14 CFR §29.969
Verified against eCFR.gov as of June 20, 2026View official text on eCFR.gov ↗
Each fuel tank or each group of fuel tanks with interconnected vent systems must have an expansion space of not less than 2 percent of the combined tank capacity. It must be impossible to fill the fuel tank expansion space inadvertently with the rotorcraft in the normal ground attitude.