Type Rating Knowledge — Cessna 172S NAV III

Cabin heating, ventilating and defrosting

Two push-pull knobs, one manifold, and a heating system whose only separation from the exhaust gas is the wall of the muffler — which is why the cabin environment module and the carbon monoxide detector are the same subject.

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Cabin heating, ventilating and defrosting

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.

Redrawn from POH 172SPHBUS §7, Cabin Heating, Ventilating and Defrosting System (Figure 7-8) — structure restated, not traced. C172S NAV III.

What actually heats the cabin air on a C172S?

Outside air passed through a shroud around the exhaust muffler. Right. It is a heat exchanger with the exhaust on one side and your air on the other, which is efficient, simple, and the reason carbon monoxide is a live concern on this type.

What does "double button locking type" tell you about the CABIN HT and CABIN AIR knobs?

They hold intermediate settings — you can select part heat, not just on or off. Right, and the POH uses that: a quarter to a half inch out for a small amount of heat, further out for more.

One manifold, three destinations — and a fourth path that bypasses it

Heated and unheated air meet in a cabin manifold just forward of the front occupants’ feet, and everything the knobs control is distributed from there. Front cabin air comes from outlet holes spaced across the manifold itself. Rear cabin air comes from two ducts, one down each side of the cabin, to outlets just aft of the rudder pedals at floor level. Windshield defrost air comes from two more ducts to outlets near the lower edge of the windshield, each with its own knob working a sliding valve, so defrost airflow is regulated separately from how much air the manifold is getting. Then there is the fourth path: the separate adjustable ventilators — one near each upper corner of the windshield and two for the rear cabin — which are fed independently and are not affected by the CABIN AIR knob at all.

Where does the windshield defrost air come from?

Two ducts leading from the same cabin manifold, each with a knob working a sliding valve at the outlet. Right. Defrost is a tap off the manifold, so it competes with cabin heat for the same air — and its own knobs decide how much of it goes to the windshield.

You push CABIN AIR fully in and the eyeball vent by your head is still blowing cold air. Why?

The adjustable ventilators are supplied independently of the manifold — the CABIN AIR knob does not control them. Right. There are separate adjustable ventilators at the upper corners of the windshield and in the rear cabin, and you close those at the vent itself.

Cabin environment controls (POH 172SPHBUS §7)
You want	CABIN HT	CABIN AIR
Ventilation	Full in	Full OUT
A small amount of heat	Out approximately 1/4 to 1/2 inch	As required
More heat	Pulled out further	As required
Maximum heat	Full OUT	Full IN
No heat at all	Full IN	As required

It is a cold morning and you want as much heat as the system will give. What do you select?

CABIN HT full out and CABIN AIR full IN. Right, and the second half is the counter-intuitive one: closing the cold air is part of getting maximum heat, because unheated air admitted to the manifold dilutes what the shroud is delivering.

The failure this system has, and what the aeroplane does about it

Because cabin heat is shroud heat, a defect in the muffler wall does not fail the heating system — it contaminates it, with exhaust gas that contains carbon monoxide and gives no warning of its own. CO is colourless and odourless, and its early effects are exactly the ones that stop you diagnosing it. So this aeroplane carries a detector: a single unit behind the instrument panel, powered by the DC electrical system and integrated into the G1000. At 50 parts per million or greater it puts a FLASHING CO LVL HIGH warning in the PFD annunciation window with a continuous tone. Pressing the softkey below WARNING silences the tone and leaves the annunciation steady; it stays until the level drops below 50 PPM, and then the alarm resets itself. Two other messages exist in the alerts window: CO DET SRVC when the system finds a problem needing service, and CO DET FAIL when the interface between the G1000 and the CO system has broken.

CO LVL HIGH is flashing on the PFD with a continuous tone. What is the first thing to do about the aeroplane, as opposed to the annunciation?

Shut off the cabin heat — push CABIN HT full in — and ventilate, because the heat path is the only route exhaust gas has into the cabin. Right. Killing CABIN HT closes the contaminated path, and every ventilating path stays available because none of it passes near the muffler.

What does the CO annunciation do once the level falls back below 50 PPM?

The alarm resets automatically. Right — it is self-clearing, which also means a cleared annunciation is not evidence that anyone fixed anything.

You move from this aeroplane to a 1970s 172 with no CO detection system. What changes?

Only the warning. The heating path is the same exhaust-shroud arrangement, so the failure is identical and nothing will announce it. Right, and that is the point worth carrying between types: the annunciation is an addition to an old failure mode, not a replacement for it.

Cabin heating, ventilating and defrosting