According to regulatory standards, what marking is required on fuel filler covers for reciprocating engine-powered airplanes?

The requirement for marking fuel filler covers on reciprocating engine-powered airplanes is specifically to ensure that pilots and ground crew can quickly and accurately identify the type of fuel that should be used in the aircraft. In this case, the correct marking is the word "Avgas" along with the minimum fuel grade.

Using "Avgas" indicates that the airplane is designed to run on aviation gasoline, which is distinct from other types of fuel, such as Jet A or other commercial fuels. Including the minimum fuel grade is critical as it informs those refueling the aircraft of the lowest octane rating that can be safely used. This is particularly important because using fuel with an insufficient octane rating could lead to engine knocking, which can cause damage to the engine.

In summary, the requirement for the marking is grounded in safety and operational efficiency. It ensures that the aircraft is filled with the correct type of fuel, thereby preventing potential issues with engine performance and reliability.