Why would a plane descend rapidly in uncontrolled manner after a steep turn?
Markus Voelter interviewed Matt Hall for an episode of his omega tau podcast. They discuss this incident in more detail at 1:13:00.

The aircraft experienced a high speed g stall, also called an accelerated stall. Stall is determined by the angle of attack, which is the angle of the airflow hitting the wing. Stall is most often thought of at low speed, because as the plane goes slower, it takes higher angle of attack to provide enough lift, until it reaches the stall angle of attack and stalls. But this can also happen at high speeds in an aircraft performing a high g turn. Much higher lift is required to make the airplane turn, meaning much higher angle of attack is needed. Trying to turn the aircraft too tightly can increase the angle of attack too far, and the airplane stalls at a high speed.

The other factor is that the airplane will tend to roll left due to torque from the engine and P-factor of the propeller. Since the plane was already in a left hand turn, this plus the stall made the airplane roll past 90 degrees and head down towards the water.

Another aggravating factor was the airplane's stability, which is discussed at 1:31:00. The CG was fairly aft, making the plane less stable. The elevators were also over-balanced, meaning they would tend to make the airplane pitch up even further as the airplane pulled more g's. These changes had helped to make the plane a bit faster but were corrected after the incident to make it safer.

On the human factors side, Matt also mentions that he had a head cold at the time, and made the decision to take a phone call right before his flight, meaning he was distracted and not at his best.

Elsewhere in the interview he mentions that when he flies in air shows, he is more conservative in his margin of safety than other pilots might be.


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