Why "redundant" isn't enough:

Pilots have always learned that redundant systems are important; that "whatever can break, will" and having a backup for important items of equipment is essential.

Nowhere is this more relevant today than electrical systems. The emergence of primary flight displays (PFDs), sophisticated autopilots and GPS means that losing electrical power is more than an inconvenience.

However, component failure is not the only threat, many failures create "shorts" of one sort or another. Bringing a redundant alternator into play simply means that the new alternator will be shorted also - possibly leaving you without any power at all.

We also need to introduce fault-tolerance and robustness into the equation: that if parts fail essential equipment is kept isolated from failures in less critical areas. You don't want to lose your PFD because of a short in a landing light system, for example.

Electrical systems with two alternators on one bus; or multiple bus systems that all connect together have excellent redundancy. They do, however, lack robustness and fault-tolerance.

Electrical Systems

The emergence of glass cockpits and other electronic systems in general aviation has changed the way we should look at electrical systems. Electricity is very much the life-blood of modern airplanes. Unfortunately, electrical system design does not always reflect this criticality.

Regulations require little more than dual sources of power (sometimes interpreted as an alternator and a battery) and backup instruments with their own power (sometimes this is vacuum or a small, dedicated battery). This ensures a level of redundancy for essential attitude, airspeed and altitude information.

The Cirrus electrical system goes much further and protects essential equipment (PFD, autopilot, GPS, etc.) by isolating it from all other equipment. Power can be drawn from any alternator or battery but essential equipment cannot be compromised by some piece of non-essential equipment or physical short elsewhere in the system. This introduces fault tolerance and robustness to the Cirrus electrical system.

Cirrus "all electric" airplanes have two alternators, two batteries and at least two buses for power distribution. Alternator arrangements vary by model with differing levels of redundancy. Robust, fault tolerant design ensures that essential equipment is supported indefinately after an alternator or main bus failure.