In constant Mach cruise control, what happens to thrust as aircraft weight decreases?

In constant Mach cruise control, as aircraft weight decreases, thrust is reduced to maintain the desired Mach number. This is due to the relationship between weight, lift, and thrust. When the aircraft's weight decreases, the amount of lift needed to maintain level flight at a constant Mach is also reduced. As less lift is needed, the engines don't have to produce as much thrust to overcome drag and maintain cruise conditions.

Moreover, at a constant Mach number, the aircraft's speed through the air increases as its weight decreases, which leads to a corresponding decrease in the thrust needed to sustain that speed. Therefore, reducing thrust helps optimize fuel efficiency and performance without compromising flight safety. Understanding this relationship is crucial for flight engineers when managing the aircraft's performance during different phases of flight, especially in cruise.