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
- 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.
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.
The CONTROL UNIT
The pressurization control unit on the center pedestal is used for both automatic and manual operation of the system. On it there is an automatic selector, a fault indicator, an AUTO/MAN switch and a manual control valve. The automatic selector is used to select:
- Cabin rates from 50 to 2,500 fpm up
- Cabin rates from 50 to 1,500 fpm down
- Landing field altitude from —1,000 to + 14,000 feet
- Barometric correction from 28 to 31 in Hg
When in AUTO position the AUTO/MAN switch electrically powers the pressure controller. The fault light monitors a self-test done by the pressure controller. The manual control valve pneumatically operates the pneumatic outflow valve. The rate at which the cabin pressure is allowed to change is controlled by the rate limit selector knob. An index position is provided which sets the rate limit at a setting of to 500 +75 feet/minute for climb and 300 ±75 feet/minute for descent. The rate limit at the extreme clockwise knob rotation is 2,500 +150 feet/minute for climb and 1,500 +150 feet/minute for descent. Setting the rate limit knob to the extreme counter clockwise position provides 0 + 50 feet/minute climb or descent. The barometric correction is adjusted at the airfield prior to takeoff via the barometric selector knob on the panel. The outflow valve manual control knob (labeled "OPEN") allows for manual modulation of the outflow valve when the system is in the manual mode. The MAN mode position switches off the 28 VDC supplied to the controller. The outflow manual control now directly controls the operation of the cabin safety valve. The fault light also illuminates if the automatic controller fails.
The digital electronic controller in the avionics rack is the primary unit in the cabin pressure control system. It processes all inputs that are used by the pressurization system. The controller has a self-test function, which performs a test cycle each time power is turned on. During the test cycle, the fault light on the control panel illuminates and remains on during the test (maximum of three seconds). If a failure is detected, the light does not extinguish. Some reasons the test might fail:
- Internal circuit failure
- Selected landing altitude off usable scale
- Selected barometric correction off usable scale
- Outflow valve torque motor circuit is open
- Static sensors not in limits
- Selected cabin rate full maximum or minimum
Modes of Operation
When the aircraft is on the ground and both power levers are retarded the system enters the ground mode. After engine starting, zero cabin pressure differential is ensured by the opening of both outflow valves. (Some aircraft may be modified so that only the primary outflow valve opens in ground mode.) The manufacturer designed the aircraft and the pressurization controller to accommodate bleeds on takeoffs which pressurized the aircraft while on the ground when takeoff power was applied. However, some operators do not perform bleeds on takeoffs. Therefore there is no cabin pressurization to control. Pressurization control begins with the flight mode described below.
During Climb: Normal transition to the climb mode occurs when the first bleed valve is opened (bleed valves are closed for takeoff). The secondary outflow valve remains closed and the primary outflow valve is regulated so that cabin pressurization doesn't exceed the selected cabin pressure rate-of-change and is maintained at the higher of the selected landing or takeoff altitudes until intercepting the autoscheduler altitude. The automatic controller limits maximum differential pressure to 7.1 psid (approximately 4500' at FL250). An aircraft altitude of 15000' will automatically activate the climb mode if the normal transition fails. One minute after the aircraft begins descent, the automatic controller enters the descent mode and cabin pressure differential begins decreasing in accordance with the autoschedule within the selected cabin rate limits.
Early Aircraft: At touchdown the primary outflow valve opens to depressurize the cabin. If the power levers are also retarded, a 20 second timer is activated within the controller. If the power levers are not retarded, then a one minute timer is started in the controller. At the expiration of either time period the controller reverts to ground mode and the primary outflow valve is fully opened to dump all remaining cabin pressurization. If the landing altitude is set higher than actual landing elevation then cabin pressurization decreases to zero differential as the aircraft descends through the set landing altitude via opening of the primary outflow valve. If the landing altitude is set below the actual landing elevation, then cabin pressurization is vented at touchdown via opening of the secondary outflow valve.
Later and Modified Aircraft: Cabin pressurization is not dumped at touchdown. Instead, the primary outflow valve is modulated to decrease pressure differential at a rate of 500 fpm until reaching zero differential. Twenty seconds after touchdown the primary outflow valve is fully opened.
The Manual Controller
The manual controller is active when the mode selector is set to the MAN position. This action completely disables the automatic controller to allow unencumbered control by the manual controller. The manual controller regulates cabin altitude via control of the secondary outflow valve. Valve opening is controlled by metering the vacuum applied to it via the control knob. Cabin pressurization is controlled by reference to the CABIN ALT and CABIN DIFF PRESS indicators. Cabin altitude is increased by turning the control knob clockwise (valve progressively opens). Cabin altitude is decreased by turning the control knob counterclockwise (valve progressively closes).
Pressure Outflow Valves
The Saab has two outflow valves: a primary and a secondary. Their opening is accomplished by vacuum. The valves close automatically without vacuum by internal springs. Each outflow valve incorporates two pressure relief functions which may be activated after controller failure or operator error. These functions limit positive pressure to 7.6 psid and negative pressure to -.5 psid by opening the valves as necessary. The primary outflow valve is mounted on a false bulkhead in the tail compartment. Discharged air is directed through a duct which vents overboard through an opening located on the underside of the tail cone. This outflow valve contains an integral vacuum metering valve (or torque motor) that is electrically controlled by the automatic controller. The torque motor valve controls vacuum entry and subsequent opening of the electro-pneumatically operated primary outflow valve. The primary outflow valve remains closed in manual mode operation.
The secondary outflow valve is located on the rear pressure bulkhead in the aft cargo compartment. Air is vented through an opening in the tail cone. The secondary outflow valve is normally only operated by the manual controller but when on the ground it will fully open if the PRESS DUMP switch is selected ON. This valve is pneumatically operated.
The pressurization is controlled by two separate systems, either automatically or manually (when auto fails).
The primary outflow valve is electro-pnuematically operated
The secondary outflow valve is pnuematically operated
While on the ground, one engine operating with its respective LP & HP bleeds open in addition to the cross valve can supply bleed air to power the both left and right air conditioning packs continuously. In flight each engine can only supply its respective pack.
During ground servicing, the aircraft can be supplied with air conditioning or heat via a ground air cart. An external connection is on the belly of the aircraft where the cart hose is connected to the aircraft. The air cart supplies both a/c distribution systems at a point downstream of both packs.
The air cycle machine "packs" in the wing root.