AFAICT voltage regs do fail periodically, and mainly due to the environment (usually forward of the firewall) i.e. heat, cold, damp, vibration… so keeping a spare is a good thing. To service them you need to understand electronics.
That’s a good online MM. Socata and most others never did that.
(the MM and the PM are online too)
I guess Cirrus had one big advantage here, because everything was drawn by CAD anyway, and the MM and PM show that.
Diamond also publishes them on their website for everyone to see. They just don’t have it one page at a time. And there are other manufacturers who do that as well.
Flyer59 wrote:
I guess Cirrus had one big advantage here, because everything was drawn by CAD anyway
That is no significant advantage. You can scan any drawing or document you want. And there is no need to do it yourself.
As someone who knows nothing about Cirrus aircraft, why doesn’t the existence of ALT2 make this a non event? I don’t understand why ALT2 didn’t prevent the low voltage condition? On one of the pictures it shows an ALT2 current of 1A, surely it can fo more than that?
With Alt 1 inop I was running on battery only becasue Alt 2 will only kick in at 1700 rpm. Until Alt 2 comes online, Alt 1 charges Batt 2 and the Essential bus voltage is equal to Alt 1 voltage. If Alt 1 isn’t working, it’s battery voltage only.
Once you are over 1700 rpm the ALT 2 light goes out
Totally dual systems are rare on piston GA. The Cessna 400 (TTX) has one: 2 alternators, 2 buses, 2 batteries, crossover switches. Like a 737 
On the SR22, is the 2nd voltage regulator set to the same voltage? Normally, a backup alternator is voltage-regulated to some 0.5V below the main one, so normally it gets a zero field current, and if the main one fails, you get 27.5V on the bus.
Neil wrote:
As someone who knows nothing about Cirrus aircraft, why doesn’t the existence of ALT2 make this a non event?
AFAIK ALT2 only powers essential avionics (transponder, for example, is not essential).
The SR 22 has that too, BAT 1/2, ALT 1/2, two busses.
The older versions (like mine) don’t have a very strong ALT 2 (20 AMPS) but the newer Persepective ones have a 70 Amps ALT 2.
here’s the desciption:
During normal operation, the alternators feed their respective distribution bus independently (ALT 1 feeds the Main Distribution Bus and ALT 2 feeds the Essential Distribution Bus). The distribution buses are interconnected by two 50-amp fuses and diodes. The diodes prevent ALT 2 from feeding the Main Distribution Bus. Additionally, since ALT 2 / Essential Bus voltage is slightly higher than ALT 1 / Main Distribution Bus voltage, ALT 1 will not feed the Essential Distribution Bus unless ALT 2 fails.
The alternators are three-phase AC generator type; each alternator has an internal rectifier, which lim- its current output to 28 VDC. Each alternator uses two diodes for each phase to rectify the output. In addition, these diodes will block reverse current. If a single output diode were to fail, the winding junc- tion will be held to the output potential, reducing the quantity of current the alternator can produce.
The alternators are regulated by the MCU and power the MCU via 6 AWG tin plated copper wires. Alternator 1 is protected by an 80-amp fuse within the MCU while alternator 2 is protected by a 40-amp fuse within the MCU. ALT 1 is connected to the Main Distribution Bus and ALT 2 is connected to the Essential Distribution Bus. Each alternator system has its own 5-amp alternator circuit breaker located in the circuit breaker panel. Each alternator is individually protected against overvoltage generation by the field control module, located within the MCU. ALT 1 is regulated to 28 volts and ALT 2 is regulated to 28.75 volts.
Voltage output of each alternator is a function of engine RPM, alternator design, and load on the alter- nator. During low RPM operation, the alternator will require higher engine RPM to provide the same voltage to increased electrical loads. With nominal loads on the electrical system, alternator 2 will drop off at approximately 1700 – 2200 RPM and alternator 1 will drop off at approximately 600 – 900 RPM. Alternator 2 drops off before alternator 1 because alternator 2 spins half as fast as alternator 1.
The drop off RPM for each alternator will change whenever the electrical system loads are altered from nominal. Any time an alternator drops off line, it will be indicated by illumination of the corresponding ALT 1 or ALT 2 annunciators. If an alternator drops off line due to low engine RPM, the alternator(s) can be restarted by simply increasing engine RPM. When alternator 2 drops off line, the ALT 2 annun- ciator will illuminate and alternator 1 will provide electrical power to the entire electrical system, includ- ing charging both batteries. When alternator 1 drops off line, the ALT 1 annunciator will illuminate and alternator 2 will provide electrical power to only the Essential Bus and BAT 1 will provide power to the Main Bus. Alternator 2 will then allow continued operation of only the flight critical instruments and charging of battery 2.
