Type Rating Knowledge — Cessna 172S NAV III
Propeller
A two-bladed, fixed-pitch, one-piece forged aluminium McCauley, 76 inches across — and everything that follows from those five words: no propeller control in the cockpit, a tachometer that reads propeller speed as well as engine speed, a static RPM band that is a certificated limit, and a blade on which a nick is a structural finding.
The propeller
Five words that settle everything
The 172S propeller is two bladed, FIXED PITCH, one-piece forged aluminium alloy, anodized to retard corrosion, and 76 inches in diameter. Each phrase closes off a whole class of system. Fixed pitch means there is no propeller control in the cockpit, no governor, no oil path to a hub, and no blue lever — the blade angle is what the factory forged into it. One-piece forged means the blades are not separately mounted and not ground-adjustable: pitch cannot be altered by a mechanic either, only replaced. And because the engine is DIRECT DRIVE, with no reduction gearbox, propeller speed equals engine speed: the tachometer is telling you about both at once.
- 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.
What is absent from the cockpit as a direct result of the fixed-pitch propeller?
A propeller control — there is no blade angle to select. Right. Power is set with the throttle alone, and RPM follows from throttle position, airspeed and altitude rather than being commanded.
The tachometer reads 2400 RPM. How fast is the propeller turning?
2400 RPM — the engine is direct drive, so propeller and crankshaft speed are the same. Right, and it is why a single instrument serves both. On a geared engine you would need the reduction ratio to answer this at all.
What the type certificate actually certificates
The propeller is a certificated part of the aeroplane, not an accessory, and the type certificate data sheet is specific. For the Model 172S the approved propeller is the McCauley Model 1A170E/JHA7660, with a spinner to Cessna drawing 0550236. Two limits go with it. The diameter must be NOT OVER 76 inches and NOT UNDER 75 inches — the range exists because a blade can legitimately lose material to dressing out damage during its life, and 75 inches is the floor past which it is no longer the certificated propeller. And the static RPM at full throttle must be not over 2400 and not under 2300 — a number you can check yourself, from the cockpit, on any run-up.
What is the certificated static RPM range at full throttle?
2300 to 2400 RPM. Right, and it is in both the TCDS and POH §2. Note it is well below the 2700 RPM maximum: static means stationary, where the propeller is least efficient.
| Item | Value |
|---|---|
| Model | McCauley 1A170E/JHA7660 |
| Type | Two bladed, fixed pitch, one-piece forged aluminium alloy |
| Finish | Anodized, to retard corrosion |
| Diameter, as installed | 76 inches |
| Diameter limits | Not over 76 in, not under 75 in |
| Static RPM at full throttle | Not over 2400, not under 2300 |
| Spinner | Cessna drawing No. 0550236 |
On the run-up, full throttle gives 2150 static RPM on a standard day. What does that suggest?
Something is wrong — it is below the certificated 2300 static minimum, so the engine or propeller needs investigating before flight. Right. The static band is a limit, not a guideline, and a low figure points at the engine not making rated power. It is a maintenance question, not a takeoff decision.
A mechanic offers to adjust the propeller pitch to improve your climb. What is your answer?
It cannot be done — the propeller is one-piece forged, so its blade angle is not adjustable at all. Right. Not fixed-in-flight but adjustable on the ground: fixed, full stop. A different pitch means a different, separately approved propeller.
An aeroplane is presented as a 172S but has a 75-inch McCauley 1C235/LFA7570. What is the most likely explanation?
It is a 172R, not a 172S. Right — the 1C235/LFA7570 is only ever an R’s propeller. Note the asymmetry: this propeller rules an S OUT, but finding the S’s 76-inch 1A170E does not rule an R out, because a kit-modified R carries the same one.
Living with a compromise
A fixed-pitch propeller is a single compromise chosen once, at the factory, for the whole flight. A propeller efficient in the climb is too coarse to let the engine reach useful RPM at cruise speed; one efficient at cruise leaves the engine unable to reach its rated RPM at low airspeed on takeoff. The 172S sits between the two, and the consequence is something you can watch on the tachometer: at a FIXED throttle position, RPM RISES as the aeroplane accelerates and FALLS as it slows, because the blade’s angle of attack is set by how fast the air is arriving. That is why RPM alone is not a power setting on this aeroplane the way manifold pressure and RPM together are on a constant-speed installation, and why the green arc’s top moves with altitude.
You hold the throttle still and lower the nose. What does the tachometer do?
RPM rises, because the increasing airspeed reduces the blade’s angle of attack and unloads the propeller. Right — the propeller is a wing whose angle of attack depends on how fast the air arrives. It is also why an overspeed is possible in a dive at a cruise throttle setting.
Why would a "climb propeller" be a poor choice for this aeroplane as certificated?
A finer pitch would let the engine reach its RPM limit early and cost cruise performance. Right — this is the compromise. Finer for climb, coarser for cruise, and the certificated propeller is the chosen middle. Only an approved propeller may be fitted in any case.
Why a nick is not cosmetic
The blades are one-piece forged aluminium alloy, and every square inch of them is under enormous cyclic load: a blade tip on a 76-inch propeller at 2700 RPM is travelling at several hundred miles per hour, and each rotation loads and unloads the blade. Surface damage matters in that environment because a nick or a gouge concentrates stress at its root, and a crack that grows from a stress concentration in a spinning blade is not a survivable failure. That is why propeller damage is assessed and dressed out by maintenance rather than filed smooth by an owner, and why the diameter limits exist as a floor on how much material may be removed in the process. The anodizing is part of the same picture: it is there to retard CORROSION, and corrosion pits are stress concentrations too.
You find a small nick in the leading edge of a blade on the walkaround. What now?
It is a maintenance item — a nick concentrates stress in a heavily loaded, high-speed part, and dressing it out is a mechanic’s job with a diameter limit to respect. Right. The right question is not "is it deep" but "who is qualified to assess it", because the consequence of getting it wrong is a blade failure in flight.
What is the anodized finish for?
To retard corrosion. Right, and it matters structurally rather than cosmetically: corrosion pits concentrate stress in the same way a nick does.