Text in force at CS25-28: from CS25-0, effective 17 Oct 2003.
Each fuel tank must have an expansion space of not less than 2% of the tank capacity. It must be impossible to fill the expansion space inadvertently with the aeroplane in the normal ground attitude. For pressure fuelling systems, compliance with this paragraph may be shown with the means provided to comply with CS 25.979(b).
ED Decision 2003/002/RM · Source