How to Build a Defensible Load Case

1. The Engineering Task

Construct a load case that defines the applied forces, moments, pressures and temperatures, together with their source, uncertainty and traceability — such that the analysis can be defended against technical review.

2. When to Use This Method

Every structural analysis requires a defensible load case. For certification work, the load case must be formally documented and traceable to the loads report or regulation from which it was derived.

3. What You Need Before Starting

• The operating scenario: manoeuvre, gust, landing, pressurisation, thermal transient
• The applicable regulation or specification: CS-25, FAR-25, DEF STAN, DNV, customer specification
• The loads report or loads model output providing the raw force and moment data
• The factor of safety: 1.5 for aerospace ultimate, or as specified by the applicable standard

4. Step-by-Step Method

1. Identify the operating scenario and the source document for the loads
2. Extract the limit loads (forces, moments, pressures) at each interface location
3. Apply the factor of safety to obtain ultimate loads: P_ult = P_limit × 1.5 (or as specified)
4. Define the load direction and coordinate system for each applied load
5. Document the load source: report number, page, table, load case identifier
6. Identify the critical load combination: are there simultaneous loads (pressure + inertia + thermal)? Are they all at their maximum simultaneously, or is a combination factor appropriate?
7. Define the load envelope: for multiple load cases, identify the governing case for each location and failure mode
8. Record the uncertainty and conservatism: is the load case a mean, a maximum, or a probabilistic extreme?

5. What to Check

• Is the load case traceable to a source document?
• Has the factor of safety been applied correctly — limit for deformation, ultimate for strength?
• Are all simultaneous loads included, or has a load combination factor been applied with justification?
• Is the load envelope complete — have all locations and failure modes been checked against the governing case?

The 1.5 factor of safety is a regulatory minimum for aerospace. Some conditions require higher factors — always check the applicable specification. Limit loads must not cause permanent deformation; ultimate loads must not cause rupture.

6. How to Interpret the Result

A defensible load case is a documented set of forces, moments and pressures with clear traceability to the source, the factor of safety, and the combination rules. The analysis report should reference the load case by its unique identifier, and the loads report should be cross-referenced.

7. Common Mistakes

• Using limit loads for a strength check — strength requires ultimate loads
• Applying the factor of safety to loads that are already ultimate
• Omitting simultaneous loads because "they are in different directions" — they may still combine critically
• Not documenting the load source — an untraceable load case cannot be reviewed
• Assuming the maximum load case is the critical one for every location — different locations may be critical under different cases

8. Further Reading

See the Loads, Load Cases & Structural Environments and Loads & Load Cases Knowledge categories for loads theory. See How to Identify the Critical Load Case for envelope determination.