Aircraft Systems — Understanding Everything Under the Cowl and Behind the Panel
A pilot who understands how aircraft systems work can identify anomalies early, manage partial failures correctly, and perform emergency procedures without confusion. This module covers the powerplant, fuel, electrical, pitot-static, vacuum, and landing gear systems — with emphasis on what can go wrong and how you handle it.
- Describe the four-stroke engine cycle and explain what happens in each stroke
- Explain the dual magneto system and interpret magneto check results
- Identify conditions that cause carburetor ice and describe recognition and corrective action
- Describe the fuel system including tank venting, sump drains, and fuel grades
- Explain the pitot-static system and the effect of blockages on each instrument
- Describe what happens when the alternator fails and how to manage battery power
- Explain what the vacuum system powers and what fails when vacuum is lost
Lesson 1 — The Reciprocating Engine
Most training aircraft are powered by horizontally opposed, air-cooled reciprocating engines — typically four or six cylinders arranged in opposing pairs. The engine produces power through a repeating four-stroke cycle: intake, compression, power, and exhaust. Only the power stroke produces useful work; the other three are mechanical setup strokes driven by the crankshaft's momentum and flywheel effect.
During the intake stroke, the piston moves down and draws a fuel-air mixture through the open intake valve. On the compression stroke, both valves close and the piston moves up, compressing the mixture to roughly 8:1. At top dead center, both spark plugs fire simultaneously (from separate magnetos), igniting the mixture. The burning gases expand rapidly on the power stroke, forcing the piston down and turning the crankshaft. Finally, the exhaust valve opens and the piston sweeps burned gases out on the exhaust stroke.
The four-stroke engine cycle — only the power stroke produces useful thrust
Lesson 2 — The Magneto System
Aircraft engines use a dual magneto ignition system that is completely independent of the aircraft's electrical system. Each magneto generates its own high-voltage current to fire one of two spark plugs per cylinder. This redundancy is critical — if the battery or alternator fails completely, the engine continues running because the magnetos are self-powered by the engine's rotation.
The magneto check during the run-up tests each magneto independently. Switching from BOTH to LEFT or RIGHT should cause a small RPM drop — typically 75–125 RPM per your POH. A larger drop indicates a problem with that magneto or its plugs. No drop at all is actually a warning sign — it may mean the magneto is not grounding when switched off (a "hot mag"), which is a safety hazard since the engine could fire when the propeller is hand-turned on the ground.
Dual magneto system — each magneto fires one plug per cylinder independently of the battery
Lesson 3 — Carburetor Ice
Carburetor icing is one of the most insidious engine hazards because it can develop rapidly in conditions that seem benign. The carburetor venturi creates a pressure drop that causes a temperature drop of up to 70°F below ambient air temperature. Combined with moisture in the air, this can cause ice to form inside the carburetor throat and block airflow — even when outside temperatures are well above freezing.
Carburetor ice is most likely when outside air temperature is between 20°F and 70°F (-7°C to 21°C) with high relative humidity. The first symptom in a fixed-pitch propeller aircraft is an unexplained RPM decrease. In a constant-speed propeller aircraft, manifold pressure drops while RPM holds steady.
The remedy is carburetor heat — a valve that routes air heated by the exhaust manifold around the carburetor. When carb heat is applied with ice present, expect the engine to run rough initially as the ice melts and passes through as water, then RPM rises above the pre-icing reading as the restriction clears. That RPM rise above the original setting confirms ice was present.
Carburetor ice forms in the venturi throat — the temperature drop can be 70°F below ambient
Lesson 4 — The Fuel System
Most training aircraft use a gravity-fed or engine-driven pump fuel system with two wing tanks and a selector valve allowing LEFT, RIGHT, BOTH, or OFF positions. For takeoff and landing, the selector is always set to BOTH so both tanks feed equally. This prevents an unbalanced fuel load and ensures maximum fuel availability during the most critical phases of flight.
