Jay Honeck wrote:
Is it illegal to subsitute Torx (or square-drive) screws in
certificated aircraft? If so, why?

Nope.


Okay, that said, why is it that everyone (that I've seen, anyway)
continues to use the (too easily strippable) Phillips screws?

Inertia? Cost? No one has thought of it? No one else cares, cuz
their A&Ps do all the work?

It just seems so bloody obvious, yet I've never seen a certificated
plane with anything but Phillips screws.

Jay,

I'm not sure if MilSpec Torx screws are readily available. I wouldn't
have a problem using N(ational)A(erospace)P(arts)A(ssociation) torx
screws on non-structural parts, but for the important bits, e.g. fuel
tanks and stressed skins, I just have never seen anything that would
qualify. I've been out of the maintenance arena for a few years,
though, maybe they are starting to show up?

Happy Flying!
Scott Skylane
N92054