Propeller, Fuel and Electrical System
These training notes cover the fundamental aspects of the fuel system, propeller, and electrical system in a Cessna 172, providing essential knowledge for new student pilots.
Fuel System
Fuel Tanks:
- Located in the wings of the airplane.
- Filled through openings on top of the wings, covered by caps.
- Cessna 172 uses "wet wings" or "integral fuel tanks," utilizing wing structure as the tank.
Fuel Lines and Vents:
- Fuel lines carry fuel from the tanks to the fuel system.
- Fuel caps are vented to allow air to replace consumed fuel during flight.
- An additional vent under the left wing allows for fuel expansion due to high temperatures.
- Possible fuel drips from the vent after topping off tanks in hot weather.
Drain Valves:
- Five drain valves on the bottom of each wing.
- Used to draw samples for checking water, sediment, and proper fuel type.
- Water or sediment settles at the bottom; sampling ensures they are drained out before reaching the engine.
Fuel Strainer and Reservoir:
- Fuel strainer filters fuel and traps water or sediment.
- Located at the lowest point in the fuel selector, fuel reservoir, and fuel strainer.
Fuel Types and Grades:
- Always use the fuel type specified by the airplane or engine manufacturer.
- Avoid using lower-grade fuel to prevent engine damage.
- Aviation gasoline (AVGAS) 100LL is blue and rated at 100 octane (low-lead).
- Jet fuel is clear or straw-colored, smells like kerosene; must not be used in gasoline engines.
- Automobile gasoline (MOGAS) may be used only if the airplane is approved for it.
Cessna 172 Fuel System Overview:
- Fuel flows by gravity from wing tanks to the three-position selector valve.
- From the selector valve to the reservoir tank, then through the electric auxiliary fuel pump.
- Auxiliary fuel pump primes cylinders before starting and serves as a backup to the engine-driven pump.
- Fuel passes the shutoff valve, through the fuel strainer, to the engine-driven fuel pump.
- Fuel/air control unit adjusts fuel flow based on air passing through it.
- Fuel distribution valve delivers fuel to injector nozzles for each cylinder.
- Fuel flow gauge displays flow rate in gallons per hour.
Propeller
Function and Design:
- Propeller is a rotating airfoil or "air screw" that produces thrust similar to how a wing produces lift.
- Blades are twisted to produce equal thrust from hub to tip.
- Blade tips travel faster than areas near the hub due to greater distance covered in the same time.
- Twist compensates for varying speeds along the blade to equalize lift.
Fixed-Pitch Propeller:
- Cessna 172 uses a fixed-pitch propeller, optimized for a balance between climb and cruise.
- Blade angle is fixed and cannot be changed by the pilot.
- Propeller is directly connected to the engine crankshaft; propeller RPM equals engine RPM.
- Throttle controls RPM and power output; more RPM means more power.
Propeller Safety:
- Never lean on or casually turn the propeller; engine could start unexpectedly.
- Hand starting an engine requires thorough training and should be avoided unless necessary.
Electrical System
Components:
- 24-volt battery is the heart of the electrical system.
- Engine-driven 28-volt, 60-amp alternator supplies direct current and charges the battery.
- Alternators are preferred over generators for better current at lower engine speeds.
Master Switch:
- Activates the electrical system; split into two switches for alternator and battery.
- Both switches are typically operated simultaneously for normal operations.
Electrical Busses and Avionics Master Switch:
- Power is distributed to electrical busses, which feed individual circuits.
- Avionics master switch controls power to avionics busses for communication and navigation equipment.
- Provides extra protection for sensitive electronic equipment.
Engine Ignition:
- Ignition is independent of the electrical system, powered by two engine-driven magnetos.
- Engine continues to operate normally if the master switch is turned off or voltage drops.
Voltmeter and Ammeter:
- Voltmeter displays system voltage; approximately 24 volts with master on, 28 volts with engine running.
- Ammeter measures system performance; positive reading indicates normal operation.
- Negative or discharge reading suggests system overload or alternator malfunction.
- Low voltage annunciator warns if voltage drops below 24.5 volts.
Circuit Breakers:
- Located on the panel to safeguard individual electrical components.
- They pop out and disable the component if a malfunction occurs.
Understanding these systems is crucial for safe and efficient flight operations. Always perform thorough preflight inspections to ensure all systems are functioning properly before flight.