10% is a commonly used guideline to determine whether a procedure has compromised the strength of a structure. For example, if you are removing corrosion from a skin, a 10% or less reduction in thickness is generally considered acceptable.
The statement "the cross-sectional AREA of the top spar cap was reduced by 10%" means that if you took a section through the top spar cap at the center of the hole, the diameter of the hole was 10% of the total section, e.g. a 1/8" hole in a 1.25" section. By making that statement they seem to imply that the hole was not large enough to significantly affect the strength of the structure relative to the design specifications. I find it interesting that they did not say "less than 10%" as I find it difficult to believe it was exactly 10%, and if it was more they certainly would have said so. -- John Skyport East www.skyportservices.net
