Heated air for warming the cabin and for windscreen defrosting and demisting is provided by a combustion heater located in the aft fuselage behind the cabin baggage area. Air from the heater is ducted forwards along the cabin floor to outlets at each seat and to the inside of the windscreen in front of the two pilots.
The combustion heater burns aviation fuel from the normal aircraft fuel tanks at a rate of one-half US gallon every hour when in use. This should be considered when flight planning.
Operation of the cabin heater is controlled by a 3-position switch, located at the front-left of a control console between the pilots’ seats, labeled CABIN HEAT/FAN. Airflow and temperature are regulated by the two levers on the console. Another switch, marked DEFROST controls an additional blower, located under the windscreen, that boosts the airflow to the inside of the windscreen in front of the pilots.
- To allow air to enter the heating system at the heater inlet (item 6 in the diagram), the AIR INTAKE lever must be partially or fully set to OPEN.
- Whilst in flight, cool ram air would now enter the system, ventilate the cabin and serve to defog the windscreen. The air can be individually regulated at each cabin floor outlet.
- Whilst stationary on the ground, the CABIN HEAT switch can be rocked rearwards to FAN in order to activate the ventilation fan and draw external air through the system.
- If heated air is required, the CABIN HEAT switch should be rocked forwards. This will simultaneously start fuel flow to, and ignite, the combustion heater. To circulate the air, it will also activate the ventilation fan when the aircraft is on the ground.
- The flow of windscreen defrost air can be boosted by selecting the DEFROST switch to ON.
Two safety switches are activated when the AIR INTAKE lever is CLOSED to shut down the heater and deactivate the ventilation fan.
When the landing gear is retracted, the ventilation fan is deactivated so that, whilst in flight, the cabin air is circulated only by ram-air.
An overheat switch is located in the heater unit and acts as a safety device to shut down the heater should a malfunction occur. If this happens, a red HTR OVRHEAT warning is displayed on the CAS. A red reset button on the heater shroud can be reached through the bulkhead access panel at the rear of the cabin.
In order to prevent activation of the overheat switch during a normal shutdown of the heater:
- During ground operations, set the CABIN HEAT switch to FAN for 2 minutes with the AIR INTAKE lever in the OPEN position before selecting the CABIN HEAT switch to OFF.
- During flight, leave the AIR INTAKE lever OPEN for a minimum of 15 seconds after selecting the CABIN HEAT switch to OFF.
Whilst performing the last action of the After Landing checks, students often mistakenly open the overhead air intake, but (for the reasons given in the last paragraph) it is the cabin heater AIR INTAKE lever that must be opened.