6 21 fcom sa227 air conditioning

AIRCRAFT SYSTEMS

AIR CONDITIONING SA227‐ SERIES/FCOM/VTM 6.21.1 THE AIR CONDITIONING ENVIRONMENTAL CONTROL SYSTEM

System supplies cold air and conditioned air to the cabin and cockpit. Two independent systems are provided. Bleed air is supplied by each engine to drive cooling turbines which provide cool air for the aircraft. Hot bleed air is routed to the center section of the aircraft and mixed with cold bleed air to provide conditioned air. Either bleed air system may be operated on the ground when the respective engine is operating. Ducts within the fuselage distribute the air to the passengers and crew. An automatic temperature control system senses and regulates the temperature within the aircraft. Fresh air is supplied by a blower and motor assembly located in the nose baggage compartment. The aircraft cabin is pressurized. The pressurization system automatically compensates for increasing aircraft altitude by maintaining cabin altitude as near to selected elevation as possible. The cabin altitude will remain at sea level, when selected, until aircraft altitude reaches approximately 16,800 feet pressure altitude. The pressurization system provides automatic over pressurization and negative pressure relief.

Cabin pressure can be dumped manually in emergency situations. The air conditioning system, the pressurization control system and the fresh air system comprise the environmental control system (ECS).

Figure 6.21‐1

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AIRCRAFT SYSTEMS AIR CONDITIONING TEMPPERATURE CONTROL UNIT

SA227‐ SERIES/FCOM/VTM

6.21.2

Figure 6.21‐2

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AIRCRAFT SYSTEMS

AIR CONDITIONING SA227‐ SERIES/FCOM/VTM 6.21.3 TEMPERATURE CONTROL

The electrically powered temperature control system is used to maintain the cabin temperature at pilot selected levels. The system may be operated in either of two modes, automatic or manual. In the automatic mode, the pilot selects a desired cabin temperature. The temperature control system will monitor the cabin temperature and adjust the temperature of the conditioned air introduced into the cabin. In the manual mode, the pilot controls the temperature of conditioned air supplied to the cabin.

The principal system components are as shown in Figure 6.21‐2.

HEATING SYSTEM

Bleed air supplied by the engines, mixed with cold air from the cooling turbines, is used for heating the cockpit and cabin. The amount of engine bleed air introduced into the conditioned air ducts is controlled by two hot air mixing valves. These mixing valves are positioned electrically. Signals controlling the valve positioning come from the temperature control system.

COOLING SYSTEM

An air cooling turbine is installed near the nacelle in each wing leading edge. Cold, non‐conditioned air from each cooling turbine enters the cabin and is ducted fore and aft to cold air outlets at crew and passenger stations. Formerly a customer option, a cold air dump valve is located in the cold air ducting under the floor on each side of the aircraft, aft of the wing. The dump valves allow the cold air to be ducted to the cabin and cockpit cold air outlets (dump valve closed) or to flow rearward through ducts which terminate in the tail cone and dump a major portion of the cold air near the outflow valve (dump valve open). The cold air dump valves are mechanically controlled flapper valves which can be modulated between full open and full closed by push‐pull cables. The cable controls are located on the floor at the entrance to the cockpit.

MOISTURE AND CONTAMINANT CONTROL SYSTEM

Moisture and other contaminants are removed from engine supplied bleed air before the air enters the cabin. The cooling turbine output air, depending upon ambient conditions, will usually be below freezing in temperature and contain some percentage of water. A water separator is installed downstream of the cooling turbine to remove water from the air. Since the air exiting the turbine is below freezing, some of the water in the air will begin to freeze, usually inside the water separator. If continued freezing is allowed, the water separator will eventually become blocked. A deicing system is installed to prevent blocking of the water separator. The deicing system consists of a slave valve that controls the introduction of hot bleed air into the cold air duct and a master valve that controls the operation of the slave valve. The master valve senses the outlet air temperature at the water separator. The master valve will control the slave valve as necessary to prevent freezing of the water separator.

