In order to maintain aircraft safety, the useful load should never exceed which measurement?

The proper limit for the useful load is the maximum takeoff weight. The useful load is defined as the total weight of the payload—including passengers, cargo, and usable fuel—along with any other operational weight that the aircraft can safely carry beyond its basic empty weight.

The maximum takeoff weight (MTOW) is a critical safety parameter as it represents the total weight limit that an aircraft can safely handle during takeoff. Exceeding this weight can adversely affect the aircraft's performance, impacting its ability to generate sufficient lift, control, and adhere to structural limitations during the flight.

Other measures such as basic empty weight, maximum landing weight, and zero fuel weight serve different purposes in the context of flight operations and safety. Basic empty weight refers to the weight of the aircraft with all necessary equipment but without usable fuel or payload. Maximum landing weight is the maximum weight allowed for landing, which considers structural integrity and landing performance rather than takeoff conditions. Zero fuel weight is a limit set to ensure the structural safety of the aircraft without regard to the weight of usable fuel. Hence, the correct answer focuses on the threshold that directly influences the safety and operability of the aircraft during takeoff.