During a descent at a constant Mach number (assume zero thrust and standard atmospheric conditions), what happens to the angle of attack?

In a descent at a constant Mach number, it is essential to consider how the dynamics of the aircraft change with altitude and speed. As the aircraft descends while maintaining a constant Mach number, its true airspeed (TAS) will increase due to the decrease in air density at lower altitudes. However, the key point here revolves around the relationship between pitch angle, descent angle, and angle of attack.

When maintaining a constant Mach number, the aircraft has to manage its speed relative to the changing atmospheric conditions. As this descent progresses, and the TAS increases relative to the true Mach requirement, the angle of attack decreases. This is because, to sustain the same Mach number at a lower altitude (where the speed of sound is lower) during a descent, the aircraft will need to adjust its attitude to maintain the required lift. A lower angle of attack is required at the higher TAS to keep the lift generated by the wings balanced with the weight of the aircraft during the descent.

Therefore, as the aircraft descends at a constant Mach number, the angle of attack must decrease to maintain steady flight conditions without increasing lift excessively, which could lead to other aerodynamic issues or potential stall if the aircraft were to pitch up too much.