switch. If either of these switches is open, or the vacuum source or electrical power is lost, the safety valve will close
to atmosphere except at maximum pressure differential of 6.5 ± 0.1 PSI. A negative pressure relief diaphragm is
also incorporated into the outlow and safety valves to prevent outside atmospheric pressure from exceeding cabin
pressure during rapid descent.
h. Drain. A drain in the outlow valve static control line is provided for removal of accumulated moisture. The
drain is located behind the lower sidewall upholstery access panel in the baggage section of the aft compartment.
i. Flow Control Unit. A low control unit, located forward of the irewall in each engine nacelle controls bleed
air low and the mixing of ambient air to make up the total air low to the cabin for pressurization, heating, and
ventilation. The ENVIRO & PNEU BLEED AIR valve switches on the overhead control panel (Fig. 2-13) control an
integral electric solenoid irewall shutoff valve. A solenoid, operated by the landing gear safety switch, controls the
introduction of ambient air to the cabin upon takeoff. Both the ambient airlow control valve and the bleed airlow
control valve are motor driven.
(1) The unit receives bleed air from the engine into an ejector which draws ambient air into the venturi of
the nozzle. The mixed air is then forced into the bleed air line routed to the cabin.
(2) Bleed air low is controlled automatically. When the aircraft is on the ground, circuitry from the landing
gear safety switch prevents ambient air from entering the low control unit to provide maximum heating.
(3) The bleed air irewall shutoff valve in the control unit is a spring loaded, bellows operated valve that is
held in the open position by bleed air pressure. When the electric solenoid is shut off, or when bleed air diminishes
on engine shutdown (in both cases the pressure to the irewall shutoff valve is cut off), the irewall valve closes.