21A Air Supply

TTP	B1-L3	ATA 21A
Beech 90 Series	B2-L2	Air Supply

Air for pressurizing the cabin, heating the cabin and cockpit, and operating the aircraft pneumatic system is obtained by bleeding P3 compressor air from each engine (refer to the MSM). This engine bleed air is ducted from the engine to the flow control unit on the lower engine mount trusses. A pressure supply line, to operate the pneumatic system, tees off the bleed-air line just aft of the first fire seal forward of the firewall. The bleed air from either flow control unit provides adequate air for pressurization, heating, and the pneumatic system should the opposite engine fail.

The bleed air and ambient air from the cowling intake  are  mixed  together  by  the  flow control units to produce a total airflow of 14 pounds per minute from both the right and left engine units. Bleed air comprises as much as 10 pounds of the total airflow on cold days and as little as six pounds on hot days. The air from each flow control unit is ducted aft through the firewall along the inboard side of each nacelle, inboard through the center section forward of the main spar, and into the fuselage. The heat in the air may either be retained for cabin heating or somewhat cooled down as the air passes through the center section to the fuselage. To cool the bleed air, it is routed through an air-to-air heat exchanger where heat is transferred from the bleed air to outside ambient ram air. The ram air enters a duct on the wing leading edge and is dumped overboard through a vent on the bottom of the wing. A variable bypass valve, adjacent to each heat exchanger, allows the hot air to be ducted through, or around, the heat exchanger controlling the amount of heat sent into the cabin air ducts.

The right and left bleed-air lines meet at a tee on the right side of the fuselage under the cabin floor. Two flapper valves prevent the loss of pressure should either engine fail. The bleed-air line from the tee is routed forward along the right side of the fuselage through a muffler to a mixing plenum underneath the copilot rudder pedals.

A venturi has been plumbed into the bleed-air line immediately forward of the muffler. Two test lines, tapped off from the high- and low-pressure areas of the venturi, are routed to immediately aft and below the static system drains beside the copilot. A manometer can be connected to the end fittings of the two lines to measure bleed air flow.

 

The bleed-air lines from the engine compartment to the plenum are wrapped with insulation and aluminum tape to reduce heat loss. Air from the top of the plenum is routed through ducts behind the instrument panel to outlets on each side of the cockpit and to the defroster outlets for the wind- shield. Valves to each outlet and the defroster duct control the flow of heated air into the cockpit. These valves are regulated by push–pull controls on the subpanels. A low-pressure duct from the aft side of the plenum extends aft under the right- hand seat deck of the cabin and distributes the heated air through the floor outlets on each side and aft of the cabin.

FLOW CONTROL UNIT

SNs LJ-1 through LJ-501

In the flow control unit, engine bleed air is blended together with ambient air and is then routed through, or around, an air-to-air heat exchanger. This modified bleed air from the right and left engine is then joined at a flapper valve in the cabin and sent into the cabin and cockpit ducting.

RIGHT–LEFT BLEED AIR VALVE switches, on the copilot left subpanel, control each flow control unit. When the switch is positioned to OPEN, a solenoid valve on the flow control unit opens to allow bleed air into the unit. This control air passes through a filter and then continues to the refer- ence regulator where it is regulated to a constant pressure of 18 to 20 psi. This reference pressure is directed to the components within the flow control unit to regulate air output to the cabin.

 Each flow control unit has three pneumatically actuated control valves as follows:

•  A firewall shutoff

•  Integral bleed-air ejector flow control

•  Ambient air-modulating valve

 

A check valve is placed downstream of the ambient air-modulating valve to prevent bleed air from back flowing through the ambient air intake duct.

Each flow control unit is also equipped with two sensing devices, which react to atmospheric pres- sure and temperature to automatically maintain optimum air delivery through the flow control unit. The bleed-air flow through the flow control unit is controlled as a function of atmospheric pressure and temperature. Ambient airflow is controlled as a function of temperature only.

