SECTION X. HEATING, VENTILATION, COOLING, AND ENVIRONMENTAL
2-72. HEATING SYSTEM.
a. Description. Warm air for heating the cockpit, mission equipment compartment, and warm windshield de-
frosting air is provided by bleed air from both engines. Engine bleed air is combined with ambient air in the heating
and pressurization low control unit in each engine nacelle. If the mixed bleed air is too warm for cockpit comfort, it
is cooled by being routed through an air-to-air heat exchanger located in the forward portion of each inboard wing. If
the mixed bleed air is not too warm, the air-to-air heat exchangers are bypassed. The mixed bleed air is then ducted
to a mixing plenum, where it is mixed with cabin re-circulated air. The warm air is then ducted to the cockpit out-
lets, windshield defroster outlets, and to the loor outlets in the mission equipment compartment. The environmental
(1) Bleed air low control unit. A bleed air low control unit, located forward of the irewall in each engine
nacelle controls the low of bleed air and the mixing of ambient air to make up the total airlow to the cabin for
heating, windshield defrosting, pressurization and ventilation. The unit is electronically controlled with an integral
electric solenoid irewall shutoff valve, controlled by the bleed air switches located on the overhead control panel
(Fig. 2-13) and a normally open solenoid valve operated by the right landing gear safety switch.
(2) Pneumatic bleed air shutoff valve. A pneumatic shutoff valve is provided in each engine nacelle to
control the low of bleed air to the surface, antenna and brake deice systems. The bleed air valve switches located
on the overhead control panel controls these valves (Fig. 2-13).
(3) Bleed air valve switches. The bleed air low control unit shutoff valve and pneumatic bleed air shutoff
valves are controlled by two LEFT and RIGHT switches placarded ENVIRO & PNEU BLEED AIR PNEU ONLY
ON , located on the overhead control panel (Fig. 2-13). When set to the ON position, both the environmental
low control unit shutoff valve and the pneumatic shutoff valve are open; when set to the PNEU ONLY position, the
environmental low control unit shutoff valve is closed, and the pneumatic bleed air valve is open; in the ENVIRO
& PNEU position, both are closed. For maximum cooling on the ground, turn the bleed air valve switches to the
PNEU ONLY position.
(4) Cabin temperature mode selector switch. A switch placarded CABIN AIR MODE SELECT OFF
AUTO AC COLD OPN 10° to -25° C MAN COOL - MAN HEAT located on the overhead control panel, controls
cockpit and mission avionics compartment heating, and air conditioning. When the cabin temperature mode se-
lector switch is set to the AUTO position, the heating and air conditioning systems are automatically controlled.
Control signals from the temperature control box are transmitted to the bleed air heat exchanger bypass valves.
Here, the bypass valves controlling the amount of air bypassing the heat exchangers regulate the temperature
of the air lowing to the cabin. When the temperature of the cabin air, has reached the temperature setting of
the cabin temperature control rheostat, the automatic temperature control allows hot air to bypass the air-to-air
exchangers admitting hot air into the cabin. When the bypass valves are in the fully closed position, allowing no
air to bypass the heat exchangers, the air conditioner begins to operate, providing additional cooling. When the
cabin temperature mode selector switch is set to the A/C COLD OPN position, the air conditioning system is in
continuous operation. The cabin temperature control rheostat, in conjunction with the cabin temperature control
sensor, provides regulation of cockpit and mission equipment compartment temperature. Bleed air heat is added as
required to maintain the temperature selected by the cabin temperature control rheostat.
(5) Cabin temperature control rheostat. A control knob placarded CABIN AIR TEMP CONTROL INCR
located on the overhead control panel (Fig. 2-13), provides regulation of cabin temperature when the cabin temper-
ature mode selector switch is set to the AUTO or the AC COLD OPN position. A temperature-sensing unit in the
cabin, in conjunction with the setting of the cabin temperature control rheostat, initiates a heat or cool command to
the temperature controller for the desired cockpit and mission equipment compartment environment.