower weight allows higher optimum altitude at constant Mach
- Basis for step-climb cruise procedures
- Optimum altitude corresponds to best lift-to-drag ratio
- Differs from maximum (service) ceiling, which is thrust-limited
Memory trick: Lighter plane, higher home — burn fuel, climb higher for best mileage