Question Where does MAX GROSS come from?

[Neal]

I find it interesting (odd) that various aircraft seemed to be stamped with max gross such as 2000 lbs or 2300 lbs which does not seem to be the capability of the aircraft but an FAA certification number? For example in my CubCrafters Carbon Cub FX-3 the max gross is 2000. My empty weight dry is 1300 lbs. (BRS, cargo pod, etc.). Two "2026 edition" adults and full fuel, you're at max gross with not even dog fur for cargo. We all know the 186 HP Lycoming, 83" constant speed prop, airframe, etc. can haul more, but what drives these numbers and limits us? How does this apply in Experimental vs Certified?

 

[Waldom]

As regards Standard Category aircraft, the maximum gross weight is set by the designer to meet required minimums of performance and structural limits. These limits are set forth in 14CFR FAR 23 for smaller aircraft and FAR 25 for transport category aircraft.

I'm not going to research how or if these limits are determined for Experimental Category aircraft, but I suspect that is left up to the builder's discretion. Most builders not being aeronautical engineers, it would seem their best guidance would come from the above FARs and the designer / kit manufacturer.

Will airplanes fly when operated above the weight limits specified for them? Of course, that happens frequently. It is simply that there is no assurance that the airplane will have adequate performance and structural margins.

 

[Cactus Charlie]

My answer - Max gross is a structural limit determined by analysis and static testing.

Mods to increase the max gross of the PA-18 have included addition fuselage bracing tubes and additional wing ribs. I think there may have been a rear spar modification too.

For the experimental CubCrafters aircraft the max gross it determined by the designer. Few builders would have the knowledge, skills, or resources to safely increase the designers limit.

In some cases the max gross is based on a certification rule not set by a structural limit. Have you ever seen a pictures of a Sport Cub with 2 people and camping gear and wondered about their actual loaded weigh? Maybe you also wondered about that 5 minute restriction on takeoff power.