Thing that I know for sure:

SCAT never had a 20% rule, in fact we never had any rule since at the time SCAT was operating we neither felt the need to have a rule, and for that matter neither did IMAC. The 10% rule was written out for an AMA rules cycle when research indicated that large fraction of airplane could not meet the current rule and the rule itself was a joke. It was later reinstated when some were fearful of being overrun by stretched airplanes.

The TOC and the IMAC rules were 10%, but there the similarity ended. An airplane could be built to be TOC legal, but not IMAC legal, because the TOC did not have a fuselage cross section requirement in back of the firewall, but IMAC did. The TOC rules were somewhat of a farce, as Billy Hempel was asked on year to glue a piece of 3/8 balsa under the tail of his AW 540T to meet the letter of the rule. Mfrs that paid attention to the 10% rule were concerned with the TOC, several that I talked during my own research didn't know or care about the IMAC rules, their planes were labeled IMAC legal because of marketing, not that they had been measured. There are plenty of planes out there today that I know would fail if tested, which is why the rule was repealed in the first place.

Things I think

I do seem to remember a 20% fuselage cross section rule that went away in around 2000, but the wingspan has always been 10%. In any case, I have always regarded the scale rules as ridiculous, as enforcement of one means enforcement for all, and neither competitors or CDs are equipped to handle the task. It does make for interesting dialogue.


Paul Kopp, once upon a time
SCAT CD
IMAC Rules Committee
IMAC Sequence Committee