This standard covers both general type and quick-donning type mask assemblies in the following classes: a. Class A@ oronasal@ demand b. Class B@ oronasal@ pressure-demand c. Class C@ full face@ demand d. Class D@ full face@ pressure-demand PURPOSE: The purpose of this standard is to establish optimum standards for crew demand and pressurebreathing oxygen mask assemblies for use by crew members in civil aircraft.