Carb heat
TLDR: Yes it’s required for airworthiness - FAA CFR 33.35(b) and required by many POH’s and the aircraft parts catalog. It’s also your alternate air if primary air intake becomes clogged (PHAK 7-10.)
If you are bringing an aircraft with a carbureted engine to your check ride, it is important take time to understand it’s design, when to use it and why it’s purpose makes it a requirement for airworthiness.
Usage during maneuvers - Many aircraft, Cessna models come to mind, have requirements that carb heat be utilized any time operating outside of “green arc” RPM. We will find ourselves outside of that arc many times during a checkride, please know that it is the applicants responsibility to ensure they are properly using carb heat during the entire checkride.
Usage during a simulated emergency - As stated above, many Cessna models require carb heat on any time RPM is outside of the green arc. During a simulated powerplant failure, I will bring the throttle to idle, putting us well outside of the green arc. It is the applicant’s responsibility to turn on carb heat themselves as soon as practicable. Many times I see the applicant completely skip this step or even say “I would turn the carb heat on” without actually doing it. In post exam briefings, some CFI’s have mentioned they will pull carb heat for their students when simulating powerplant failures, this is creating a crutch for students. CFI’s should have students turn it on since carb heat doesn’t come on automatically.
As for aircraft that do not have the “outside of green arc” requirement for carb heat, most notably Piper models: your owners manual/POH likely will outline only 3 times where carb heat should be applied. During an engine failure, during your run up and if carb icing is suspected. It is the applicants discretion if they want to actually apply carb heat during a simulated emergency. There are risks involved with actually applying it, but those risks are associated with forgetting to turn it off and applying full power. At the very least, an applicant should mention applying and consider touching the carb heat while troubleshooting a simulated powerplant failure.
Another common knowledge gap is that carb heat acts as alternate air. In some fuel injected engines, alternate air will come on automatically when the primary intake is clogged. But in carbureted engines, it requires pilot action to get air back to the engine in the event of an air intake failure. There have been instances where off field landings occurred after a bird strike clogged the air intake and all the pilot had to do was turn on carb heat. This is why turning on carb heat is part of the emergency checklist to troubleshoot a failed engine, even if carb ice is not suspected as the reason for the failure.
