Here http://www.straightdope.com/classics/a3_144.html, Cecil says, among other things, that if you toss a baseball upwards while in an airplane in flight, the weight of the plane doesn’t change because you exerted an equal amount of force downward. OK, fine. What if you just drop the ball, so that it’s in motion toward the floor? 
azcat
The plane would have to lighten and rise to meet the ball, and equalize when the ball came to a stop.
So, working backwards, the plane must have been holding up the ball, and releasing it caused the lightening.
There’s a famous story about this is Charles Lindburgh’s book We.
He has a fly in the open cockpit and can’t decide if it’s a drag on the plane when it’s in the air.
I know he got it wrong, but I forget which way he picked!
That’s it exactly.
It helps if you imagine things a lot heavier than a baseball. Think of a plane dropping 1-ton bombs. When the bomb is released, the plane is immediately 1 ton lighter. (And the pilot has to correct for this.) What if you dropped that same bomb with the bomb bay doors closed? Assuming the bomb doesn’t explode or go through the doors, the plane would be 1 ton lighter while the bomb is falling, then go back to its original weight when the bomb hits the floor. (Of course, even if the bomb doesn’t explode or go through the doors, all that weight crashing around would definitely give the pilot problems.)