Type Rating Knowledge — Cessna 172S NAV III
Engine
The Lycoming IO-360-L2A as installed in the Cessna 172S: 180 horsepower at 2700 RPM, fuel injected, air cooled and wet sump. Its controls, the G1000 engine indications and the limits behind their coloured bands, the oil circuit and the bypass valve that keeps oil flowing when the filter blocks, and the alternate air door that costs you ten percent of your power.
The engine itself
Eight words that describe the whole engine
The 172S is powered by a direct drive, horizontally opposed, four cylinder, overhead valve, air cooled, fuel INJECTED engine with a wet sump lubrication system: a Lycoming IO-360-L2A rated at 180 horsepower at 2700 RPM. Each of those words rules something out. Direct drive means the propeller turns at engine speed, with no reduction gearbox — which is why the tachometer is also your propeller speed. Air cooled means there is no radiator or coolant to check, and cooling depends on airflow, so it depends on airspeed. Fuel injected means there is no carburettor and therefore no carburettor ice and no carburettor heat control to find in the cockpit. Wet sump means the oil lives in a sump under the engine and returns there by gravity, with no separate oil tank.
- STARTER — Starter, on the front of the engine — the one accessory here that CONSUMES electrical power rather than serving another system.
- ALTERNATOR 60A — Belt driven 60 A alternator on the front of the engine — belt driven, which is a failure mode a gear-driven accessory does not have.
- PROPELLER 76 IN — Two bladed fixed pitch McCauley 1A170E/JHA7660, 76 inches. Direct drive: propeller speed equals engine speed, so the tachometer reads both.
- IO-360-L2A 180 HP @ 2700 RPM — Direct drive, horizontally opposed, four cylinder, overhead valve, air cooled, fuel injected, wet sump. Everything on this diagram is driven by it.
- DUAL MAGNETOS — Two engine driven magnetos on the rear accessory case — self-generating, which is why the engine keeps running with the master switch off.
- VACUUM PUMP — Engine driven vacuum pump on the rear accessory case — it drives the standby attitude indicator, so it is the engine’s stake in the instrument system.
- FUEL PUMP — Engine driven fuel pump on the rear accessory case — the fuel system’s normal supply, needing no electrical power.
- FULL-FLOW OIL FILTER — Full flow oil filter on the rear of the accessory case, holding the ninth quart of oil.
Which cockpit control does a 172S NOT have, and why?
Carburettor heat — the engine is fuel injected, so there is no carburettor to ice up. Right, and this is the single most common wrong-aeroplane mistake in a 172 cockpit. Injected engines have their own induction icing concern — the alternate air door — but no carb heat control.
A standard 172R and a 172S both have the Lycoming IO-360-L2A. What differs?
The rating: 160 hp with a 2400 RPM limit on the R, 180 hp at 2700 RPM on the S. Right for a standard R — same engine model, different certificated rating and engine speed limit. Note the word standard: certain R serials modified by Cessna kit MK172-72-01 are approved at 180 hp and 2700 RPM, which is why an engine model number is never a complete answer about a 172.
What is bolted to it
The accessories matter more than the engine internals for systems purposes, because each of them is another system’s single point of failure. On the FRONT of the engine: the starter, and the BELT DRIVEN alternator — belt driven, which is a failure mode a gear-driven alternator does not have. On the rear accessory case: dual magnetos, the vacuum pump, the engine driven fuel pump, and the full flow oil filter. That list is the map between this module and four others: the alternator is the electrical system’s primary source, the vacuum pump drives the standby attitude indicator, the engine driven fuel pump is the fuel system’s normal supply, and the magnetos are why the engine keeps running with the master switch off.
The master switch is turned off in flight. What happens to the engine?
It keeps running — the magnetos are engine driven and need no electrical power. Right. That is exactly why MASTER OFF is a usable load-shedding action after an alternator failure, and also why the MAGNETOS switch must be OFF before anyone touches the propeller.
Throttle, mixture, magnetos and starter
Two knobs, and how to tell them apart without looking
Engine power is set with the THROTTLE: a smooth black knob at the centre of the instrument panel below the standby instruments, open forward and closed full aft. A friction lock at its base is rotated clockwise to increase friction and counterclockwise to decrease it — it stops the throttle creeping, and it is worth setting rather than fighting. Mixture is the RED knob immediately to the right of the throttle, with raised points around its circumference so it is identifiable by feel alone: full forward is rich, full aft is IDLE CUTOFF. It has a lock button in the end of the knob; press it for rapid or large movements, and rotate the knob for small adjustments — clockwise moves it forward, counterclockwise aft. Note what idle cutoff means: the normal way to shut this engine down is with the mixture, not the magnetos.
