Air/Pnuematic

Pneumatic air is used for three aircraft systems: air conditioning, pressurization and the boot deicing. Bleed air is taken off both engines to supply compressed air to these systems. Each engine has a 5th stage, low-pressure bleed valve and one compressor discharge for high-pressure (HP) bleed air. Low-pressure (LP) bleed is the normal source for compressed air to feed these systems. However, when LP bleed air is insufficient for pneumatic system operation, it is supplemented by (HP) bleed air depending upon the gas generator rpm and airplane altitude when the HP switch is in the AUTO position.

Low-pressure bleed air is bled off the engine at aft of the 5th stage compressor. A check valve prevents reverse flow into the engine. High-pressure air operates at a pressure about three times greater than the LP air and is bled from a point behind the compressor section. HP air is supplied only if the HP valve switch is in AUTO mode and centrifugal compressor discharge pressure is below 76 psi (80-85% Ng on the ground). Check valves ensure that high-pressure bleed air flows in only one direction.

There are two air conditioning packages or "packs" for air conditioning and pressurization and two distribution systems: left and right. The left is entirely for the cabin and the right is for the cabin and cockpit. Each distribution system has fans and filters in the recirculation ducting to clean and condition the air. The packs are located under the cabin floor in the wing fairings Each pack operates independently of the other. The packs are supplied by their respective engines with regulated and temperature controlled bleed air via the pnuematic system.

Each "pack" includes:

• An Air Cyle Macine/ACM
• Heat Exchanger
• Ducting
• Monitoring and control

Engine bleed valves will close automatically when:

• Bleed air is too hot or pressure is too high
• the A/C pack overheats
• the distribution duct overheats
• the fire handle is pulled

High Pressure/HP bleed valves close automatically close when:

• Bleed air over temps or if an over pressure occurs
• main DC bus loses power
• Engine shutdown
• <80% Ng

Two sources of air are used for cooling: ram air taken from a scoop on the engine nacelle is used in flight and on the ground, airflow is augmented by a "jet pump". The low-pressure bleed air used does not require precooling, unless it is to be routed to the boot system, because of its lower temperature and it bypasses the ram-air heat exchanger. Bleed air from the heat exchanger is ducted to the pressure regulating/shutoff valve before entering the air conditioning system.

A precooler in each engine nacelle lowers the HP bleed air temperature to a usable temperature before the air is routed into the pneumatic manifold. The manifold connects the left and right engine bleed systems and when the cross valve (X VALVE OPEN) is selected open, it interconnects the two sides. To operate the X VALVE, one bleed air switch must be in the closed position as differential pressure is used to move the valve.

Part of the precooler is also used to further cool the bleed air routed to the leading edge boot deice system. A jet pump activated by a solenoid valve in series with landing gear weight-on-wheels switches assists airflow through the heat exchanger when the airplane is on the ground.

Temperature regulation is accomplished via a bypass valve and a pack valve in the ACM. The bypass valve controls the bypass of hot bleed air around the pack and the pack valve controls bleed air entry into the ACM. When one valve opens, the other closes thereby permitting cabin temperature control by the mixing of hot and cold air from the ACM. Temperature is normally controlled in the AUTO mode but can also be done via the manual modulation of the temp select switch between hot and cold.

BLEED VALVE OPERATION

The LP bleed valve regulates outgoing pneumatic system pressure as a function of altitude. The valve acts as a nonreturn valve and is located downstream of the high-pressure/ low-pressure connector point. Under normal operating conditions with the HP bleed valve closed, the LP valve opens in the same direction as the low pressure from the compressor bleed point. With the highpressure bleed-air valve open, the low-pressure check valve, located upstream of the HP/LP connector, closes to prevent any reverse flow of high pressure bleed air into the low-pressure system. Valve operation is normally automatic; however, a three-position switch on the cockpit overhead panel permits closing of the valve.

The bleed valve closes automatically if any of the following conditions occur and must be reset before it can be opened:

• The fire handle is pulled
• The bleed-air temperature > 288° C
• The air-conditioning pack compressor is >225° C
• The duct temperature > 82° C
• The regulated pressure exceeds 43.5 psi

The HP valve opens below 76 psi or <80-85% Ng. It closes when Ng exceeds this

value. The valve also automatically closes and must be reset if:

• The fire handle is pulled
• The bleed-air temperature >288° C

This valve can also be selected closed. The valves are pneumatically actuated and require

a minimum of 10 psi for operation. The valve is normally closed and is located upstream of the

ram air precooler/heat exchanger. If the HP valve fails to close after a close signal was given, the HP HIGH light will illuminate on the overhead panel along with a master caution chime and AIR COND light on the CWP. If bleed pressure or temperature becomes too high, a BLD FAULT light on the overhead panel, the AIR COND caution light and MASTER CAUTION lights illuminate accompanied by a single stroke chime.

BLEED LEAK DETECTOR

There is a bleed leak detection system that runs the length between the engine nacelle and the cross valve on each side. If a temp >205 degrees is detected by the bleed leak detector loop, a master caution alert L/R BLD AIR LEAK is triggered, along with the AIR COND on the CWP and a single stroke chime. If the air leak cannot be eliminated by closing the bleed valves, shutdown of the engine may be necessary.

CABIN PRESSURIZATION

On the Saab 340 aircraft pressurization and air-conditioning is accomplished with conditioned air. Pressurization of the cabin is achieved by controlling the outflow of the conditioned air. The outflow of the air is at a rate that achieves and maintains the selected or preprogrammed cabin altitudes on the pressurization controller on the center pedestal. The pressurization system also changes the cabin air at regular intervals for a better cabin environment.

The cabin is pressurized by the L & R packs and is pressure regulated via one the of two outflow valves. The outflow valves are supplied with servo vacuum pressure from the pneumatic system via a jet pump. One outflow valve is electrically controlled by the pressure controller and is normally used for automatic pressure regulating. The other outflow valve is pneumatically controlled from the cockpit control panel and functions as a manually operated standby system.

Cabin pressure is either automatically controlled by a pressure control unit or manually controlled

from the pressurization panel.

Normally, the AUTO mode of the control system is used. The system gets input from power lever position, static pressure, cabin pressure, preselected airfield altitude and control panel pressure values. Inputs are processed by the control unit which maintains the correct cabin pressure by modulating the automatic outflow valve. The manually controlled system is an all pneumatic system. When manual cabin pressurization is necessary, the pilot can operate the pneumatic outflow valve by adjusting the control valve setting on the controller. Cabin pressure can be dumped by opening both outflow valves when on the ground. The electrically controlled outflow valve can also be opened via the emergency dump switch on the overhead panel. When emergency dump is selected ON inflight, the primary outflow valve is immediately opened. When selected on the ground, the primary and secondary valves are both opened. There is a pressurization display in the cockpit on a panel that has separate indicators for differential pressure, cabin altitude, and vertical speed. The CABIN PRESS light on the CWP illuminates if cabin pressure altitude is above 10,000ft or differential pressure is above 7.5 psi. The MASTER WARNING also illuminates and a triple stroke chime is sounded.