Stab trim handles allow direct mechanical backup control of what component if electronic control fails?

The stab trim handles provide direct mechanical backup control for the horizontal stabilizer in the event of an electronic control failure. The horizontal stabilizer is critical for maintaining pitch control of the aircraft, and the ability to manually adjust the stabilization allows pilots to maintain control and stability during flight. The stab trim system is designed to trim the aircraft for various flight conditions, and having a mechanical override ensures that even if the electronic system encounters a failure, pilots can still make necessary adjustments to the horizontal stabilizer, thereby ensuring safe operation of the aircraft. This feature is particularly important in maintaining the aircraft’s control characteristics, especially during critical phases of flight.