What must the fuel storage system of an airplane be designed to prevent?

The primary purpose of a fuel storage system in an airplane is to ensure that fuel is stored safely and efficiently, which includes the requirement to prevent fuel contamination. Fuel contamination can occur through various means, such as the introduction of water, particulates, or other foreign substances into the fuel. This contamination can lead to significant performance issues, engine failure, and safety hazards.

By preventing fuel contamination, the design of the storage system ensures that the fuel remains clean and pure, thus maintaining engine efficiency and safety. While improper filling, evaporation, and overflow are concerns, the most critical aspect that directly impacts the aircraft's operational reliability and safety is the prevention of contamination. Hence, a well-designed fuel storage system prioritizes this aspect to protect the aircraft's performance and integrity.