FAA Airline Transport Pilot (ATM)Transport Aerodynamics and High-Altitude OperationsHard
An airline's flight manual specifies a Long-Range Cruise (LRC) Mach number rather than the Mach number for maximum specific range (Mmr). LRC is defined as the Mach number that provides:
- AThe minimum drag speed for the current weight and altitude
- BThe absolute maximum specific range achievable at that weight and altitude
- CApproximately 99% of the maximum specific range, at a somewhat higher Mach, trading a small fuel penalty for reduced flight time
- DThe maximum true airspeed the airplane can achieve regardless of fuel consumption
Show answer & explanationAnswer & explanation
Correct answer: C. Approximately 99% of the maximum specific range, at a somewhat higher Mach, trading a small fuel penalty for reduced flight time
The specific range curve is relatively flat near its peak (Mmr), so flying slightly faster than Mmr only reduces specific range by about 1%, while noticeably reducing flight time. LRC is chosen as this favorable trade-off point, sacrificing a small amount of fuel efficiency for meaningfully faster cruise speed.
Why the other options are wrong
- A. Minimum drag speed corresponds to L/Dmax, a different specific speed than LRC.
- B. That describes Mmr (max specific range Mach), not LRC.
- D. LRC is not about maximum achievable speed; it's a fuel-efficiency-based cruise speed.
Long-Range Cruise (LRC) Mach
A cruise Mach number slightly higher than the Mach for maximum specific range (Mmr), chosen because it provides ~99% of maximum range while flying noticeably faster.
- Specific range curve is flat near its peak
- LRC trades ~1% range loss for reduced flight time
- Higher Mach than Mmr
- Common published cruise schedule in flight manuals
Memory trick: LRC: 'lose a little range, cut the ride short'