Flight Loads Testing – Design Criteria Review (EN)

This article corresponds to the Design Criteria Review stage in the Flight Loads Testing process, where engineers define the load basis and verification parameters for test planning.

1. Role and Importance of the Design Criteria

The Design Criteria Document underpins load, strength, fatigue, and aerodynamic performance evaluations. Depending on the aircraft type, it is based on FAA, FAR, JAA, JAR, or military MIL-Spec standards. All measured data validate the limit loads defined in this document.

2. Relationship with Aircraft Specifications

Aircraft specifications are developed based on FAA, FAR, JAA, JAR, and MIL-Spec requirements and are categorized as follows:

System Specification
Product Specification
Process Specification
Material Specification

The Development Specification includes the aircraft, training systems, and logistics documentation. For the aircraft itself, it consists of the General Airframe Specification and the Subsystem Specification. These establish the basis for the Mach–Alt Diagram and Nz–W Curve, essential for defining test load envelopes.

3. Understanding the Mach–Alt and Nz–W Diagrams

Flight load design involves two key relationships: Mach–Altitude (Mach–Alt) and Load Factor–Weight (Nz–W).

3-1. Mach–Alt Diagram: Key Design Speeds

V_LF: Limit speed in takeoff/landing configuration
V_A: Design maneuvering speed (gust/control input reference)
V_H: Maximum level flight speed
V_L: Limit speed (absolute boundary)

3-2. Nz–W Curve: Load Factor vs Weight

The Nz–W Curve defines the maximum design load factor (Nz) at specific gross weight. Generally:

Gross Weight ↑ → Nz ↓

Heavier aircraft have lower allowable load factors due to reduced structural margins.

3-3. Design Notes for Nz–W

For load analysis relative to BFDGW (Basic Flight Design Gross Weight):

If analysis is performed above BFDGW, the applied load factor must be smaller than the maximum design load factor.
If analysis is performed below BFDGW, the applied load factor cannot exceed the maximum load factor.

This ensures compliance since the Maximum Load Factor itself is a design criterion.

4. Development vs Operational Basis

During development: Design limit basis
After certification: Operational envelope basis

The first ensures safety margins; the second defines the actual flight envelope. Together, they confirm operational safety.

5. Summary and Next Post

Flight Loads Testing is the physical validation of the design logic. The Mach–Alt and Nz–W diagrams act as a visual framework defining where the aircraft operates safely. Understanding this is sufficient before moving to the next stage.

Next post: Aircraft Loads and Classification.

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