The SA227 family of aircraft are low wing monoplanes, built by Fairchild
Aircraft Corporation. These aircraft include the Merlin IIC, Merlin IVC, and the Metro III. These three aircraft are powered by two turboprop engines equipped with constant speed, full feathering, four blade, reversible propellers made by Dowty-Rotol, or McCauley propellers.
The cabin is pressurized, heated and cooled by bleed air which is ducted through valves, heat exchangers, and refrigeration unit. The air conditioning system supplies cold, hot, and fresh air to both the cockpit and the cabin. These aircraft have two independent systems; both are capable of complete air conditioning. Bleed air is supplied by each engine to drive cooling turbines that provide the cool air. The bleed air is routed to the center section of the aircraft and mixed with cool air to provide hot air. Fresh air is supplied by a blower and motor assembly located in the nose baggage compartment. Either bleed air system may be operated on the ground when the respective engine is operating. Ducts within the fuselage distribute the airflow to the passengers and the crew. An automatic temperature control system senses and regulates the temperature within the aircraft.
The cabin is pressurized to increase passenger comfort. The pressurization system automatically compensates for increasing aircraft altitude by maintaining cabin altitude as near to field elevation as possible. The cabin altitude will remain at sea level until airplane altitude reaches approximately 17,000 feet. The pressurization system also provides for the emergency dumping of cabin pressurization, automatic over pressurization relief, and negative relief.
The pressurization system, within the limits available, maintain 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. A safety dump valve is used to manually dump cabin pressure. The entire fuselage, with exception of the nose baggage compartment, is pressurized. The outflow valve 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.
The heating and cooling air conditioning system, the pressurization control system, and the fresh air system comprise the environmental control system (ECS).
Bleed air supplied from the engines, mixed with cold air from the cooling turbines, is used for heating the cabin and cockpit. The heating system consists of two hot air mixing valves controlled by the temperature control system. Each valve controls the introduction of hot air into the cold air duct.
An orifice plate is installed on all later model aircraft at the hot air entry side of the Hot Air Mixing Valve, between the hot air line flange and the valve gasket. The orifice reduces noise by restricting airflow.
By reducing the airflow, the cabin will take longer to get warm. If you need to heat the cabin faster during the winter months, remove the orifice plate and reinstall it during the warmer months. The orifice plate may be removed permanently.
A cooling turbine is installed in each wing. Bleed air from the flow control valve drives the turbine. Cooled bleed air is used to cool and pressurize the cabin. Other systems remove moisture from the cooled air and help prevent ice accumulation in the system. Air enters the turbines and passes through an air-to-air heat exchanger. The cooling turbine inlet and outlet are located in the wing near the turbine. RAM air enters the heat exchanger and cools the bleed air. The inlet transition duct features a spring-loaded door to supply air to the heat exchanger in the event the inlet is blocked. Bleed air is routed through the heat exchanger and directed onto the turbine wheel. The turbine wheel and impeller are connected to a common shaft. The rotation of the impeller draws air through the inlet and over the heat exchanger coils. Air passing over the heat exchanger coils cools the bleed air. A sump housing assembly contains oil to lubricate the turbine wheel and impeller bearings.
No regularly scheduled maintenance is required on the hot air mixing valve. The operation of the valve may be checked by placing the mode control in AUTO and moving the auto temperature control from COLD to HOT. The mixing valves should move near full travel with a smooth steady motion. Watch each valve for evidence of hang up or jerky movement.
The cooling system is to be check every 200 hours, or at recommended inspection intervals, inspect the turbine for security of installation. Check the hoses and lines attached to the turbine for condition a