Finally, I smiled when looking at the source of this carb heat "trick" discussion. Step Turn, isn't this "trick" the sort of non-POH procedure you've been deprecating in other contexts?
Fair enough, I knew this would come up at some point
. That is the case, and that criticism is fair. This "trick" is on the edge of guidance to operate the aircraft in a manner not described in the Flight Manual. So, a few thoughts as to why I made the post of this edgy topic!...
I do not believe that leaning after carb heat application is
counter to any flight manual procedure, and I do believe that it equates to flight manual procedures for higher altitude operation. It is not counter to good airmanship. It is a "touchy feely" thing you can do with a running engine to keep it running. Aside from leaning too much, to power reduction, it can be instantly reversed without affecting the flying characteristics of the aircraft. Retracting flaps on final
will affect the flying qualities of most aircraft, and mostly in a negative way, in a low power, approach configuration.
I was inspired to think more about the effectiveness of carb heat systems while assiting a freind with his modification of a C185 to have a carburetted engine. Included in that effort was the requirement to build a carb heat system, which would meet the FAA requirement of:
Sec. 23.1093
Induction system icing protection.
(a) Reciprocating engines. Each reciprocating engine air induction system must have means to prevent and eliminate icing. Unless this is done by other means, it must be shown that, in air free of visible moisture at a temperature of 30° F.--
(1) Each airplane with sea level engines using conventional venturi carburetors has a preheater that can provide a heat rise of 90° F. with the engines at 75 percent of maximum continuous power;....................
.
We had a heck of a time providing enough heat to rise the 90F heat rise, as measured in the carb venturi. We succeeded, but during the process, also developed the leaning trick, and we
did describe it in the changed flight manual, as we had to do that to pass the test for the approval. As you can see from the data I provided on two aircraft, one certified, one not, the carb air temperature rise is not even close to the required value for FAA approval, yet the 150 is approved. The FAA considered this 90F heat rise important enough to specify it, so I suppose any action a pilot can take to get closer to it when needed is a good thing - at least as viewed for certification!
I wanted people to think about this. If after thinking about it, you don't agree, don't do it! But, you'd have to agree that people are thinking and discussing it, and for this topic, I opine that discussion which promotes systems understanding is good.
Yes, I'm the flight manual proponent, nanny state guy. Pilots seem to not want to read them much, and I guess that the manufacturers know this, so they try and keep them thin (though not so much for brand new Cessnas!). So little gems of knowledge are set aside (like not slamming the throttle closed on a cold day at altitude, and shock cooling the engine).
But after all is said and done, this rather good discussion was worth taking a black eye in my inconsistency with respect to following flight manual procedure
. But, this does not mean that I have now gone soft on retracting flaps for a continued landing, nor waterskiing landplanes!
