Engine Bleed-Air Bypass and Shutoff Valve

The engine bleed-air bypass and shutoff valve, located in the cross-bleed manifold, is normally closed. It is open for engine starting and during single-engine, wing-deicing operations. (See figure 4-4.) When the solenoid (1) is energized, the shuttle valve (7) senses the higher pressure air from the right and left pressure inlets (3 and 5) and directs it through the solenoid to chamber A (6) to open the butterfly (4). When the solenoid is de-energized, air bypasses the solenoid and enters chamber B (2) to assist the spring in closing the engine bleed-air bypass and shutoff valve.

Bleed-Air Flow Control and Shutoff Valve

The bleed-air flow control and shutoff valve is a normally closed valve with two flow schedules: fixed and inlet pressure regulated. (See figure 4-5.) The valve is electropneumatically controlled and pneumatically actuated.

The venturi inlet (17) and throat pressure (18) are routed to the delta-P servo diaphragm (22). As the inlet pressure to the bleed-air flow control and shutoff valve is increased, regulated pressure routed to the actuator diaphragm (13) causes the butterfly (15) to open. When the resultant venturi delta-P reaches the predetermined value, as set by the calibration spring (12), the delta-P servo diaphragm moves. This causes the flexure beam (11) to lift off the servo valve and seat (20). This decreases pressure downstream of the control orifice (3), which closes the butterfly to a position that maintains the desired venturi delta-P. This delta-P corresponds to the desired high-flow setting when solenoid A (26) is de-energized.

When solenoid A is energized, regulated pressure acting on the high-flow low-flow reset diaphragm (7) moves the reset lever to the lowflow stop (10) and reduces the calibration spring load on the delta-P servo diaphragm. This causes the delta-P servo diaphragm to regulate the airflow at low condition. Solenoid A is operated electrically by an altitude switch (25). As the venturi inlet pressure increases, the inlet pressure compensating piston (5) moves against the reset lever (9) and modulates the air flow to a low value. The inlet pressure compensating spring preload and rate are selected to provide a prescribed schedule. When solenoid B (27) is energized, actuator pressure is vented to ambient, and the butterfly valve closes.

Figure 4-4.-Engine bleed-air bypass and shutoff valve.

Figure 4-5

Bleed-Air Transmitter

The pressure transmitter senses the bleed-air pressure in the duct upstream from the bleed-air flow control and shutoff valve. The pressure transmitter transmits the pressure indication to the bleed-air pressure indicator on the environmental panel.