To maximize the aircraft's cargo space, designers consider the contour of the aircraft hold and create a ULD that fits that contour optimally. The contour of the ULD is determined based on the dimensions of the aircraft hold, its shape, and the type of cargo that will be stored in it. The ULD design should be flexible enough to accommodate multiple types of aircraft and cargo. To decide on the most suitable contour, designers typically use data and analysis of past cargo loads, dimensions of the aircraft, and industry standards.

Since ULD's are often transferred between different types of aircraft, this gives rise to three types of fit: optimal fit, non-optimal fit and not fit:  

• Optimal fit: the ULD’s contour is the same as the aircraft’s hold;
• Non-optimal fit: the ULD’s contour does not exactly match the hold’s contour;
• Not fit: the ULD does not fit the aircraft’s hold.

The contour of the ULD is typically determined by the dimensions of the most frequently used aircraft in the fleet and the intended cargo it will carry. The manufacturer takes into consideration the volume and weight of the cargo, as well as the aircraft specifications, to determine the most suitable ULD contour. The manufacturer also considers safety and ease of handling for loading and unloading the ULD. The decision of which contour is most suitable depends on a cost-benefit analysis, balancing the cost of having a ULD that fits the contour of each type of aircraft against the potential loss of cargo space in aircraft that do not have an optimal fit.

How do you decide which is the most suitable contour?

The contour you select depends on a few factors:

• Which airline do you use?
• Which aircraft does the airline use?
• Is the ULD also going to be used for land/sea transport?
• What type of cargo is being transported? 
• Do you need to use more than one aircraft or airline (interlining)?