only. If the left or right engine bleed air is secured, or if a rupture occurs in the left or right bleed-air manifold, the appropriate check valve closes. This allows the air-conditioning and pressurization subsystems to operate from the opposite bleed- air source. An open engine bleed-air bypass and shutoff valve allows bleed air to bypass the check valves and flow from the left-to-right or right-to-left manifolds. The engine bleed-air bypass and shutoff valve is open during engine starting. It is also open when operating the deicing system with one engine secured. Bleed-air pressure is sensed by the bleed-air pressure transmitter located in the bleed-air supply duct downstream of the crossover duct isolation check valves. The pressure is displayed on the bleed-air pressure indicator on the environmental panel. Bleed air from the left and right manifolds flows through the crossover duct isolation check valves to the bleed-air flow control and shutoff valve. The bleed-air flow control and shutoff valve is electrically controlled and pneumatically actuated to modulate the bleed-air flow to the air- conditioning and pressurization systems in response to predetermined flow schedules. Two alternate air supply sources, APU air and ground start air, connect to the left and right bleed-air manifold. The APU air duct supplies bleed air through two check valves to the left manifold. The ground start duct supplies high- pressure air through a check valve to the right manifold. These alternate air supply sources are used primarily for starting engines and for ground operation of the air-conditioning system. APU Bleed Air Bleed air flows from the APU compressor through two one-way check valves in the APU duct to the left one-way check valve in the cross- breed manifold. Bleed air is also supplied to the bleed-air shutoff valve, the left side of the engine bleed-air bypass and shutoff valve, empennage deice valve, the left wing deice valve, and the ram air anti-icing valve. With the bleed-air switch in the ON position, the left bleed-air shutoff valve is opened. In the open position, bleed air is supplied to the left engine starter control valve. To provide bleed air to the right side of the cross- breed manifold, the engine bleed-air bypass and shutoff valve is opened to allow bleed air to the right bleed-air shutoff valve and to the right wing deice valve. With the bleed-air engine No. 2 switch set to ON, the right bleed-air shutoff valve is opened. In the open position, bleed air is supplied to the right engine starter-control valve. SE Ground Start Air When support equipment is the source of air, the ground air start hose is connected to the ground start connection nipple located in the right wheel well. External high-pressure air flows through the engine starting duct check valve into the right cross-bleed manifold. Normal flow is through the right one-way check valve in the cross- breed manifold to the bleed-air flow control and shutoff valve. High-pressure air is available to the right bleed-air shutoff valve and the right wing deice valve. Opening the right bleed-air shutoff valve provides air to the right engine starter- control valve. To provide SE air to the left cross-bleed manifold, the engine bleed-air bypass and shutoff valve is opened. When the valve is open, air flows around the cross-bleed check valves to the left bleed-air shutoff valve, the left wing deice valve, the ram air anti-icing valve, and the empennage deicing valve. To provide air to the left engine starter-control valve, open the left bleed-air shutoff valve. SYSTEM COMPONENTS Now that you are familiar with the operation of the system as a whole, let’s look at its components and their operation. Knowledge of the individual components makes troubleshooting easier and faster. To aid you in locating parts of the components, numbers within parenthesis ( ) are included that correlate to the numbers on the illustrations. High-stage Bleed-Air Regulator Valve The high-stage, bleed-air regulator valve is a normally closed, differential-pressure regulator. 4-3