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Larry, Jerry and I all fly in heavy icing country. Larry flys quite a bit in lake-effect near Michigan, Jerry flys there and all over, and my flying is quite a bit in the Pacific Northwest.
So it is with some trepidation that I offer a slightly different opinion than Jerry's.
It has long been said that one should wait until you have a good load of ice (1/2 to 3/4 of an inch) before excercising the boots. Recently, one of the AOPA authors, I think it was Horne, did some research to see where that rule came from, and although oft repeated, Horne could not find the source of the rule, and could find no evidence that more frequent actuation of the boots removes less ice, or really causes the boots to form a pocket inside the ice on the leading edge, as is said. In my own experience, actuating the boots with a 1/4 inch of ice causes no problems. And icing is a very frequent occurence during a winter flight in the Pacific Northwest.
However you decide to go, I want to point out something that I have never read anywhere else:
Sometimes, the boots don't work.
One time, on a training flight, my instructor and I let 1" of ice develop on my P337, because I wanted to see how the airplane would act with that much of a load. On that particular day, it did quite well, losing only 20 knots of airspeed (but that will be different for every icing encounter, it depends on the type and rate of icing). Anyway, with an inch of ice on the airplane, I pressed the boot button, and nothing happened.
I immediately experienced a high level of stress, since we were at 16000' over a 10000' MEA airway through the Siskiyou mountains (between Medford, Oregon and Red Bluff, California, near Fort Jones). We turned around and landed in Medford, but a successful completion of that flight was made possible by the freezing level being at 9000', well above the field elevation of Medford. I would not want to have to carry that load of ice to the ground.
The boot system had sprung a leak since the last use, making it impossible for the pumps to pressurize the boots.
So, what I learned from that is to actuate the boots sooner to see if they work, to realize that they could fail any time, and to not fly myself into a situation where the boots HAVE to work.
There is a downside to more frequent actuation of the boots. The boot system on a Skymaster is a weak sister, and every time you actuate the boots you take a chance that you will kill your vacum pump. Now, there I go passing on what could be a wives tale, as I have never lost a pump that way. But my instructor, who has 500 hours in 337s in icing country, says it be true.
Those are my policies, you should make your own. If you fly in the central and eastern US, you have many, many more immediate alternates available than in the west. On the other hand, the freezing level goes all the way to the ground in winter in most of the US (except the balmy West Coast), meaning that any ice you accumulate, you are going to keep, unless you have boots (that work). In an unbooted airplane, with freezing levels on the ground, as Larry says, ice is an emergency, and you need to go where you KNOW there is no ice (either an ice free altitude, on top or below the freezing level, or turn around) *immediately*. In my opinon.
Blue skies (and no ice),
Kevin
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