What type of fuel system is most commonly found on older aircraft?

Bladder systems are indeed commonly found in older aircraft due to their historical prevalence and effective design for fuel containment. These systems utilize flexible, removable bladders that are typically made from rubber or similar materials. This design allows for easier fuel tank replacement or repair, which is especially useful in older aircraft that may have less integrated structures compared to modern designs.

The use of bladder systems in older aircraft is often linked to their ability to conform to the shape of the aircraft's fuselage or wings, maximizing fuel capacity while minimizing weight. As aircraft technology has advanced, many newer models have moved towards integral fuel systems or other modern designs that eliminate the need for such bladders, focusing on reducing the number of components and improving safety by preventing the risk of leaks that might occur with older bladder systems.

While other types of fuel systems exist, bladder systems stand out for their historical context and operational characteristics in older aircraft.