I’m training in an Evektor SportStar and want to have a better understanding of my checklist concerning carb heat checks. Here’s a copy of my school’s engine runup checklist (I have made my own, but it’s based on this one):
For Item 10 (“ON/CHECK/OFF”), with the throttle set to about 4000 RPM, I pull the carb heat ON, look for a small RPM drop, and then push it OFF. It’s my understanding that this check is done to confirm the operation of a diverter gate - one that feeds the carburetor with hot air that is being pulled over the exhaust system, rather than cold air coming in through the normal filtered path. Basically, this is confirming that the carb heat mechanism isn’t broken or jammed.
For item 13 (“IDLE CHECK”), with the throttle pulled back to full idle (engine runs at about 1750 RPM), I pull the carb heat ON, listen and watch, and then push it OFF – basically as before except at low RPM rather than high RPM. I understand that I am still looking for a small RPM drop and for the engine to keep idling.
Am I performing the checks correctly?
I’m mostly curious about item 13 because the Item 10 check is in the official “Before take-off” list in section 4.5.5 of the POH, but the item 13 check is not.
Why do you suppose the item 13 check is not in the POH, but is in my school's checklist? Am I literally trying to make sure the engine won’t shut off at low power settings with carb heat applied? Is that a real risk? I’ve heard experienced aviation folks suggest that it isn’t a risk because the plane, once flying, will windmill the prop at low power settings and it will never be down at such a low RPM. Thoughts? Comments?
Joe