In the Aircraft Flight Manual of the Bombardier CRJ700, the Minimum Flight Weight is listed as 42,000lbs. What is the meaning of Minimum Flight Weight?
This is the weight in an operational condition(with engine oil, hydraulic fluid, unusable fuel in the tanks[maybe also with a specific usable amount like 30 minutes], and some other misc items), zero cargo, with minimum crew.
The minimum weight at which the airplane can fly. You may also have maximum zero fuel weight, maximum landing weight, maximum takeoff weight, and maximum taxi weight.
After little digging found EASA Part 25/FAR 25.25 (same text):
CS 25.25 Weight Limits
(b) Minimum weight. The minimum weight (the lowest weight at which compliance with each applicable requirement of this CS–25 is shown) must be established so that it is not less than –
The lowest weight selected by the applicant;
The design minimum weight (the lowest weight at which compliance with each structural loading condition of this CS–25 is shown); or
The lowest weight at which compliance with each applicable flight requirement is shown.
So the aircraft has to meet all of its structural and performance design criteria at all approved weights. As you can see from the wording, the chapter it self is quite ambiguous and includes the entire regulation within its scope. This would include for example controllability and gust limits etc. For example, aircraft being too light would lose its controllability in engine failure or might overstress in sudden gust.
The Bombardier CRJ 700 is a has two rear jet engines with a T tail and a long forward fuselage. It's MTOW is around 75,000 lbs, allowing for around 33,000 lb combinations of passengers, cargo, and fuel. The 42,000 lb "minimum weight" (also listed as 44,000 lbs) would be a useful tool to determine safe loading of the aircraft.
But one other criteria for loading is Center of Gravity. Many CG diagrams look like a lopsided rectangle, especially for the utility category. Heavier loads favor more rearward placement. It occurred to me that some aircraft actually require a brace to keep the tail from dropping when completely unloaded.
So, "minimum weight" (for safe flying) may be different from "empty weight" because empty CG is too far aft. Further reading into the CRJ 700 POH with attention to CG charts may be in order.