Engine Fuel, Induction, Cooling, and Exhaust Systems; Propellers
Engine Fuel, Induction, Cooling, and Exhaust Systems
Fuel
Avgas grades are identified by octane/performance number (100LL is common); turbine fuel is Jet A or A-1. Detonation is uncontrolled, explosive combustion after normal ignition - it raises CHT and can damage cylinders. Preignition is ignition before the spark, often from a hot spot or glowing carbon deposit, and can destroy a piston quickly. Higher octane resists detonation. Watch for vapor lock in hot conditions, especially with fuel injection.
Induction
Naturally aspirated engines draw air through a filter and air box; alternate air supplies unfiltered air if the filter ices or clogs. Turbocharged systems use exhaust-driven turbines, a wastegate to control boost, and often an intercooler. An overboost can exceed the manifold pressure limit very quickly.
Cooling
Cylinder fins, baffles, and cowl flaps manage airflow. Cowl flaps are normally open for low-speed, high-power climbs and closed or trailing in cruise. CHT is the limiting gauge - commonly 435 degrees F maximum, with much cooler values desired in cruise. Avoid abrupt cooling on descent.
Exhaust
Know the exhaust stacks, collector, muffler, turbocharger, and wastegate. Exhaust leaks are dangerous: carbon monoxide can enter the cabin heat system, and a cracked exhaust can start a fire.
Traps
Confusing detonation with preignition is common on the exam. Turbocharged engines also require careful mixture management at altitude.
Rule of thumb
Keep CHT below the red line and generally in the 350-400 degrees F range in cruise; if CHT climbs with cowl flaps closed, open them.
Propellers
Types
Fixed-pitch, ground-adjustable, and constant-speed propellers. A constant-speed propeller uses a governor with flyweights and a speeder spring to hold RPM by varying blade angle with engine oil pressure. In the common single-engine design, oil pressure drives blades toward low pitch (high RPM), while counterweights and a spring drive them toward high pitch if oil pressure is lost. Many twin installations are designed so loss of oil pressure drives the blades toward feather.
Know the terms: blade angle, pitch, slip, geometric versus effective pitch, feathering, reversing (beta range), unfeathering accumulator, and propeller synchronization and synchrophasing.
Traps
Governor questions are common: increasing the RPM setting compresses the speeder spring, the flyweights move inward, and the pilot valve directs oil to decrease blade angle and raise RPM. Governor failure can cause overspeed (blades driven to low pitch) or underspeed.
Worked example and rule of thumb
Track is checked by rotating the propeller and comparing blade tips to a fixed reference; a typical field tolerance for a metal propeller is about 1/16 inch. Blade angle is measured at a specified station (often 75 percent radius) with a propeller protractor. Track or balance errors produce vibration at propeller RPM.
Inspection notes
Look for nicks, cracks, corrosion, and damaged cuffs or hub bolts. Wooden propellers require checks for delamination and hub bolt tightness. Any propeller strike normally requires an engine teardown inspection per the manufacturer.