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PilotSchool Knowledge Base

Carburettor Heat and Alternate Air Systems

Understanding the differences and applications of carburettor heat and alternate air controls is essential for pilots, particularly concerning engine safety and efficiency. This section will explore these controls in the context of the CASA RPL pilot license theory exam.

Difference between Carburettor Heat and Alternate Air

Carburettor Heat

  • Purpose: Used in carburetted engines to prevent or eliminate ice buildup in the carburettor, ensuring consistent air-fuel mixture for optimal engine performance.
  • Function: Mechanically diverts heated air from the exhaust manifold shroud into the carburettor via a cable-operated butterfly valve.
  • Operation: Typically results in a slight drop in engine RPM when activated, indicating proper function.
  • Conditions: Useful during conditions prone to icing, such as descent with reduced power settings in humid or moist environments.

Alternate Air

  • Purpose: Provides a secondary air intake path for fuel-injected engines if the primary route is blocked, usually by ice.
  • Function: Bypasses the primary intake using an auxiliary intake located under the cowling.
  • Operation: Can be manual or automatic, depending on system design, and may involve unfiltered and lower pressure air, impacting engine efficiency.

Key Differences

  • Engine Type: Carburettor heat is for carburetted engines, whereas alternate air is for fuel-injected engines.
  • Air Source: Carburettor heat uses heated exhaust air; alternate air uses an intake bypass under the cowling.

Checking Carburettor Heat Before Take-Off

  • Conduct checks at approximately 2,000 RPM, alongside magneto checks.
  • Apply carb heat to observe RPM drop, indicating the operation of the carburettor heat system.
  • Turn off carb heat; observe an RPM rise confirming ice clearance.
  • Return engine power to 1,000 RPM post-check.
  • Avoid ground usage due to unfiltered air risks unless conditions specifically require it.
  • Always follow the guidance in the Pilot Operating Handbook (POH) or Flight Manual.

Use of Carburettor Heat

During Ground Operation

  • Minimize use on the ground to prevent unfiltered air ingestion, which can introduce contaminants to the engine.

Anti-Icing

  • Preemptively apply carb heat during potentially icing conditions, particularly at lower power settings like descent.
  • Regular checks and short heat applications can prevent ice buildup, crucial in humid, cool conditions.

De-Icing

  • Apply carburettor heat if symptoms of icing occur, such as unexplained power reduction or RPM drop.
  • Engine performance may initially decline as ice melts and enters the engine, but this typically resolves as the normal air-fuel mixture resumes.

Effects of Carburettor Heat on Engine Performance and Instrument Indications

  • Performance: Activating carburettor heat can cause a temporary loss of engine power, up to 15%, due to the less dense, heated air and enriched mixture.
  • Engine Instruments:
    • Fixed-pitch propeller engines may show a decrease in RPM.
    • Constant-speed propellers often exhibit a drop in manifold pressure.
  • Adjustments: Monitor and adjust manifold pressure and RPM to counterbalance power loss. Continuous observation prevents ice buildup and maintains engine efficiency.

Understanding and managing carburettor heat are vital for safe aircraft operation, particularly in variable weather conditions or during different flight phases. Pilots should integrate these practices with detailed knowledge from the aircraft’s POH to ensure optimal engine performance and safety.