Performance Class A (CS-25): Cruise Theory
Cruise Strategies
Maximum Range Cruise (MRC)
- Speed: Corresponds to $1.32 V_{MD}$ (for jets).
- Objective: Maximum distance per kg of fuel.
- Disadvantage: Speed can be slow, sensitive to speed instability.
Long Range Cruise (LRC)
- Speed: Typically 4% faster than MRC (99% of max specific range).
- Objective: Provides 99% of the range of MRC but with significantly better speed stability and higher block speed.
- Historical standard before Cost Index.
Cost Index (CI)
- Modern Flight Management Systems (FMS) optimize speed based on the ratio of Time Cost to Fuel Cost. $ CI = \frac{\text{Cost of Time (\$ per hour)}}{\text{Cost of Fuel (\$ per kg)}} $
- CI = 0: Minimizing fuel cost (Max Range). Speed close to MRC.
- CI = Max: Minimizing time (Max Speed). Flying at $V_{MO}/M_{MO}$.
Buffet Margins
At high altitudes, the speed range is limited:
- Low Speed Buffet: Caused by stall onset (high Angle of Attack). Occurs at $V_{S1g} \times \text{load factor}$.
- High Speed Buffet: Caused by shock waves (Mach effects).
- Load Factor: Turning increases load factor (g), which increases stall speed, bringing the buffets closer.
Operational Requirement: A minimum margin of 1.3g load factor protection to buffet onset is usually required for cruise altitude selection. This ensures the aircraft can bank (turn) without stalling or overspeeding.