Cool quality of most aircraft wings? (14)
I believe the answer is:
unflappability
I believe this is a double definition.
'cool' is the first definition.
(as in keeping your cool, being unflappable)
'quality of most aircraft wings?' is the second definition.
(aircraft wings don't flap)
(Other definitions for unflappability that I've seen before include "Composure" , "the skill of rigid wings?" .)