What functions are lost when PRI stab trim is switched to SEC trim?

When the PRI stab trim is switched to SEC trim, the functions related to autopilot and certain specialized trims are lost. This is because the primary (PRI) trim system provides more comprehensive control capabilities, including the ability to adjust based on various flight parameters such as Mach number and configuration. When transitioning to the secondary (SEC) trim, these more advanced features are disabled, leading to a reduction in the aircraft's automated stability and handling capabilities.

This includes the loss of functionalities that manage how the aircraft responds to speed variations and the specific requirements for maintaining optimal flight conditions through the autopilot system. The SEC trim is more basic and does not incorporate these sophisticated adjustments, therefore limiting the aircraft’s overall flight performance and control.

The other options refer to other aspects of flight control that are either not directly impacted by the trim switch or relate to primary flight control functions that remain active. Understanding these distinctions helps clarify the implications of switching from primary to secondary trim settings in terms of aircraft management and control performance.