Being an Aeronautical Engineer, i sought out the origin of this saying,
FLYING BY THE SEAT OF YOUR PANTS
Well, there's a pretty simple explanation.
Before
airplanes had sophisticated instruments and
flight control systems, and
even today, planes are piloted by feel.
Pilots can feel the reactions of the
plane in response to their actions at the controls.
Being the largest point of contact between pilot and plane, most of the feel or
feedback comes through the seat of the
pants.
If you are "
flying by the seat of your pants" your are responding to the
feedback received.