How is the mixture control identifiable in the dark, by hand alone?
It is the knob with raised points around its circumference, to the right of the smooth black throttle. Right — shape and position, not colour, are what your hand reads. The red is for your eyes; the raised points are for your fingers.
Two magnetos, four plugs, one rotary switch
Ignition comes from two engine driven magnetos firing two spark plugs in each cylinder. The split is deliberate and diagonal: the LEFT magneto fires the upper left and lower right plugs, and the RIGHT magneto fires the lower left and upper right. Normal operation is on BOTH, because dual ignition burns the mixture more completely — which is why a single-magneto run shows an RPM drop rather than no change. The MAGNETOS switch is a rotary switch on the left switch and control panel, labelled clockwise OFF, R, L, BOTH and START. R and L are for checking and emergency use only. Turning to the spring-loaded START position, with the MASTER switch ON, closes the starter contactor and cranks the engine; releasing it returns the switch to BOTH automatically.
- ENGINE DRIVE — The engine itself drives both magnetos. They generate their own current, so the ignition system needs nothing from the aeroplane’s electrical system.
- MAGNETOS SWITCH — Rotary MAGNETOS switch on the left switch and control panel, labelled clockwise OFF, R, L, BOTH, START. R and L are for checking and emergency use only.
- LEFT MAGNETO — Left magneto — fires the UPPER LEFT and LOWER RIGHT spark plugs.
- RIGHT MAGNETO — Right magneto — fires the LOWER LEFT and UPPER RIGHT spark plugs.
- UPPER L + LOWER R PLUGS — One plug in each of the four cylinders, on the upper left and lower right positions — the left magneto’s diagonal.
- LOWER L + UPPER R PLUGS — The other plug in each cylinder, on the lower left and upper right positions — the right magneto’s diagonal. Two plugs per cylinder, four cylinders, eight plugs.
Why does the START position need the MASTER switch on, when the magnetos do not?
The starter is an electrical load fed from the battery; the magnetos generate their own spark. Right. Cranking needs the battery, running does not. It is the cleanest illustration of where the electrical system stops mattering to this engine.
A magneto check shows no RPM drop at all on L or R. What does that suggest?
A fault — a missing drop suggests a magneto that is not being switched off, which means an ignition circuit that may be live regardless of the switch. Right. Dual ignition means selecting one magneto should ALWAYS cost some RPM, because you have made the burn less complete. No drop is a finding, not a pass.
Engine indications and limits
The EIS: one strip, three pages
Engine indications come from the G1000 Engine Indication System, a vertical strip shown on the left of the PFD during engine start and on the MFD in normal operation — and if either display fails, the EIS moves to the survivor. There are three pages, selected with the ENGINE softkey. The ENGINE page carries tachometer, fuel flow, oil pressure, oil temperature, EGT, vacuum, fuel quantity, engine hours, bus voltages and battery currents. The LEAN page shows EGT and CHT for ALL FOUR cylinders at once, which is the page you lean on. The SYSTEM page gives numbers for the parameters the ENGINE page shows only as indicators, plus fuel used and fuel remaining. All of it comes through the engine and airframe unit forward of the instrument panel, which collects the sensor signals and feeds the displays — one box between every engine sensor and everything you see.
- RPM SENSOR — Speed sensor on the engine tachometer drive accessory pad — sends a digital signal, not a mechanical cable drive.
- FUEL QTY — One fuel quantity sensor per tank. Their measurement range ends at approximately 24 gallons, which is why a visual check of each tank is required.
- OIL P / T — Oil pressure transducer on the forward oil pressure port, and the oil temperature sensor in the oil filter adapter.
- EGT / CHT — A thermocouple in each cylinder head and in each cylinder’s exhaust pipe — four of each, which is what the LEAN page displays.
- FUEL FLOW — Turbine type fuel flow transducer between the fuel servo and the flow divider — the source of FFLOW GPH, and of the calculated GAL USED and GAL REM.
- LOW OIL SW — A SEPARATE low oil pressure switch. It drives the OIL PRESSURE annunciation directly, independently of the indicating chain — a second opinion by design.
- ENGINE + AIRFRAME UNIT — The engine and airframe unit, forward of the instrument panel. It receives every engine and system sensor signal and feeds the displays — one box between all of them and you.
