Chapter 17: Induction Icing and Ignition Systems

17.1 Induction System Icing

• Ice can form in the engine's induction system, restricting airflow and causing a loss of power.
•  Types of Icing:
• Carburetor Icing: The most common and dangerous type. It is caused by the temperature
• drop from fuel vaporization and the pressure drop in the venturi94. This can cool the moist air
• enough to form ice on the throttle plate and venturi walls, even with outside air temperatures
• as high as +22°C.
• Impact Ice: Forms on air filters and bends in the induction system in visible moisture and
• freezing temperatures95.
• Fuel Icing: Water dissolved in the fuel freezes at the low temperatures in the carburetor96.
•  Indications of Icing:
• For an aircraft with a fixed-pitch propeller: A gradual drop in RPM97.
• For an aircraft with a constant-speed propeller: A gradual drop in manifold pressure98.
•  Corrective Action: The pilot must apply carburetor heat. This directs hot, unfiltered air
• from a shroud around the exhaust manifold into the carburetor intake, melting the ice99.
• When heat is applied, a drop in RPM or manifold pressure will be observed initially because
• hot air is less dense. If ice was present, the power will then gradually increase back to normal
• r above normal as the ice melts.
• Using carb heat causes a richer mixture and a loss of power (approx. 15%) and should not be
• used during takeoff or at power settings above 80% due to the risk of detonation100100100100.

17.2 The Ignition System

• Aircraft piston engines use a dual magneto ignition system for redundancy and improved
• engine performance.
•  Dual Ignition: Each cylinder has two spark plugs, and each set of plugs is powered by a
• separate, independent magneto101. This provides redundancy in case one magneto fails and
• also promotes more complete and even combustion by igniting the mixture from two
• points102.
•  Magnetos: A magneto is a self-contained, engine-driven electrical generator that produces
• the high-voltage spark for the spark plugs103. It requires no external power source (like a
• battery) to operate once the engine is turning.
•  Ignition Switch: The ignition switch in the cockpit does not switch the magnetos "on."
• Instead, in the "OFF" position, it
• grounds the primary circuit of the magneto, preventing it from producing a spark104104104104.
• This is why a broken ground wire can lead to a "hot mag" that is live even when switched off.
•  Spark Augmentation: At the slow cranking speeds during engine start, a magneto does not
• turn fast enough to produce a strong spark. To overcome this, a spark augmentation system is
• used:
• Impulse Coupling: A mechanical, spring-loaded device that momentarily spins the magneto
• at a high speed to produce a strong spark and also retards the timing for starting105.
• Booster Coil: An external electrical coil that supplies a high-energy spark for starting106.