14 CFR § 25.975

Fuel tank vents. Each fuel tank must be vented from the top part of the expansion space so that venting is effective under any normal flight condition. In addition—

Each vent must be arranged to avoid stoppage by dirt or ice formation;

The vent arrangement must prevent siphoning of fuel during normal operation;

The venting capacity and vent pressure levels must maintain acceptable differences of pressure between the interior and exterior of the tank, during—

Normal flight operation;

Maximum rate of ascent and descent; and

Refueling and defueling (where applicable);

Airspaces of tanks with interconnected outlets must be interconnected;

There may be no point in any vent line where moisture can accumulate with the airplane in the ground attitude or the level flight attitude, unless drainage is provided;

No vent or drainage provision may end at any point—

Where the discharge of fuel from the vent outlet would constitute a fire hazard; or

From which fumes could enter personnel compartments; and

Each fuel tank vent system must prevent explosions, for a minimum of 2 minutes and 30 seconds, caused by propagation of flames from outside the tank through the fuel tank vents into fuel tank vapor spaces when any fuel tank vent is continuously exposed to flame.

Carburetor vapor vents. Each carburetor with vapor elimination connections must have a vent line to lead vapors back to one of the fuel tanks. In addition—

Each vent system must have means to avoid stoppage by ice; and

If there is more than one fuel tank, and it is necessary to use the tanks in a definite sequence, each vapor vent return line must lead back to the fuel tank used for takeoff and landing.