Bill Denton wrote:
Consider the following aircraft:

Seats ---------------------------------------------- 2

Empty Weight (typical) -------------------- 920 lbs.

Maximum Gross Weight -----------------1320 lbs.

Useful Load ----------------------------------- 400 lbs.

Payload with Full Fuel (18 Gals) -------- 292 lbs.

Suppose I wanted to add additional equipment to this aircraft.
Obviously, this would raise the Empty Weight, and lower the Useful
Load and the Payload.

Is there any sort of "minimum" Useful Load or Payload that must be
maintained when equipment is added, or is it simply a matter of
operating it within the "weight envelope"?

I guess for day VFR you would have to have at least a useful load weight =
your weight + the weight of 30 min of fuel. That gives you ZERO range and if
you added a few pounds over the holiday you are over gross.