Training and Safety Tip: Deice your engine
Most pilots rent aircraft during some or all of our flying life, and we renters need to learn the differences between various aircraft. Even two aircraft of the same make and model at a single flight school may have different panel layouts, engines, or other controls, and procedures vary based on these differences.
One example of varying procedures relates to the use of carburetor heat. The use of engine heat when icing is suspected or expected is similar across many carburetor-equipped general aviation aircraft. (Fuel-injected engines have no carburetor to ice up, though they have their own issues.)
When power is reduced, especially in humid conditions, ice is more likely to form inside the carburetor of some aircraft, based on factors including position of the carburetor on the engine, and the airflow around the engine.
Some aircraft manuals call for carburetor heat to be applied when descending, and to remain on through the landing. Others require carburetor heat to be applied briefly, and then turned off, during the descent, to confirm that it is operating normally. Still others don’t require checking the carb heat or applying it during flight—unless carburetor ice is encountered or suspected. The procedures vary because some engines are more prone to carburetor ice than others. And there might be different carburetor heat procedures for the same engine mounted on different aircraft because of subtle differences in the induction system.
It’s important to understand that when carburetor heat is applied, some of the hot exhaust gas coming out of the cylinders gets diverted into the carburetor, and that hot gas is less dense (and contains less oxygen) than the ambient air. That is why the application of carburetor heat causes a slight drop in revolutions per minute. When checking the carb heat during runup prior to takeoff, the rpm drop is an indication that the carb heat is working. Some engine monitoring systems include a very useful carburetor temperature indication function.
If an engine is running rough or losing rpm because ice has formed in the carburetor, and carburetor heat is applied, the engine may get worse before it gets better. Ice fragments can clog the air passage briefly on the way out, or until the hot exhaust gas turns them into vapor. It’s important to be proactive with applying carb heat and not wait until the carburetor is fully iced up and air stops flowing through the engine, at which point the engine will stop and carburetor heat will no longer have any effect.
Ultimately, it’s up to the pilot to know and follow the aircraft manufacturer’s recommendations as to how and when to use carburetor heat.
For more information about carburetor systems, see Chapter 7 of the FAA’s Pilot’s Handbook of Aeronautical Knowledge.
