J.Kahn wrote:

I don't think capital gains applies there. The materials weren't
purchased as an "investment". I believe that the IRS or (Revenue Canada
up here) would have to see a pattern of construction and sales of
aircraft that would indicate that the activity is professional in nature
before they would consider the profits from the sale as income.

In the U.S., it is most definitely taxable if sold at a
gain. The tax law does not distinguish between personal,
business, or investment assets (except that a loss on sale
of a personal asset is not deductible).

Fred F.