What markings are required for fuel filler openings in reciprocating engine-powered airplanes?

The requirement for fuel filler openings in reciprocating engine-powered airplanes is to include the fuel grade along with the word "AVGAS." This marking plays a crucial role in ensuring that anyone handling the fuel is aware of the specific type of aviation gasoline (AVGAS) intended for use in reciprocating engines. Proper identification helps prevent the accidental use of the wrong fuel type, which could lead to engine performance issues or even catastrophic failures.

In contrast, markings associated with jet fuel or just a general term like "Gasoline" do not apply to reciprocating engine aircraft, since these airplanes specifically require AVGAS. Thus, it's vital that the specific grade is stated alongside "AVGAS," promoting safety and proper fueling practices in aviation maintenance and operations.