These recommendations are written to cover the subject of engine exhaust gas to air type heat exchangers under the following classifications: CABIN HEATING: (All occupied regions and windshields heating). Wing and empennage heating. ENGINE AND ACCESSORY HEATING: (When heater is installed as part of the aircraft). These recommendations are not intended to cover the design of secondary exchangers which may be required when engine exhaust gas to air type heat exchangers are used in supplying ventilating air.