- PFD — Primary flight display — carries the EIS strip during engine start, and the annunciation window at all times.
- MFD — Multi-function display — carries the EIS strip in normal operation, with its ENGINE, LEAN and SYSTEM pages.
Which EIS page shows exhaust gas temperature for every cylinder?
The LEAN page. Right — EGT and CHT for all four cylinders, which is what makes it the leaning page. The ENGINE page shows a single EGT indicator with the hottest cylinder’s number in the pointer.
The green arc on the tachometer moves with altitude
The tachometer’s pointer runs 0 to 3000 RPM with a digital value in 10 RPM increments, and the top of its green arc is not a fixed number. For standard-day conditions the upper limit of the normal operating range is 2500 RPM from sea level to 5000 feet, 2600 RPM from 5000 to 10,000 feet, and 2700 RPM above 10,000 feet. The reason is that a normally aspirated engine cannot make full power at altitude, so a higher RPM is permissible where less power comes with it. Meanwhile the certificated maximum engine speed is 2700 RPM for all operations, and the display turns the pointer, value and label RED at 2780 RPM or more, flashing, and jumps back to the ENGINE page from wherever you were. There is also a static figure worth knowing for the power check: at full throttle on the ground, the static RPM range is 2300 to 2400 RPM.
You are level at 7000 feet. What is the top of the tachometer green arc?
2600 RPM. Right — 2500 up to 5000 feet, 2600 from 5000 to 10,000, 2700 above that. The arc moves because available power falls with altitude.
| Parameter | Range and markings | Limit |
|---|---|---|
| RPM | Pointer 0–3000; green tops out at 2500 / 2600 / 2700 by altitude | 2700 RPM max; red at 2780+ |
| Static RPM, full throttle | — | 2300–2400 RPM |
| Fuel flow | 0–20 GPH, green 0–12 GPH | — |
| Oil pressure | 0–120 PSI; red 0–20, green 50–90, red 115–120 | Min 20 PSI, max 115 PSI |
| Oil temperature | 75–250°F; green 100–245°F, red 245–250°F | Max 245°F (118°C) |
| Cylinder head temperature | 100–500°F; normal 200–500°F | Red line 500°F |
| EGT | 1250–1650°F, 50°F graduations | — |
| Fuel grades | 100LL (blue), 100 (green) | 100/100LL minimum grade |
The OIL PRESSURE annunciation appears on the PFD. Where did that come from?
A separate low oil pressure switch, not the oil pressure indicator. Right — the annunciation has its own switch, so it is an independent second opinion at 0 to 20 PSI rather than an alert derived from the gauge.
At idle with warm oil, indicated oil pressure sits just below the green band. Acceptable?
Yes — with the engine at normal oil temperature and at or near idle, below the green but above the lower red band is acceptable. Right, and the POH says so explicitly. What must not happen is an indication in the lower red band, and in cruise or climb below the green band is a different matter entirely.
Lubrication
A full pressure wet sump, drawn as the loop it is
Lubrication is a full pressure, wet sump system. Oil is drawn from the sump through a filter screen on the end of a pickup tube to the engine driven oil pump, then through the full-flow oil filter, a pressure relief valve at the rear of the right oil gallery, and a thermostatically controlled remote oil cooler. From the cooler it is circulated to the left gallery, and the engine parts are lubricated from the galleries. After that the oil RETURNS TO THE SUMP BY GRAVITY — no scavenge pump, no separate tank; that is what "wet sump" means. One component in the diagram exists purely for a failure case: the filter adapter carries a bypass valve, which sends lubricating oil PAST the filter if the filter becomes plugged, or if the oil is extremely cold.
- SUMP 8 QT — Wet sump on the bottom of the engine — eight quarts, with one more quart in the oil filter. Never operate on less than five quarts.
- DIPSTICK/FILLER — Oil dipstick and filler tube at the right rear of the engine case, reached through a door in the right side of the cowling.
- PICKUP SCREEN — Filter screen on the end of the pickup tube — the first thing the oil passes on its way out of the sump.
- OIL PUMP — Engine driven oil pump — the pressure source for the whole system.
- FULL-FLOW FILTER — Full-flow oil filter on the rear of the accessory case, holding the ninth quart. Its adapter carries the bypass valve.
- BYPASS VALVE — Bypass valve in the filter adapter — sends oil PAST the filter if the filter plugs or the oil is extremely cold. Unfiltered oil beats no oil.
