When does lift begin to decrease during an increase in angle of attack?

Lift begins to decrease during an increase in angle of attack due to the aerodynamic phenomenon known as stall. As the angle of attack is increased, the airflow over the wings becomes more aggressive, generating increased lift up to a certain critical angle. However, beyond this critical angle, which typically falls within the range of 12 to 20 degrees for most airfoil shapes, the flow can no longer adhere to the wing surface and begins to separate.

In many aircraft, this critical angle often falls around 16 degrees, making it a common reference point. Beyond this angle, the lift is compromised because the airflow detaches from the upper surface of the wing, leading to a reduction in lift and potential stall conditions. While different aircraft may have varying stall angles, the 16 degrees mark is recognized in many cases as the point where lift starts to reduce. The gradual failure of lift continues as the angle of attack is increased even further, highlighting the significance of maintaining an optimal angle for effective flight.