Performance Class A (CS-25): Cruise Theory

032-04-03Pablo Asensio Martínez2026-04-011 min

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.