The environmental system provides heat and ventilation
for the helicopter. The system consist of a heater and
fan mounted between an air inlet duct and exhaust
pipe, an ignition unit, control switches, a temperature
controller and thermostat, cockpit and cabin ducting,
and fuel delivery components.
The heater is located in a compartment on the right
forward end of the cabin, ahead of the cabin door. It is
capable of 200,000 btu/hr output.
A fuel control unit draws fuel from the right side fuel
system and controls the pressure and flow to the heater
The fan draws air from outside through the air inlet duct
and forces it through the heater. Some air is mixed with
fuel and ignited in the heater combustion chamber and
other air is heated and sent through the duct network.
The fan can also supply fresh air for ventilation. Exhaust
gases are sent through the exhaust pipe and discharged
outside, above and aft of the air inlet.
The ignition unit supplies a high voltage current that
produces a continuous spark in the heater combustion
chamber to ignite the fuel and air mixture. Unburned
fuel is drained overboard.
The thermostat maintains a steady temperature in the
cabin by causing a relay in the temperature controller to
open or close as cabin temperature fluctuates. In this
way, the controller controls the electrical circuit to the
heater fuel system, interrupting or resuming fuel flow,
The heater system is protected from overheating by
three thermostatic switches. A differential air pressure
switch shuts down the heater when there is not enough
air for safe operation.
Switches on the overhead panel control heater
operation. A temperature controller adjusts the cabin
thermostat setting. Push-pull controls in the cockpit
regulate air flow through the ducting.
THEORY OF OPERATION
The heater and fan are both controlled by a control
switch and a start button on the cockpit overhead
panel. The switch has three positions, labeled OFF,
VENT BLOWER ONLY, and HEATER ON. The button is
spring-loaded and is labeled HEATER START. When the
switch is set to HEATER ON and the button is pressed
and released, the system is set in operation as follows:
Relay 082K3 is energized by 28 volts from
the No. 2 dc bus through the normally closed
(NC) contacts of overheat switch 082A5. This
action energizes relay 082K2 and fan relay
082K1, connecting 115 volt 3-phase ac from
the No. 2 ac bus to the heater fan.
When the fan has been in operation for 10
seconds, time-delay (TD) relay 082K4 and
air pressure switch 082A4 will close. The
delay allows time for the fan to purge the
heater of combustible vapor and to build up
enough air pressure to activate the switch.
The closed TD relay and switch allow voltage
to close relay 082K5, operating master fuel
valve 082L1, the fuel pump (located in fuel
control unit 082A2) and ignition unit 082A3.
The closed relay also connects power to the
temperature controller and thermostat (See
The open master fuel valve unit allows fuel to
be delivered from the boost pump in the right
main tank to the fuel control unit. In the fuel
control unit, the fuel passes through a filter, a
pressure regulator and a pump. The pump
raises fuel pressure to 100 psi, controlled by
a pressure relief valve, and delivers it through
a solenoid valve to the heater. The solenoid
valve is energized through the temperature
controller and thermal cycling switch (See
Fuel enters the heater combustion chamber,
where it is atomized by a spray nozzle and
mixed with air provided by the fan through
the combustion air tube. The mixture is
ignited by a spark plug supplied with high
tension voltage from the ignition unit. The
remainder of the pressurized air flows around
the combustion chamber, picking up heat as
it does. The heated air is then distributed
through ducting to the cockpit and cabin. Air
flow direction and volume is controlled by
manual operation of valves in the ducting.
The temperature of the heated air is
controlled by the combined action of
CABIN TEMP SELECTOR switch 082S1,
temperature controller 082A1, and cabin