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To add my two cents worth to the discussion on carb ice:
I have picked up a load of carb ice more than once while taxiing out to take off. One of the things that I look for when I run the engine up is for the RPM to return to exactly the same value before and after applying carb heat. If you run the engine up at exactly 1700 RPM, and then apply carb heat for 10 seconds or so, and then push the carb heat knob back in, if there was carb ice built up, the RPM will end up at 1750 to 1900 RPM depending on the amount of ice that was formed. This is a pretty good indication that there was ice in the carb. If I see this increase in RPM (above 1700), I then apply carb heat for an additional 10 or 15 seconds to make sure that the ice is gone completely. Pulling on carb heat, seeing an RPM drop to verify that the heat is working and then immediately pushing it back in is not a good practice in my opinion. If the carb is iced up, it takes time for the carb heat to clear the ice. Pushing in the carb heat knob immediately upon seeing an RPM drop can leave the carb with ice in it.
As far as applying carb heat during the actual takeoff, I don't do that. If the carb heat was applied immediately before taxiing onto the runway and taking off, the carb should be cleared of ice. Ice shouldn't form fast enough with a wide open throttle setting to cause a problem. If the engine is making normal RPM at full throttle at the beginning of the takeoff run prior to liftoff, then ice shouldn't be a problem. If the engine is not making normal RPM at the beginning of the takeoff run, for any reason, then the takeoff is aborted.
My procedure upon starting the takeoff run is to push in full throttle, then verify all three (throttle, mixture, carb heat) are full forward by giving each a shove, then glance at the oil pressure gauge to make sure it's in the middle of the green, and then glance at the airspeed to make sure it's increasing normally. I know that you can lean for max rpm at higher elevations, but I've never flown from any airports that are high enough to warrant doing that.
On landing, I pull on full carb heat on downwind adjacent to the touchdown point, reduce throttle to about 1500 RPM several seconds later, and the carb heat stays full on until rolling out on the runway. On a go around, I go to full throttle, and then push the carb heat back in 1 or 2 seconds later. I turn off carb heat on roll out to reduce the amount of time that the carb is sucking in unfiltered air.
FWIW, my neighbor who was an instructor in the USAF (and now flies for Delta and flies a Cherokee 140 for fun) always has a spot on the runway and an airspeed picked out for every takeoff. I forget what the numbers are, but it's something like - if he is not at 70% of rotation speed at 50% of the runway, he aborts the takeoff. I've never been able to consistently force myself to keep that in my head during takeoff but it's probably a good idea to do so. His reasoning is that the only way that you can get that far down the runway and not have that speed is if something is wrong, or if the runway is too short.
OK, that was 8 cents worth! Comments?
Best Regards,
Wayne DelRossi
Alon N5618F
Hours logged since restoration: 142.3
Wayne DelRossi
Alon N5618F
Hours logged since restoration: 142.3
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