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.