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Cruise Optimization and Limitations
Document Analysis: Altitude Optimization & Limitations
Getting to Grips with Aircraft Performance January 10, 2026
Getting to Grips with Aircraft Performance Cruise Performance January 10, 2026 1 / 11 Table of Contents 1 Altitude Optimization 2 Maximum Cruise Altitude 3 En route Maneuver Limits 4 Step Climb 5 FCOM Tables
Getting to Grips with Aircraft Performance Cruise Performance January 10, 2026 2 / 11 3.1. Optimum Cruise Altitude Basic Principle
When flying at a constant Mach number, for each weight, there is an altitude where the Specific Range (SR) is maximum.
This altitude is referred to as the "Optimum Altitude".
At optimum altitude, the aircraft operates at the maximum Lift-to-Drag ratio (CL/CD)
corresponding to the selected Mach.
Rule: As aircraft weight decreases (due to fuel burn), the optimum altitude increases. Relationship Formula Weight = constant Ps
(Ps: Outside static pressure at optimum altitude)
Getting to Grips with Aircraft Performance Cruise Performance January 10, 2026 3 / 11 Wind Influence
Wind (Headwind/Tailwind) affects the ground Specific Range (SR).
Wind force can vary at different altitudes. The Trade-off
Cruising lower than optimum altitude generally decreases aerodynamic SR.
However, if a lower altitude has a sufficiently favorable wind (Tailwind), the ground SR may improve.
Conclusion: Under such conditions, it may be more economical to cruise at a lower altitude.
Getting to Grips with Aircraft Performance Cruise Performance January 10, 2026 4 / 11 3.2. Maximum Cruise Altitude
Definition: The maximum altitude an aircraft can maintain at maximum cruise thrust for a given weight and fixed Mach number.
Dependent on: Weight and Temperature (ISA deviation). Temperature Influence Altitude Influence
As temperature increases → Maximum engine
The higher the altitude → The more thrust is
thrust decreases → Maximum altitude decreases.
required to maintain a given Mach number.
Getting to Grips with Aircraft Performance Cruise Performance January 10, 2026 5 / 11
3.3. En route Maneuver Limits (Buffet Onset)
Buffet Phenomenon occurs at two limits: 1
Low Speed Buffet (Stall):
As speed decreases, Angle of Attack (AoA) must increase to maintain lift.
Excessive AoA causes airflow separation → Buffeting and Stall. 2
High Speed Buffet (Compressibility):
At high speeds, shock waves form on the upper wing surface.
Airflow separates behind the shock wave → induces buffeting similar to low speed stall.
Getting to Grips with Aircraft Performance Cruise Performance January 10, 2026 6 / 11 Load Factor Limits (n)
In level straight flight: n = 1g.
During turns, load factor increases: n = 1 . cos(bank angle)
1.3g Operating Limitation (Buffet Margin)
To ensure safety during turbulence or maneuvering:
An acceptable load factor limit is generally set to 1.3g.
1.3g corresponds to a bank angle of approximately 40◦.
The altitude where 1.3g can be sustained without buffeting is called the 1.3g Buffet Limited
Altitude (or Buffet Ceiling).
Getting to Grips with Aircraft Performance Cruise Performance January 10, 2026 7 / 11 Maximum Recommended Altitude
The FMGS calculates the maximum recommended altitude based on the lowest of the following four values: 1
Maximum Certified Altitude: Structural design limit. 2
Maximum Cruise Altitude: Thrust limited. 3
1.3g Buffet Limited Altitude: Aerodynamic safety limit. 4
Climb Ceiling: Performance limit.
Getting to Grips with Aircraft Performance Cruise Performance January 10, 2026 8 / 11
3.4. Cruise Optimization: Step Climb
Ideal cruise follows the optimum altitude (which rises as weight drops). Efficiency
Reality (ATC): Flights must maintain constant flight
With proper step climb profiles, Specific levels. Range (SR) achieves: Method: Step Climb.
Level segments are typically flown within ±2, 000 ft of SR ≥ 99%SRmax the optimum altitude.
Note: Steps are usually 2,000 ft (in RVSM areas) or 4,000 ft.
Getting to Grips with Aircraft Performance Cruise Performance January 10, 2026 9 / 11 4. FCOM Cruise Tables
FCOM provides tables to determine cruise performance. Input Parameters:
Mach Number (e.g., M.78, M.80...). Weight. Flight Level (FL). Temperature (ISA deviations). Output Data: N1 (%) - Engine rating. Fuel Flow (KG/H/ENG). Specific Range (NM/1000KG).
Getting to Grips with Aircraft Performance Cruise Performance January 10, 2026 10 / 11 Summary Key Takeaways
Cruising at Optimum Altitude maximizes fuel efficiency.
As weight decreases → Optimum Altitude increases.
Consider Wind when opting for altitudes lower than optimum.
Always stay below the 1.3g Buffet Ceiling for aerodynamic safety.
Step Climb technique maintains near-maximum efficiency (≥ 99%) in controlled airspace.
Getting to Grips with Aircraft Performance Cruise Performance January 10, 2026 11 / 11
Document Outline

• Altitude Optimization
• Maximum Cruise Altitude
• En route Maneuver Limits
• Step Climb
• FCOM Tables