FAA Airline Transport Pilot (ATM)Transport Aerodynamics and High-Altitude OperationsMedium
A transport jet departs at a gross weight requiring an optimum cruise altitude of FL330. After three hours of cruise, fuel burn has significantly reduced the airplane's gross weight. For maximum specific range at this new, lighter weight, the optimum cruise altitude will:
- ADecrease, since a lower weight requires a lower altitude to maintain the same lift
- BIncrease, since the lighter weight allows the same optimum lift coefficient to be flown at a higher, thinner-air altitude
- CBecome undefined until the airplane reaches its landing weight
- DRemain the same regardless of weight change
Show answer & explanationAnswer & explanation
Correct answer: B. Increase, since the lighter weight allows the same optimum lift coefficient to be flown at a higher, thinner-air altitude
For maximum specific range, the airplane is normally flown near the AOA/lift coefficient giving best L/D at a given Mach. As fuel burns off and weight decreases, less lift is required; to maintain the same optimum CL and Mach, the airplane must fly in thinner air, i.e., at a higher altitude. This is why crews request 'step climbs' as the flight progresses.
Why the other options are wrong
- A. This reverses the correct relationship; lighter weight allows a higher, not lower, optimum altitude.
- C. Optimum altitude is calculable at any weight, not just at landing weight.
- D. Optimum altitude is weight-dependent, not fixed.
Step Climb / Optimum Altitude
As fuel burn reduces gross weight during cruise, the optimum altitude for maximum specific range increases, prompting crews to request step climbs to progressively higher flight levels.
- Optimum altitude rises as weight decreases
- Based on maintaining optimum lift coefficient/Mach
- ATC step climbs implement this in practice
- Improves fuel efficiency over the flight
Memory trick: Lighter jet = higher home — climb as you burn fuel