When a trim air valve fails closed, optimized regulation is lost, and the trim air valves are driven to the fully closed position. In this case:

• Pack 1 controls the cockpit temperature to the selected value.
• Pack 2 controls the cabin temperature (both FWD and AFT) to the average of the selected temperatures.

This ensures some level of temperature regulation, but without the finer adjustments normally provided by the trim air valves.