And you thought all the possible carb heat questions had been asked! Well, if they have, I've forgotten the answers to these....
1) As many of you know, the C150 is particularly prone to carb icing, especially in the sort of warm, damp weather we've had recently. Ours has shown evidence of it before takeoff, straight after levelling off after take-off, during the cruise - almost any time except during the climb with full power applied, in fact.
As a result, we use carb heat extremely frequently, and if flying near the coast, or over mountains where the options are limited in the event of engine failure, we fly with it on all the time. This meant that last week, dodging low cloud around the coast from Sleap to Caernarfon, we flew almost the whole route with full carb heat. Now, we all know you're not supposed to use partial carb heat. But in that sort of situation, why not? It would seem logical to apply full carb heat at around five minute intervals for about 30 seconds, which you tend to do in a C150 anyway (not that this is an absolute rule, but it seems to make sense), but then to push it in partially - to save fuel and have a bit more power. Could we do this? And if not, why not?
2) Having the carb heat on all the time like this uses more fuel. But how much more? Since our fuel gauges are of course hopelessly inaccurate, what percentage more fuel are we likely to be using if we have full carb heat applied except when climbing out from the airfield (or occasionally climbing for any other reason) during a cross country flight?
3) There are two schools of thought about use of carb heat in the circuit. One says that you apply it when you reduce power on base, and then leave it on. The second says that you put it off at around 200 ft on final, so that you have full power if needed for a sudden go-around. I always thought this was down to personal preferences, or often to instructor's preferences or what you were taught. I'm now wondering if it should be different in different aircraft. Certainly in the C150 I'm not pushing the carb heat knob in at 200ft; I'm quite sure it could get carb ice in that time.
A friend who instructs on C152s says they always teach carb heat on and leave it on. I can't find anything in the POH about this, but does it vary for different aircraft? Or is it just down to common sense and personal preference?
Jeez...roll on fuel injected engines for all aircraft!