PLT189: Carburetor Heat and Icing
What code PLT189 covers, why the FAA tests it, and how to study Carburetor Heat and Icing before your checkride.
Recall carburetor - effects of carburetor heat / heat control
What this means for your checkride
Carburetor heat routes warm air into the carburetor to melt or prevent ice that forms when fuel vaporizes and pressure drops, which can happen even on a warm day. Because the warm air is less dense, applying carburetor heat richens the mixture and causes a small drop in power.
Carburetor icing can quietly starve the engine, so the FAA tests when to expect it, when to apply heat, and why the RPM drops and then may rise as ice clears.
Pilots wait for rough running before using heat. Carburetor ice is most likely at low power with high humidity and temperatures well above freezing, so the time to apply full heat is before it builds, such as on descent.
Where to study it
Practice questions on this code
Real questions from the Aviator IQ bank that test this exact area. Answer each to see the correct choice, the full rationale, and the FAA source.
Once a power loss caused by carburetor ice is noticed in flight, what immediate action does the handbook specify?
Oral checkride defense
A code on your report is an area the examiner can revisit. Here is how an examiner might probe it, and how to explain it out loud.
When and why do you use carburetor heat?
I apply carburetor heat when I suspect or want to prevent carb ice, especially at low power settings like descent, and as a check during runup. It routes warm air to melt or prevent ice. Because warm air is less dense it richens the mixture, so expect a small power drop, which is normal.
How does carb ice show up differently in a fixed-pitch versus a constant-speed prop airplane?
With a fixed-pitch prop the first sign is a drop in RPM. With a constant-speed prop the governor holds RPM, so ice shows as a drop in manifold pressure. In both, full carb heat may run rougher briefly as the ice melts, then smooth out.
A code on your test report is an area of deficiency. An authorized instructor must review it with you and endorse that review before you take the practical test, and the examiner may revisit it during the oral. Study it until you can explain it out loud, not just recognize the answer.