- PRESSURE RELIEF — Pressure relief valve at the rear of the right oil gallery — what holds indicated oil pressure inside the 50–90 PSI green band.
- OIL COOLER — Thermostatically controlled remote oil cooler — bypassed by its own thermostat until the oil is warm enough to need it.
- OIL GALLERIES — Left and right oil galleries — cooled oil enters the left gallery and the engine parts are lubricated from both.
- ENGINE — The bearings, cams and cylinder walls the galleries feed. Oil returns to the sump by gravity — no scavenge pump, which is what "wet sump" means.
The oil filter becomes plugged in flight. What does the system do?
The bypass valve opens and the engine receives unfiltered oil. Right — the design decision is that unfiltered oil is much better than none. Debris circulating is a maintenance problem; oil starvation is an immediate one.
Eight quarts, nine with the filter, never below five
The sump holds EIGHT quarts, with ONE ADDITIONAL quart in the oil filter — so a full engine holds nine, but the dipstick reads the sump. The dipstick and filler tube are at the right rear of the engine case, reached through a door in the right side of the cowling. Three operating numbers go with that: the engine should not be operated on less than FIVE quarts; fill to eight quarts for normal flights of less than three hours; and for extended flight, fill to eight quarts by dipstick indication. Filling above that simply loses oil through the breather, which is why "topped right up" is not a virtue here. (The type certificate lists the capacity as 8.0 quarts with 3.0 usable — that is a weight-and-balance figure, not a quantity you manage.)
The dipstick reads six quarts before a two-hour flight. What now?
Add oil to eight quarts — the POH’s figure for flights under three hours. Right. Six quarts is above the five-quart floor, so it is not unairworthy, but eight is the number the POH gives for this flight.
Induction, exhaust and cooling
The alternate air door, and the ten percent it costs
Induction air enters as ram air through an intake in the lower front of the cowling and passes through an air filter that removes dust and foreign matter. Behind the filter is an air box fitted with a SPRING-LOADED ALTERNATE AIR DOOR. If the induction filter becomes blocked — by ice, slush or debris — the suction created by the engine opens that door by itself and draws unfiltered air from inside the lower cowl area. Note what that means: there is no cockpit control and no annunciation for it. The evidence is a power loss of approximately 10% at full throttle. After the air box, induction air passes into the fuel/air control unit under the engine and is ducted to the cylinders through intake manifold tubes.
- INDUCTION INTAKE — Ram air intake in the lower front of the engine cowling — the induction system’s only normal air source.
- COOLING INTAKES (2) — Two ram air intake openings in the front of the cowling, feeding engine cooling — separate from the induction intake.
- AIR FILTER — Induction air filter — removes dust and foreign matter. Blocking it is what opens the alternate air door.
- ALT AIR DOOR — Spring loaded alternate air door in the air box. Engine suction opens it if the filter blocks, drawing UNFILTERED air from inside the lower cowl. No cockpit control, no annunciation.
- BAFFLING — Baffling that directs cooling air from above the engine, around the cylinders and the rest of the engine.
- AIR BOX — Air box behind the filter, carrying the alternate air door — where filtered and unfiltered air paths meet.
- FUEL/AIR CONTROL — Fuel/air control unit under the engine. Induction air passes through it, and it meters fuel in proportion to that airflow — which is why less air means less power.
- COWL EXIT — Opening at the bottom aft edge of the cowling where cooling air leaves. Cooling depends on airflow, so it depends on airspeed.
- CYLINDERS (4) — The four cylinders — fed induction air through intake manifold tubes, cooled by the baffled airflow, and exhausting through a riser each.
- MUFFLER + SHROUD — Common muffler below the engine, overboard through a single tailpipe. A shroud around it forms the heating chamber — the muffler wall is the only separation between exhaust gas and cabin air.
- CABIN HEAT — Outside air, heated by the muffler shroud, supplied to the cabin. All cabin heat on this aeroplane is exhaust-shroud heat.
The induction air filter ices over in flight. What do you do?
Nothing — the alternate air door opens on engine suction by itself, at a cost of about 10% power. Right. It is automatic and there is no cockpit control for it, which is why the symptom to recognise is an unexplained power loss of roughly ten percent.
