What if the Control Unit accidentally turns on the starter during flight?
Even though the solid-state switches are rated for 2 billion cycles, the system must be designed to anticipate that some will fail prematurely in the field. When these fail they usually fail closed, which is a non-event for some devices but a big deal for other devices. The starter, trim, and flap circuits are designed with two solid-state switches in series (one after the other), and each switch is controlled by a unique set of commands from the processor. This type of circuit provides redundant safety both on the switch and the commanding of that switch. Further, the starter circuit requires a heartbeat signal from the Display Unit to remain engaged. If the communication fails between the Control Unit and the Display Unit, the starter will disengage.
