At 09:50 AM 9/22/99 -0400, Karl wrote: > My coupe has a Stromberg so this may not be much of an issue. However >here is my reasoning. If you have carb heat on when you enter the pattern, >and on downwind, it has already melted any ice that has formed and if not >it surely will not do much for you on final approach.
It may keep new ice from forming. Carb heat is a preventative measure as much as it is anything else. (Note: I'm not terribly interested in the odd counter-example where carb heat has actually CAUSED worse ice by bringing the throat up to condensing temperature. That's just one of life's little jokes). The preventative nature of carb heat is why you turn it on when you reduce power, and keep it on while power is reduced. Since further reductions are likely to happen on base and final, it stands to reason that it becomes ever more necessary. In fact, they told you in ground school that carb heat may not be effective if you wait until ice is formed to use it. >With the carb heat on during final, you are already running a little rich. >If you have a carb with an accelerator pump, and you go around and add >power rather fast, you could get a too rich condition and the engine will >sputter at least. I was always taught that heat comes off just before or simultaneously with powering up. Full throttle and carb heat is to be avoided because of risk of detonation. Cessnas have a nice arrangement where an extended thumb shoves the carb heat off as the throttle goes forward. >I have seen accidents caused by this. So that is why I >do it. Agreed. Engines have sputtered when given full throttle and carb heat. Or failed to put out full power. That's why the heat comes off as a matter of routine IF and WHEN you open the throttle into the 'green' arc. You MUST turn it off when it should be off. But you also MUST turn it on when it should be on, and keep it on. I think you're far more likely to have an accident caused by needing the engine and finding it iced up due to no carb heat. These little Continenatals make ice, they make a lot, and they make it fast. More than one student 150 pilot has ended up in the weeds as a result of a single neglect of carb heat, just from abeam the numbers until he decided to go around. >It has worked rather well for me for a lot of years. One other item, >with the carb heat on, it is not going through the air filter so if you >land on a dusty strip you could ingest whatever dust is kicked up from the >landing. Do you agree? On that rare occurrance, you might consider shutting off the heat when you come over the fence and have pretty much decided you are definitely going to land. On the other hand, during most of the landing regime, any dust will remain behind you. It is taxiing where 'kicking up dust' that remains in a position to be ingested is really a concern. And I'd prefer a bit of dust in the motor to a bit of ice in the carb throat. So I guess I don't agree. I believe that on a C-series Continental, you need to err on the side of keeping the heat on in all situations where the RPMs are below the green arc, and in all situations where there is any suspicion that carb ice exists, except where the manufacturer has clearly told you that it is inappropriate. Of course, the old-timers who were low-timers when our planes were new will fold their arms and tell you that if you HAVE to have any power to make the runway after being abeam the numbers, you're taking your life in your hands by flying too wide a pattern. Unfortunately, the students at N85 seem to fly patterns halfways to La Guardia, then land hot and dawdle on the runway. So there is sometimes no choice in the matter. Greg
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