Hot Air Balloon Pilot Practice Test

During propane filling, tanks are limited to 80% of their capacity to allow for liquid expansion. An overfill protection system uses a vapor-bleed valve to vent vapors as the liquid fills. When the tank reaches about 80% full, this valve opens to vent vapor, signaling that the fill should stop. That venting mechanism is specifically designed to indicate the 80% limit, making the vapor-bleed valve the one that shows the tank has reached that capacity.

The other valves have safety or control roles but do not indicate fullness. The pressure relief (or relief) valve vents gas if pressures get too high, protecting against overpressure. The shutoff valve simply controls whether gas can flow to the regulator and does not provide a fill-level indication.