14 CFR § 27.973

Each fuel tank filler connection must prevent the entrance of fuel into any part of the rotorcraft other than the tank itself during normal operations and must be crash resistant during a survivable impact in accordance with § 27.952(c). In addition—

Each filler must be marked as prescribed in § 27.1557(c)(1);

Each recessed filler connection that can retain any appreciable quantity of fuel must have a drain that discharges clear of the entire rotorcraft; and

Each filler cap must provide a fuel-tight seal under the fluid pressure expected in normal operation and in a survivable impact.

Each filler cap or filler cap cover must warn when the cap is not fully locked or seated on the filler connection.