Which of the following describes the limitation for the use of FMS performance speeds?

The use of Flight Management Systems (FMS) has various limitations based on the operational requirements necessary for ensuring safety and accuracy in navigation. One significant limitation is that in many operational contexts, two FMS units must be available to provide redundancy. This is crucial because it ensures that if one system fails for any reason, the other can continue to function effectively, maintaining operational integrity and safety.

Having two FMS systems allows for cross-checking of data and calculations, which is important for flight planning, navigation, and performance management. It enhances reliability and reduces the risk of errors that could arise from relying on a single system. This is particularly important in commercial aviation, where safety is paramount, and multiple systems provide a safety net against possible electronic or data failures.

In contrast, other options like good weather conditions, requirement of a full flight crew, or minimum fuel load pertain to situational operational conditions rather than specific limitations on the use of FMS performance speeds. Each of these options addresses other aspects of flight operations rather than the structural and functional requirements of FMS.