Limit and Ultimate Loads
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You may have heard these terms, ‘Limit and Ultimate Loads’. What in the world do they mean? Well, I will take the smoke screen off for you. Let us dive right in then.
The lower of limit and ultimate loads, limit load is defined as the maximum load that an aircraft is expected to see at any point in the service life of that aircraft.
Limit loads can be expected in any given direction (forward or down etc.) or a combination of more than one directions (example forward+up etc.). Under limit load conditions, the FAA regulations require that the aircraft components support those loads without any permanent detrimental deformations and that the stresses remain below the critical yield point.
For flight operations under the control of a pilot, usually there are flight limitations built into the design to ensure the loads never exceed the design limit loads. But is it always possible to limit the loads on the aircraft?
Although the aircraft is not expected to see any loads above the limit loads, there will need to be an additional factor of safety to account for unexpected events such as a sudden and severe gust load in a storm on a wing for example, or an emergency landing or crash condition. The higher of limit and ultimate loads, these loads are beyond the maximum expected limit loads for any of the taxi, take off, in flight or landing conditions. Such loads are defined as ultimate loads.
Although statistical and historical gust load velocities are assumed during design based on many decades of experience and flight data, it is quite possible that certain load conditions might be even higher than these expected loads. In such situations, the ultimate load factor of safety ensures that the aircraft does not fail and allow the aircraft to land safely.
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This ultimate load safety factor is generally assumed to be 1.5 times the limit load.
The ultimate load safety factor is also important to account for any other assumptions or possible variations in the material properties, fabrication or inspection variations, approximations and assumptions in aerodynamics and stress theory, and of course the possibility of the most dreaded crash conditions.
The FAA requires that under the ultimate loading conditions, the aircraft components are allowed to yield but not to the point that it is detrimental to the safe landing or exit of the passengers and the crew. Although yielding is permissible, the aircraft must be designed to not fail under ultimate load conditions.
The Hudson river emergency belly water landing is a good example. The inertia loads in those conditions may very well have been higher than the design limit loads. Here is another article that sheds some more light on the loads.
Check out this awesome video of an airplane wing taken to ultimate load and beyond, 1.54 times the limit load:
Many decades of flight hours and experience resulted in the currently required limit and ultimate loads and corresponding safety factors. The ultimate safety factor of 1.5 is not something that is a be all end all factor that will cover any possible load condition. However, it is unlikely given the history of aviation, a particular aircraft will see higher limit and ultimate loads. The FAA regulations are there for good reasons.