chicagorandy wrote: Why carb heat in some planes must be switched on and off?
I already explained in an earlier post why you really don't want to keep carb heat on all the time. But, not all planes even have a carb heat control - and among those that do, there are different requirements for when and how you use it.
For example, some engines are particularly vulnerable to carb ice. A notorious case in point are the small-bore Continental engines with Marvel Schebler carbs used in many antique aircraft (including the legendary J3 Cub built right here on my airport in Lock Haven PA). In those installations, the carb attaches to the bottom of the engine, and is hanging right out there in the airflow. Those planes tend to be landed with power all the way at idle - which basically closes off a butterfly valve in the carb throat, maximizing pressure differential in the venturi, which then makes the carb extremely sensitive to blockage from ice crystals. So, we are taught that, upon power reduction on downwind, we pull on
full carb heat
for every landing. This certainly protects against carb ice, but in the event of a go-around, carb heat will prevent us from getting full power (as mentioned in my earlier post), so climb is anemic. For that reason, one has to remember to set full carb heat on downwind, and then turn carb heat completely off on short final (in case it becomes necessary to convert the landing to a takeoff).
Other engines are relatively immune to carb ice. A good example here is the Rotax 912 ULS that powers many of the LSAs we all fly. So, unlike the 65 HP Continental in the Cub, we
don't automatically pull carb heat on downwind. We don't have to. This is partly because the carbs in a 912 sit on top of the number 2 and 4 cylinders, where they are always subject to a bit of residual engine heat. (Yes, that does indeed slightly reduce the maximum power available from this particular engine.)
But, there's another reason why the Bing 64 carbs used on the 912 ULS rarely ice up. That is because there are two of them (the left hand driving cylinders 2 and 4 through a 2-port intake manifold, and the right hand carb similarly driving cylinders 1 and 3). Now, the odds of both carbs icing up at exactly the same rate are pretty close to zero. So, if the aircraft is being flown in conditions conducive to carb icing. one carb will tend to ice up first. Now the engine doesn't quit outright, because it will still run (sort of) on just two cylinders. Yes, it will lose power, RPM will drop, and it will shake your teeth loose, but it doesn't quit. And, all that vibration gives you
lots of warning that you're picking up carb ice (you'd have to be pretty inattentive to miss it). So, as soon as the plane starts shaking, you just pull on full carb heat, push the throttle, and wait for the ice to melt. When the engine smooths out, you can bring back the power, turn off the carb heat, and go on your merry way.
Now, if you happen to be flying a Rotax 912 iS engine (or the new 915), you don't
ever have to pull on carb heat. In fact, you can't, because you don't have a carb heat knob. Why not? Because these are fuel injected engines --
they don't have carbs! Probably one of the best reasons to prefer fuel injection over carbs.
One final Rotax note - the turbocharged 914 is still a carbutreted engine -- so of course it can be equipped with carb heat. You'll still very seldom have to use it, thought, because - well, two carburetors, mounted above the back jugs.