Limit Load, Ultimate Load and Factor of Safety


The limit load refers to the maximum load that an aircraft structure is expected to encounter during normal service. It encompasses the various forces acting upon the aircraft during flight, including those due to lift, drag, thrust, and weight. These forces are typically encountered in level flight conditions. However, the limit load must also account for additional loads experienced during maneuvers and when flying in turbulent air. Essentially, the limit load sets the upper boundary for the expected loads that the aircraft structure should withstand during its operational life.


The ultimate load represents the failing load of the aircraft structure. It is the maximum load that the structure can sustain before experiencing structural failure or permanent deformation. Unlike the limit load, which accounts for expected loads in normal service, the ultimate load is a measure of the structural strength of the aircraft under extreme conditions. The ultimate load serves as a critical benchmark for assessing the structural integrity and safety of the aircraft design.

It is important to understand that permanent deformation of structure is possible above Limit Load.


The factor of safety is a key parameter used to assess the margin of safety between the ultimate load and the limit load of the aircraft structure. It is defined as the ratio of the ultimate load to the limit load. In aircraft structures, the factor of safety is typically set at 1.5, indicating that the ultimate load is 1.5 times greater than the limit load. This provides a safety margin to account for uncertainties in loading conditions, material properties, and manufacturing variations. The factor of safety ensures that the aircraft structure can withstand unexpected loads or events without risking structural failure.

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