[WalterBright]

My father, AF pilot, was once flying a DC-3 over New Mexico, when he encountered clear air turbulence. The airplane shook violently. The airspeed indicator just ping-ponged around. He said there was nothing he could do but pull the airspeed back to the minimum (to reduce stress on the airframe), radio the controller to warn other aircraft, and ride it out.

After he landed, the mechanics showed him where rivets had been pulled out of the wings.

Edit: one of the reasons the DC-3 was such a robust airplane was nobody had any experience with an airplane of that size and type, so it was overdesigned. It's the same reason the B-17 was a tough bird.