 LJ-502 AND AFTER LW-1 AND AFTER

 

Each flow control unit consists of an ejector and an integral bleed air modulating valve, firewall shutoff valve, ambient air modulating valve, and a check valve that prevents the bleed air from escaping  through  the  ambient  air  intake.  The flow of bleed air through the flow control unit is controlled as a function of atmospheric pressure and temperature. Ambient air flow is controlled as a function of temperature only. When the bleed air valve switches on the copilot left subpanel are turned on, a bleed air shutoff electric solenoid valve on each flow control unit opens to allow the bleed air into the unit. As the bleed air enters the flow control unit, it passes through a filter before going  to  the  reference  pressure  regulator. The regulator will reduce the pressure to a constant value (18 to 20 psi). This reference pressure is then directed to the various components within the flow control unit that regulate the output to the cabin. One reference pressure line is routed to the firewall shutoff valve located downstream of the ejector. A restrictor is placed in the line immediately before the shutoff valve to provide a controlled opening rate. At the same time, the reference pressure is directed to the ambient air modulating valve located upstream of the ejector and to the ejector flow control actuator. A pneumatic thermostat with a variable orifice is connected to the modulating valve. The pneumatic thermostat (pneumostat) is located on the lower aft side of the fireseal forward of the firewall. The bimetallic  sensing discs of the thermostat are inserted into the cowling intake. These discs sense ambient temperature and regulate the size of the thermostat orifices. Warm air will open the orifice and cold will restrict it until, at -30ºF, the orifice will completely close. When the variable orifice is closed, the pressure buildup will cause the modulating valve to close off the ambient air source. An ambient air shutoff valve, located in the line to the pneumatic thermostat is wired to the LH landing gear safety switch. When the aircraft is on the ground, this solenoid valve is closed, thereby directing the pressure to the modulating valve, causing it to shut off the ambient air source. The exclusion of ambient air allows faster cabin warmup during cold weather operation. An electric circuit containing a time delay relay is wired to the above mentioned solenoid valves to allow the LH valve to operate 2 to 3 seconds before the RH valve. This precludes the simultaneous opening of the modulating valves and a sudden pressure  surge  into  the  cabin. A  check  valve, located downstream from the modulation valve, prevents the loss of bleed air through the ambient air intake. The ejector flow control actuator is connected to another variable orifice of the pneumatic thermostat and a variable orifice controlled by an isobaric aneroid. The pneumostat orifice is restricted by decreasing ambient temperature and the isobaric aneroid orifice is restricted by decreasing ambient pressure. The restriction of either orifice will cause a pressure buildup on the ejector flow control actuator.

COMPONENTS

 

Normally Closed Solenoid

 

When not powered, this prevents the flow-control unit from functioning. When power is supplied through the bleed-air switch, the solenoid valve opens and allows the flow-control unit to provide bleed air for heating on the ground and to mix bleed air and ambient air at various rates for heating and pressurization during airborne operation.

 Firewall Shutoff Valve

 

In the normally closed position, the firewall shut- off valve prevents any flow of air into the aircraft ducting. When the solenoid is powered, the valve reacts to 18-psi reference air pressure and fully opens to allow air from the flow-control unit into the aircraft ducting.

 Pressure Regulator

 

This regulates bleed air to 18-psi for proper flow- control unit operation.

 Ejector Flow-Control Actuator

 

This actuator automatically proportions the flow of metered engine bleed air to ambient air delivered to the cabin for pressurization and heating.

 Ambient Sense Aneroid

 

The ambient sense aneroid uses sensed barometric pressure to close off one of two bleed ports in the control pressure line connected to the ejector flow-control actuator. The aneroid valve will be completely closed at altitudes above approximately 18,000 feet.

 Dual Bleed-Port Pneumatic Thermostat (Pneumostat)

 

This temperature-sensitive actuator controls the position of two bleed ports. As the temperature gets colder, the bleed-air ports slowly close down. At –30°F the ports are fully closed. One bleed port is connected to the air pressure line to the ambient air modulating valve and, as the port closes, the valve drives toward the closed position and less ambient air is drawn into the flow control actuator. The other port is connected into the line to the ejector flow-control actuator, and, as this port closes, the actuator opens the ejector so more bleed air, and less ambient air, is sent through the flow-control actuator. By varying the position of the ejector and the ambient  air-modulating valve,  the optimum mix of bleed air and ambient airflow is achieved.

OPERATION

 

Condition 1: Aircraft on Ground, Bleed-Air Switch OFF

 

With the engines running, bleed-air pressure is delivered to a fitting at the bottom of each left and right flow-control unit. Bleed air enters the unit and stops at an unpowered, normally closed solenoid shutoff valve. This prevents the flow of bleed air from flowing into the cabin as long as the solenoid valve remains closed. The check valve prevents bleed air from back flowing through the ambient air intake duct.

Condition 2: Aircraft on Ground, Bleed-Air Switch ON

 

Left and right BLEED AIR switches, on the copilot subpanel, electrically activate each flow control unit. When the switches are turned ON, each solenoid valve is energized open. This allows unregulated engine bleed air to flow into the regulator that reduces engine bleed- air pressure down to 18 psi.

