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. …
and
(…) A diode prevents ALT 2 from feeding Main Distribution Bus 1. Additionally, since ALT 2 voltage is slightly higher than ALT 1 voltage, bus voltage is further assured
The Cirrus electrical system went through several stages of development, but still suffers from its original set-up: it started out with a weak ALT2 and a backup battery connected to an essential bus. It was NOT designed as a dual alternator-dual battery-dual bus system like, for example, the Columbia 400.
There are two hold-overs from this design (including the the present G5):
The advantage of this design is that it does not need a cross-tie switch, if ALT2 fails ALT1 takes over everything.
The disadvantage is that an ALT1 failure is more serious than an ALT2 failure, and similarly BAT2 can’t power stuff BAT1 can.
Yes, that is correct, but the newer models have a 100 Amp Alt 1 and a 70 Amp Alt 2. (The TTx has 2 × 70 Amps)
My older G2 has 100 and 20 amps, so Alt 2 can only power the really necessary stuff.
My point is – the legacy of the Cirrus design means that even if Alt2 had 100 amp (which it now has in the G5, I believe), it cannot power MAIN DIST BUS 1 because of the way it is wired. So you don’t have MAIN BUS 1, MAIN BUS 2, landing light. I don’t remember exactly what was connected to these, it included stuff like de-icing, and some of the avionics.
Hence a broken ALT1 in a Cirrus means the aircraft is grounded, a broken ALT2 means it is grounded IFR (no electrical redundancy) but it can fly VFR.
In a proper, symmetrical dual-bus design you just flip the “cross-tie” switch on, and you have everything back. In the Columbia, you can fly VFR with either alternator INOP.
Just for the avoidance of doubt – none of these slight design flaws make any difference to the utility or overall quality of an aircraft…
I see your point!
I am still at the beginning to understand the differences in the electrical systems of different planes. I think the Alt 2 of the G5 is 70 Amps.
Actually on the Perspective Cirrus, an ALT1 failure is a non-event. I remember an exercise in the simulator at Aero Poznan where they simulated an ALT1 failure on a flight from Klagenfurt to Innsbruck. I did multiple approaches and it didn’t affect anything. This is actually the checklist from the POH for Perspective systems in case of ALT1 failure. Note that only two items are affected, the air conditioning/fan and the cigarette lighter outlet:
1. ALT 1 Master Switch ………………………………………………………. OFF
2. Alternator 1 Circuit Breaker ……………………………Check and Reset
3. ALT 1 Master Switch …………………………………………………………ON
If alternator does not reset
4. ALT 1 Master Switch ………………………………………………………. OFF
5. Notify ATC of Alternator Failure
6. Autopilot ………………………………………………………………… ENGAGE
Use of autopilot will reduce work load. Expect a slight pitch change when autopilot is disengaged.
7. Air Conditioning and Fan ……………………………………………….. OFF
8. Convenience Outlet ……………………………….. Disconnect appliance
Whereas on the Avidyne avionics, there is a long list of load shedding, which includes things like the audio panel, GPS/COM 2, fuel pump, panel lights, landing light, TAWS, Stormscope, and MFD. It is also a “land as soon as practical” condition, which is not the case with the Perspective.
In a simulator anything is a non event :-)
But you are right, the later system offers more redundancy. But even in mine I would have enough time to get out of IMC and land, I think BAT 1 should last about 30-45 minutes, and then I’d still have Alt2/Bat2 for navigation and radio.
Flyer59 wrote:
In a simulator anything is a non event
In the plane, ALT1 failure would be a non-event as well.
BTW, that sim feels very realistic because it’s an original cockpit and you have the full motion. My first instructor also told me about an instance where after an engine failure in IMC, the pilot crashed the plane (instead of using the parachute), and he just asked: “Do you have kids? What if this was for real?” and sent him home only 5 minutes after the start of the lesson to think about it. So it is really not like playing around on MS Flight Simulator.
I also wouldn’t say it offers “more redundancy”, because I can well fly without air condition and my iPad charger. I would call it “fully redundant”. The list of things to switch off in the Avidyne have the potential of making me feel uncomfortable, depending on the circumstances.
even in mine I would have enough time to get out of IMC and land, I think BAT 1 should last about 30-45 minutes
Way too short… in some situations I have been in, being forced to fly an IAP within 30-45 mins would be an all out emergency.
I know that, I’ve flown many sims (ATR72, B747, Eurofighter) … unfortunately never the SR22 Sim! For 2 years now I want to fly to Poznan, and I will.
I don’t know what systems you have to switch off with the newer electric systems. Of course that’s a safer system than the older version, no doubt about it. But with the Avidyne you can fly the plane safely on Alt 2 aswell.
