Type Rating Knowledge — Cessna 172S NAV III
Cabin heating, ventilating and defrosting
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Where the heat comes from
There is no heater in this aeroplane. Cabin heat is outside air ducted through a SHROUD around the exhaust muffler, warmed by contact with the muffler’s outer wall, and then admitted to the cabin. Ventilating air is separate outside air that never goes near the exhaust system. Two push-pull knobs meter them: CABIN HT and CABIN AIR. Both are the double-button locking type, so they hold intermediate settings rather than being on or off. That is the whole architecture, and it explains both of the things worth knowing about this system: it costs the engine nothing, and the only thing between the exhaust gas and the air you are breathing is one sheet of metal.
- OUTSIDE AIR → SHROUD — Ram outside air ducted to the shroud around the exhaust muffler. All cabin heat on this aeroplane starts here.
- OUTSIDE AIR → VENT — Ram outside air for ventilation, which never passes near the exhaust system.
- MUFFLER SHROUD — Heating chamber formed by a shroud around the muffler. The muffler wall is the only separation between exhaust gas and the air you breathe.
- CABIN AIR VALVE — Valve admitting unheated ram air to the cabin manifold.
- CABIN HT — Push-pull, double-button locking knob. Full in is no heat; a quarter to half inch out is a little; full out is the most.
- CABIN AIR — Push-pull, double-button locking knob. Full out for ventilation — and pushed full IN is part of getting maximum heat.
- CABIN MANIFOLD — Manifold just forward of the pilot’s and front passenger’s feet, where heated and unheated air are mixed and distributed.
- FRONT OUTLETS — Outlet holes spaced across the manifold, forward of the front seat occupants’ feet.
- REAR DUCTS (2) — Two ducts from the manifold, one down each side of the cabin, to outlets just aft of the rudder pedals at floor level.
- DEFROSTER OUTLETS — Two ducts from the same manifold to outlets near the lower edge of the windshield, each with a knob working a sliding valve.
- ADJ VENTILATORS — Separate adjustable ventilators — one near each upper corner of the windshield, two for the rear cabin. They are fed independently of the manifold.