Regulated, 18-psi air pressure  flows from  the regulator through the firewall shutoff valve pneumatic line. This drives the firewall shutoff valve open and allows  engine  bleed-air  flow to  the cabin. An orifice on the inlet side of the firewall shutoff valve actuator controls the opening rate of the valve.

 Another pneumatic line,  mounted  in  parallel with the ambient air valve line, is connected to a pneumostat (a pneumatic thermostat). A solenoid valve in this line is used to either pass or block regulated air pressure to a bleed port in the pneumostat. The left hand (LH) landing gear safety switch electrically energizes the solenoid valve closed whenever the aircraft is on the ground, thus preventing any regulated air from venting overboard through the port. The pressure in the pneumatic line to the ambient air-modulating valve then builds to 18 psi, which drives the valve to the closed position, thus preventing any flow of ambient air into the flow control unit whenever the aircraft is on the ground. The cabin can then be heated as quickly as possible and any noxious fumes from the exhaust are prevented from entering the cabin.

 A third pneumatic line from the 18-psi regulator is connected to the ejector flow-control actuator. For normal ground operations, most of the control pressure to the ejector flow control valve is vented overboard through both the ambient sense aneroid and the pneumostat bleed port. The ejector will be in the minimum flow/maximum velocity position. In this configuration, enough engine bleed air is delivered to sufficiently heat the cabin for most ground operations.

Condition 3: Takeoff, Low Altitude

 

At takeoff, a printed circuit board, triggered by the left landing gear safety switch, removes power from the left ambient air-solenoid valve and the pressure in the line to the ambient air-modulating valve is bled off. An internal spring drives the valve open. The ejector, setting at the minimum flow/maximum velocity position, then draws ambient air through the check valve, mixes it with bleed air from the ejector, and sends it to the cabin for heating and pressurization. The additional ambient air mixed with the engine bleed air greatly increases the amount of air sent to the cabin.

 Six seconds after takeoff, the printed circuit board removes power from the right ambient air-solenoid valve. And, the right flow-control unit then operates just as the left unit did. This six-second delay between the flow-control unit operation precludes the simultaneous opening of the ambient air-modulating valves and prevents a sudden pressure surge into the cabin at the moment of lift-off. Either  flow-control  unit  is  capable  of maintaining the pressurization schedule of the aircraft during airborne operations

Condition 4: Aircraft in Flight, Medium Altitude

 

As the aircraft climbs into a colder and less dense atmosphere, the ambient sense aneroid expands and starts closing the orifice in the bleed line to the ejector flow-control actuator. Additionally,  the  pneumostat  bimetallic  discs react to the decreasing temperature and start to close off the port in the bleed line to the ejector flow  control  actuator.  Pressure  then  increases in the line and the ejector flow-control actuator piston compresses the spring and the ejector is positioned to an intermediate setting that allows higher bleed-air volume and less bleed-air velocity. This results in less demand for ambient air.

 The closing pneumostat port on the line to the ambient air-modulating valve increases the pres- sure in that line and the valve is driven to an intermediate  position that  restricts  the  flow of ambient to the flow-control unit. The result is that more hot bleed air and less cold ambient air is passed through the flow control unit.

Condition 5: Aircraft in Flight, High Altitude

As  the  aircraft  climbs  through  approximately 18,000 feet pressure altitude, the ambient sense aneroid  completely  closes  its  orifice. As the temperature goes below –30°F, the pneumostat bleed ports are fully closed. In this condition, the  ambient  air-modulating  valve  is fully closed and no ambient air is allowed into the flow-control unit. The ejector flow-control actuator drives the ejector to its maximum volume/ minimum velocity setting. Now, only hot bleed air is sent through the flow-control unit.

 

Bleed-Air Flow

 

The discharge bleed air leaving each flow-control unit flows to an air-to-air heat exchanger. Outside ram air cools any bleed air flowing through the heat exchanger.

 Variable, electrically-driven bypass valves at the out- let of each heat exchanger can be modulated from the cockpit to direct bleed air through or around each exchanger for temperature control. Each heat exchanger may be used to reduce the temperature of the hot engine bleed air flowing through it, but cannot transfer enough heat to provide cool air to the cabin. Cooling is provided by a refrigerant-type, vapor-cycle air-conditioning system.

 After leaving the heat exchangers, bleed air flows into the inlet junction of the bleed-air distribution duct under the right cabin floor, and through a double check valve tee assembly, which prevents the loss of pressure should either engine fail. From the tee, the bleed air from both engines flows through a single duct to the forward plenum chamber just ahead and below the feet of the copilot.

PTP  Beech 90 Series	B1	LOC	FOT
	B2	LOC	FOT