Where cabin heat actually comes from
Exhaust gas from each cylinder passes through a riser assembly to a common muffler below the engine and overboard through a single tailpipe. Around the outside of that muffler is a shroud, and outside air is supplied to the shroud to form a heating chamber — the air heated there is what is supplied to the cabin. That is the fact to carry out of this unit: cabin heat is exhaust-heated air, separated from exhaust gas by nothing more than the muffler wall. It is why an exhaust system defect is a carbon monoxide problem and not merely a performance one, and why the aeroplane carries a CO detection system at all. Engine cooling is separate: ram air through two intake openings in the front of the cowling, directed around the cylinders and the rest of the engine by baffling, exiting at the bottom aft edge of the cowling. Because it is airflow cooling, low airspeed at high power — a prolonged climb — is the hot case.
Why does an exhaust leak matter for the cabin, not just for engine performance?
Cabin heat is outside air warmed in a shroud around the exhaust muffler, so a muffler defect can put exhaust gas into the cabin air. Right. The muffler wall is the only separation between exhaust gas and the air you breathe, which is the reason for the CO detection system.
When is this engine most likely to run hot?
In a prolonged full-power climb at low airspeed. Right. Air cooling means cooling depends on airflow, and a climb combines maximum heat production with minimum airflow — which is why cruise climbs exist.
Fuel injection
From the engine driven pump to four nozzles
The injection system is five components: the engine driven fuel pump, the fuel/air control unit, the fuel manifold, the fuel flow indicator and air-bleed type injector nozzles. Fuel is delivered by the engine driven pump to the fuel/air control unit — the fuel servo — which proportions fuel flow to induction air flow; this is where the mixture control acts. Metered fuel then goes to the fuel manifold, also called the flow divider, on top of the engine, which uses spring tension on a diaphragm and valve to distribute fuel evenly to an air-bleed injector nozzle in the intake valve chamber of each cylinder. Between the servo and the flow divider sits a turbine-type flow transducer, and that is the thing producing the FFLOW GPH number on the EIS — worth knowing, because it measures flow to the engine, not fuel in the tanks.
- L TANK 28 GAL — Left integral wing tank — 28.0 gallons total, 26.5 usable, 1.5 unusable. Carries the check valve equipped overboard vent.
- VENT LINE — Interconnecting vent line between the tanks, with the overboard vent under the left wing. Complete blockage decreases fuel flow and eventually stops the engine.
- R TANK 28 GAL — Right integral wing tank — 28.0 gallons total, 26.5 usable, 1.5 unusable.
- SELECTOR BOTH/L/R — Three-position fuel selector — BOTH for takeoff, climb, landing and any prolonged slip or skid; LEFT or RIGHT is for level cruise only.
- RESERVOIR TANK — Fuel reservoir tank — everything above it is fed by gravity alone; it also receives the fuel return line.
- AUX FUEL PUMP — Electrically driven auxiliary fuel pump — priming, vapour suppression, and enough flow for maximum continuous power if the engine driven pump fails.
- SHUTOFF VALVE — Fuel shutoff valve, downstream of the auxiliary pump.
- FUEL STRAINER — Fuel strainer — one of the five sampling points drained before flight.
- ENG DRIVEN PUMP — Engine driven fuel pump — the normal supply for the injection system, needing no electrical power at all.
- FUEL/AIR CONTROL — Fuel/air control unit (fuel servo) under the engine — meters fuel in proportion to induction air flow. The mixture control acts here.
- FLOW SENSOR — Turbine type fuel flow transducer between the servo and the flow divider — the source of FFLOW GPH on the G1000 EIS.
- FLOW DIVIDER — Fuel distribution valve (flow divider) on top of the engine — spring tension on a diaphragm and valve distributes fuel evenly to the nozzles.
- INJECTOR NOZZLES — Air-bleed type injector nozzle in the intake valve chamber of each of the four cylinders.
Where does the mixture control physically act?
At the fuel/air control unit, which meters fuel in proportion to induction air flow. Right. The servo is where the fuel-to-air ratio is set; everything downstream just divides and delivers what it has metered.
A new engine is already run in
The engine run-in was accomplished at the factory, so a new 172S is ready for the full range of use — there is no restricted period to observe. What the POH does suggest is cruising at 75% power as much as practicable until a total of 50 hours has accumulated, or until oil consumption has stabilised, to ensure proper seating of the piston rings. That is a recommendation about ring seating rather than a limitation, and the distinction is the point: the number that constrains you is 2700 RPM, not 50 hours.
A club aeroplane has a factory-new engine with 12 hours on it. What applies?
Nothing limits you, but the POH suggests cruising at 75% power until 50 hours or until oil consumption stabilises. Right — a suggestion for ring seating, not a limitation. The run-in itself was done at the factory.
Read more on this system
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