This specification is written to cover heating and ventilating equipment under four main classifications, namely; A HEATING AND VENTILATING EQUIPMENT - GENERAL - Dealing with features applicable to all makes and uses. B HEATING AND VENTILATING EQUIPMENT - MILITARY - Dealing with features applicable only to military aircraft. C HEATING AND VENTILATING EQUIPMENT - COMMERCIAL - Dealing with features applicable only to commercial aircraft. D DESIRABLE DESIGN FEATURES - General information for use of those concerned in meeting requirements contained herein.