The reason for the max. weight rule...
is to prevent certification of an LSA with an unrealistic useful load.
If someone was trying to certify a 950 lb. airplane with a Rotax 912S engine as an LSA compliant airplane, it would not be allowed (even if it meets all of the other performance requirements).
Since the max. gross is 1320 lbs., if the seats each had passengers that weighed only 150 lbs, it would still be limited to less that 12 gal of fuel.
If the weight in the seats totaled at least 350 pounds (Two 175 pounders, probably still low for what might be typical) you would now be down to less than 4 gal of fuel.
The FAA knows that people would over gross (above 1320 lbs) an airplane such as this, so they have minimum useful load weights based on a reasonable fuel payload depending on the size engine installed (related to how thirsty the engine is).
KPmarc posted a formula which is correct, except it is for calculating the minimum useful load, not maximum empty weight as he stated.
It uses the new FAA standard passenger weight of 190 lbs per passenger and .5 lbs of fuel per HP, to have at least a minimal range.
When calculating the minimum useful load, any LSA using the Rotax 912 ULS (100 HP) would have to have a useful load of at least 430 lbs or a max empty weight of 890 lbs. As you can see the example I used at the beginning of the post would not qualify.
As for certifying a new experimental as amateur built and then flying it as an LSA pilot, it is true you can do that if it meets the requirements.
What I have not yet been able to find out yet, is how the FAA determines if it meets them. The EAA web site has a list of airplanes that can be flown under LSA rules. It is not clear who made up that list.
The other thing that is not yet clear to me, is if I apply for certification of a new experimental amateur built airplane listing a gross weight of 1320 lbs, but it has an empty weight of 950 with a Cont O-200 installed, is the FAA going to issue an airworthiness cert. for it. It does not meet the minimum useful load requirements so it is highly likely that it would be flown at a gross weight of above 1320 lbs.
Mel, has the FAA issued any guidelines to DAR's?