BLEED AIR AND CONTROL

Air is extracted from a pad on the left hand side of each engine case for use in the environmental control system, surface deice system, and vacuum system. Part of the surface deice system provides pressure for the inflatable door seals. Before passing through the firewall, the bleed air is routed to a heat exchanger. The heat exchanger lowers the temperature of the bleed air approximately 100°F (38°C),

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AIRCRAFT SYSTEMS

AIR CONDITIONING SA227‐ SERIES/FCOM/VTM 6.21.4 thereby increasing the service life of the ECS components. Bleed air for use in the engine anti‐icing system is routed and controlled through separate lines and valves.

A flow control valve is used to regulate the bleed air flow. Since the extraction of bleed air will cause a loss in engine power, the amount of air extracted must be carefully regulated. The flow control valve is calibrated to extract a preset amount of bleed air from the engine under all operating and ambient conditions. When the cockpit bleed air switch is moved to the OFF position, the flow control valve will close to stop bleed airflow to the ECS. The bleed air supplied to the engine anti‐icing and vacuum systems is not controlled or affected by the flow control valve.

FRESH AIR SYSTEM

A blower motor and associated distribution plumbing supply fresh air to the cockpit. The blower is located under the nose baggage compartment left hand floorboard. A check valve prevents the escape of cabin pressurization in flight. A switch actuated by the nose gear during retraction serves as a safety interlock to prevent operation of the fresh air fan in flight. An override position of the fresh air fan switch allows this interlock to be defeated in case the fan is needed in flight. The fan should not be operated with the cabin pressurized.

FRESH AIR BLOWER AND CHECK VALVE

Figure 6.21‐3

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AIRCRAFT SYSTEMS

AIR CONDITIONING SA227‐ SERIES/FCOM/VTM 6.21.5 PRESSURIZATION SYSTEM

The pressurization system, within the limits available, maintains the cabin of the aircraft at any selected pressure altitude equal to or lower than the aircraft altitude. In normal operation, the system controls the increase or decrease in cabin pressure and the rate at which these changes in pressure take place.

Safety features prevent the cabin from exceeding the maximum pressurization limit and from maintaining a negative pressure (cabin pressure less than ambient pressure). A safety dump valve is used to manually dump cabin pressure. The entire fuselage, with the exception of the nose baggage compartment, is pressurized. The outflow valve, which controls air leaving the fuselage, is located on the aft pressure bulkhead. The emergency dump valve is located on the forward pressure bulkhead. Normal airflow through the aircraft is rearward and out the outflow valve. With the emergency dump valve open, the airflow is forward.

PRESSURIZED VESSEL

Figure 6.21‐4

PRESSURIZATION SYSTEM COMPONENTS

CABIN ALTITUDE WARNING

The cabin altitude warning system is used to inform the pilot that cabin altitude has exceeded approximately 11,000 feet. Above this altitude supplemental oxygen will be required. Illumination of the cabin altitude warning light usually indicates a problem with the pressurization system.

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LOW SUCTION CABIN ALTITUDE GPU PLUG IN L SRL OFF R SRL OFF NOSE STEER FAIL

AIRCRAFT SYSTEMS

AIR CONDITIONING SA227‐ SERIES/FCOM/VTM 6.21.6 DUMP VALVE

The pressurization dump valve can be used to quickly depressurize the cabin should a malfunction of the pressurization system occur. The dump valve, located on the left hand side of the forward pressure bulkhead, is connected to a static source, the vacuum system, and the dump valve electrical circuitry.

Power is supplied to open the valve when the aircraft is on the ground. After takeoff, with the dump switch in NORMAL, the valve will close. Vacuum is supplied to the valve causing the valve to close slowly, thus preventing a pressure bump immediately after takeoff. Electrical power for the valve may be obtained from either essential bus by using the transfer switch. The dump valve also contains internal over pressurization relief that will cause the valve to open should the pressure differential between the cabin and ambient exceed 7.30, plus or minus .10 psi.

