← All explained questions · Supplemental · II — Preflight Procedures

Carburetor heat applied during the run-up should produce

Choices

a noticeable RPM rise.
Carb heat REDUCES RPM (less dense charge).
✓ a slight RPM drop, confirming that hot.correct
less dense) air is being delivered to the carburetor and the system is functional. Carb heat introduces warmer, less dense air to the carburetor → richer mixture → slight RPM drop (typically 50-150 RPM). The drop confirms the heat valve is working. No drop suggests a stuck valve or duct disconnection. A large drop with rough running may indicate ice that's now being melted/ingested.
no change in RPM.
No change indicates a malfunction.
engine roughness.
Roughness alone is abnormal; the expected response is a small smooth RPM drop.

Why

less dense) air is being delivered to the carburetor and the system is functional. Carb heat introduces warmer, less dense air to the carburetor → richer mixture → slight RPM drop (typically 50-150 RPM). The drop confirms the heat valve is working. No drop suggests a stuck valve or duct disconnection. A large drop with rough running may indicate ice that's now being melted/ingested.

FAA source: FAA-H-8083-3C, AFH Ch. 2, before-takeoff check / engine run-up; aircraft POH/AFMbrowse the reference library →

Covered in Supplemental · II — Preflight Procedures — study the lessons free, then practice with grading and mastery tracking.

Original study question written for this course — representative of FAA knowledge-test topics, not an actual current FAA exam question.