Technical data

13773-001
21-40
Page 1
HEAT AND DEFROST SYSTEM
1. DESCRIPTION
Heated air is available at floor level for all occupants. Positive pressure inside the engine cowling causes
airflow from the inlet at the rear engine baffle to the engine exhaust muffler. The airflow is then ducted to
the heat box on the firewall. When the heat box damper is closed, the heated air remains in the engine
compartment. When the heat box damper is open, heated air flows from the heat box into the cabin air ple-
num mixing chamber. The heated air is conditioned in the cabin air plenum by mixing it with fresh air (if
desired) from the fresh air inlet on the right wing. From the cabin air plenum, the conditioned air is ducted
to either the defroster or passengers. Defroster outlet holes are located in the glareshield just aft of the
windshield. Forward passenger conditioned air enters the cabin through outlets in the kick panel at each
position. Conditioned air for rear passengers is ducted to outlets on the sidewalls near floor level.
The amount of heated air allowed into the air mixing plenum is controlled by rotating the Cabin Heat Con-
trol, located inboard of the Cabin Air Selector. The control is mechanically linked to a damper in a heater
box between the heater muff and the cabin air plenum mixing chamber. Rotating the control full counter-
clockwise (HEAT OFF) bypasses heated air from the heater muff into the engine compartment. Rotating
the control clockwise opens the door in the heater box allowing heated air to enter the mixing plenum. The
proportion of heated air to ventilation air mixed in the cabin air plenum mixing chamber is regulated by
manipulating the Cabin Heat Control and Cabin Cold Control to obtain the desired temperature and flow.
The pilot and co-pilot control the conditioning (mixing) of the heated and fresh air.
2. MAINTENANCE PRACTICES
A. Heat Box
The heat box is mounted in the engine compartment on the forward right-hand side of the firewall.
(1) Removal - Heat Box
(a) Remove engine cowling to gain access to the heat box. (Refer to 71-10)
(b) Remove hose clamp securing duct to heat box. Remove duct.
(c) Disconnect heater cable from heat box.
(d) Remove screws and washers securing heat box to the firewall. Remove heat box.
(2) Installation - Heat Box
(a) Acquire necessary tools, equipment, and supplies.
(b) Solvent clean heat box and firewall mating surfaces. (Refer to 20-30)
(c) Apply silicone sealant (fay seal) to the back side of heat box. (Refer to 20-10)
(d) Secure heat box to the firewall.
(e) Rotate control knob to the full heat position.
(f) Position heat box flap in the down position.
(g) Connect heater core wire to heat box cable stop.
(h) Secure duct to heat box with hose clamp.
Description P/N or Spec. Supplier Purpose
Cable Ties 8 inch Any source Retain air duct
Dow Corning Silicone Seal-
ant
RTV 736 Any Source Provide air-tight
seal between cabin
and engine com-
partment
30 Nov 2000