Mxsmanic wrote
How far off can aircraft weight be, percentage-wise, before it makes a
potentially hazardous difference in flight characteristics?

1%-2% would not even be noticed except in total fuel burn on a long
flight. 5% would be noticed by the pilot, but only takeoff distance,
climb rate, and maximum altitude attainable. 10% would certainly
cause performance problems. However, most flights are not weight
limited by the maximum certificated T.O. weight, but by some other
performance limit...RW length, obstacles, landing weight, etc.

Bob Moore
ATP B-707 B-727
PanAm (retired)