That said, experiencing light chop on a modern large airplane presents no danger to the airframe or properly secured passengers. You really should be strapped in, though, especially if you're on a small plane. Wake turbulence, for example, actually does present a significant risk to smaller aircraft.
Other people will correct me if I am wrong, but I believe the last time a large commercial airliner was lost to turbulence was 1966. https://en.wikipedia.org/wiki/BOAC_Flight_911
Wild how far we've come.
A 40 foot drop is pretty scary; by comparison, an NCAA dive platform is 33 feet.
Aircraft have to be built light for the sake of efficiency (or even, just, being able to get off the ground.
There are certainly factors of safety... but not 3x+. Probably closer to 1.5.
https://www.law.cornell.edu/cfr/text/14/25.337
"(b) The positive limit maneuvering load factor n for any speed up to Vn may not be less than 2.1 + 24,000/ (W + 10,000) except that n may not be less than 2.5 and need not be greater than 3.8—where W is the design maximum takeoff weight.
(c) The negative limit maneuvering load factor—
(1) May not be less than −1.0 at speeds up to VC; and "
The 2.5 number is important. That right there is the +2.5 to -1.0 requirments for transport (i.e. seating more than 19 passengers) category aircraft.
Here's a quote from Boeing: "Our airplanes are built to withstand 3.75 G load before there is any kind of damage — that's almost four times gravity,” said Doug Alder, a spokesman for Boeing. “Some of the worst turbulence gets in the range of 2 to 2.5 G's, well below the damage tolerance.”
3.75 is not nearly enough to cause a blackout. It's also exactly 1.5 (typical airplane factor of safety) times 2.5
He's not saying the drop was 40 feet instantaneously, he's saying the turbulence and the subsequent recovery only caused a 40 foot deviation from the assigned altitude.
Just for reference, a descent rate on a standard flight is pretty normal at 40 feet per second. Some descent profiles can double that. The NCAA diver will hit the water at 46 feet per second.