Underfire - I'm pretty much with you. Gallup reports that U.S. men self-reported weights that averaged out to 191 lbs (women, 159 lbs). One may think that even those are low (due to vanity) but the same groups reported that their
ideal weight would be about 20 lbs less (they were honest enough to admit they were a bit - chubby). 25% of men admitted to 200+ lbs; 29% of women admitted to 175-200+ lbs.
So the FAA numbers are about right -
IF one excludes clothes and carryons, and only counts flesh. And 20 pounds shy with those added (in my case, the real extra weights are below the assumptions most of the time, if not, sadly, the body weight
I weighed my winter overcoat - 3 lbs)
OTOH, let's take, oh, a nice average 737-800 with 2-class seating. 160 people plus babes-in-arms and crew = 180 x 20 = 3600 lb error. Given a MTOW for the 738 of 174,200 lbs, that is a
2% error. 157 feet/50 m on a nominal MTOW takeoff run, negligible in terms of exceeding MTOW unless one habitually flies with full tanks.
Redo the math as needed, for an Islander or an A380.
Nevertheless, I'm all for some updated statistics from which to "average."