OUTFLOW VALVE

The outflow valve, installed on the aft pressure bulkhead, is used to control the flow of air out of the aircraft pressure vessel. The valve responds to pressure commands supplied by the pressurization control system through the pneumatic relay. If the maximum differential pressure between the aircraft cabin and ambient pressure exceeds 7.15 psi (49.3 kPa), the valve will open regardless of the command being supplied by the pressure control system. Also incorporated into the outflow valve is automatic vacuum relief capability. If the pressure within the aircraft cabin is lower than the ambient pressure, the outflow valve will open to equalize the pressure.

PNEUMATIC RELAY

The pneumatic relay is used to speed up the reaction time of the outflow valve to commands given by the pressurization control system. Small changes in pressure to the control connection of the relay produce large but corresponding changes in pressure to the outflow valve connection. The relay is located in the aft fuselage area.

MODE SELECTOR AND MANUAL CONTROL

Manual pressurization controls consist of the pressurization mode selector and manual rate control. The mode selector is a two position valve that controls the connection of the aircraft vacuum system to the pressurization system. In the AUTO position, vacuum is supplied directly to the ATMOS 3 port of the pressurization controller. In the MANUAL position, vacuum is supplied to the manual rate control.

The manual pressurization rate control is a needle valve that controls the amount of vacuum supplied to the outflow valve (through the pneumatic relay). Opening the valve (turning counterclockwise) supplies more vacuum to cause the outflow valve to open and decrease cabin pressure. Closing the valve (turning clockwise) supplies less vacuum to cause the outflow valve to close and increase cabin pressure. In comparison to the pressurization controller, the manual rate control provides very coarse adjustment.

When using the manual system, small adjustments must be made with the manual rate control knob and adequate time must be allowed for them to take effect. Opening the manual rate control with the aircraft pressurized can produce a very rapid depressurization that may be uncomfortable for passengers.

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AIRCRAFT SYSTEMS

AIR CONDITIONING SA227‐ SERIES/FCOM/VTM 6.21.7 PILOT’S OPERATING TIPS

CABIN PRESSURE CONTROLLER OPERATION

The cabin pressure controller operates to keep the cabin pressure at the pressure altitude selected on the controller. It is important to realize that the controller recognizes only standard day altitudes. Thus, if the controller is set to pressurize 100 feet above field elevation on a day when the altimeter setting is well above 29.92 in. Hg. (1013.2 milibars), the airplane will be several hundred feet in the air after takeoff with bleed air ON before pressurization begins. Similarly, with the same controller setting during landing approach to the same airport, the airplane will be depressurized well above touchdown and the crew and passengers will experience pressure build‐up at a rate equal to the rate of descent during the landing approach. Selecting a cabin altitude slightly below field elevation on high pressure days will eliminate the high rate of pressure increase during final approach.

Conversely, if it is a day with an altimeter setting well below 29.92 in. Hg. (1013.2 milibars), the selected cabin altitude should be higher than field elevation to avoid an objectionable pressurization bump immediately after liftoff with bleed air ON and to avoid touchdown with the airplane still pressurized.

During takeoff the bleed air may be turned off. Therefore, the pilot should set the cabin altitude after considering the altimeter setting and the altitude where he predicts he will be when he turns the bleed air switches ON, or AWI switch OFF, after takeoff. Turning the bleed air switches ON one at a time before reaching the altitude where pressurization will begin should prevent the “bump” associated with sudden pressurization.

With sea level selected on the cabin pressure controller, the cabin will maintain that altitude until the aircraft exceeds 16,800 feet pressure altitude.

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AIRCRAFT SYSTEMS AIR CONDITIONING

SA227‐ SERIES/FCOM/VTM

INTENTIONALLY LEFT BLANK

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6.21.8