SR-22 Electrical System

[M&Maviation]

3 Guys that train at the FBO I work at asked me to look over the SR-22 Electrical System for them and answer some questions about it. They are considering a partnership in a used SR-22.

The system is a 2 alternator system. The weird thing is if Alternator 2 takes a dump you can continue the flight with no problem, all systems are powered just fine. Alternator 1 pumps out 60 amps and 28 volts. Now Alternator two on the other hand only pumps out 20 amps and 28.75 volts, and there are diodes inbetween Alternator 1 and 2 that restrict alternator 2 from powering the systems on alternator 1's side of the buses. Alternator 2 contains the "essential bus". Once alternator 1 goes you really only have the primary instruments available to you. You lose the flaps, you lose a GPS, not that you need 2 in that situation. What I don't understand is why you would restrict an alternator from the ability to power the entire system. Doesn't really seem the purpose of redundancy to me, no?

Secondly the batteries Battery 1, 24 volts and a whopping 10 amp/hrs. Battery 2, 2 12 volt batteries wired in series, and 7 amp/hrs. With all the electrical powered instrumentation you would think there would be beefier batteries backing up the system in case of a total dual alternator failure. You don't really have much time to load shed anything with that little power getting sucked up by 2 GPSs, AP, MFD, EHSI, and who knows what else. Oh yeah, every other instrument in the cockpit, since there is no vacuum system.

So there are my questions, can anybody with some experience in these things help me out here.

Thanks,

M&M

 

[A Squared]

I don't have any experience with Cirruses (Cirrii ???) but I might be able to shed some light on the situation. I would imagine that the reason that the busses are seperated by diodes is to keep you from overloading the # 2 alternator if you lose the #1 alternator. I'm speculating here, as I don't know the CIrrus system at all, but it's likely that with all the toys running the draw exceeds the continuous output rating of the alternator. Let's say that you lose #1 alternator. Of course you, being the heads up individual that you are would immediately catch it and start shedding load to only what you really need....and you continue on no problem on the #2 alternator. OK lets say it's being flown by somone less perceptive, and they don't catch that the #1 alternator has become a boat anchor ... they keep on going with all the avionics and the pitot heat going and they gradually draw down the batteries, and overheat the # 2 alternator because of the amperage draw, so the # 2 alternator goes T.U....you can see where this is headed.....

Or, maybe on the Cirrus is is impossible not to notice that the 1# alternator has died, the primary display flashed "alternator failure" in red letters accross the MFD. OK, so this guy says "no problem" I got another alternator and keeps going, without shedding load down to somehtin that is managable by the #2 alternator .... same result as in the first scenario.

In short, they have designed the system so that you don't have to shed electrical load, it does it for you, even if you don't take any action.

 

[NuGuy]

I agree with ASquared. It sounds like to save a bit of weight/space/insert reason here they went with a smaller alternator. Since it has limited capactiy, it can't operate the entire electrical system on it's own, so they set up the electrical system to shed non-essential components.

This is very common with larger aircraft. You lose a generator, and the electrical system will shed non-essential items to prevent the remaining generators from overloading and tripping off (which is obviously worse).

Nu

 

[mtrv]

In addition to whats already been said. The standby alternator is usually bolted to an accessory pad, which in this case, is most likely where the vacuum pump would have been installed. Space is limited, along with physical size & amperage output of the alternator.

I'm familiar with a number of installations of this kind, but not the SR22.

 

[PropsForward]

Aircraft that I have seen with the 20amp alternator were equipped for flight into known icing. (The second alternator was part of the certification).