DA 42 electrical circuit structure / redundancy

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salim
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DA 42 electrical circuit structure / redundancy

Post by salim »

Hello,

I find the DA 42 electrical system relatively poorly explained in the POH, the diamond training handouts and by the instructor.

In particular, no explanations are given about the failure modes, the redundancy etc.

Given the reliance of the engines on electrical power, I find this particularly important.

Some of the questions I ask myself :

- You have two main bus (LH and RH), both connected to the battery bus via breakers
* what is the point of the LH/RH BUS design ?
* having both LH/RH voltage indications on the G1000 is thus most of the time useless as they are physically connected. You would have a different reading only if a circuit breaker pops. Correct ?
* What level of protection the breakers between busses provide ? Do you have a possible scenario where a short on the RH bus kills it, but thanks to the breaker the LH BUS would remain good ?
* would it be desirable in some situations to voluntarily split the system by pulling a breaker ? (probably not as it is not in the checklists...)

- The red electric master control switch commands three key relays :
* Battery to battery bus
* LH alternator connection to LH BUS
* RH alternator connection to RH BUS
Thus it looks as a single point of failure. But is it ? is there any likelihood of such a switch failing ?

- Has there ever been a total electrical failure on a DA42 ? (leaving aside the crash of the plane which took off with an empty battery and which led to the ECU backup batteries introduction)

Thanks a lot for your answers !
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Re: DA 42 electrical circuit structure / redundancy

Post by CFIDave »

My understanding of splitting electrical system consumers between LH and RH main busses is the idea of "don't put all your eggs in one basket," i.e., failure/shorting of one bus would still leave equipment/systems operational on the other bus sufficient to safely land the plane.

The dual voltage indications are tied to alternator outputs, not directly to LH vs. RH main busses (although the alternators also connect to each main bus before connecting to the shared battery bus). Each engine's alternator has its own voltage regulator for controlling output current, so the G1000 indicators let you see if these systems are working correctly. One of the alternator voltage regulators acts as "master" and the other "slave" so that they work together for automatic load balancing. If one of the alternators were to fail in flight (as actually happened to me in my DA42), the other alternator detects this and automatically provides greater output to power everything -- no action is required by the pilot.

If you shut off the red ELECTRICAL MASTER switch in flight (or it were somehow to fail in your scenario), the panel would go dark but the engines would continue to operate off of the backup ECU batteries. And you could then use the backup gauges (with yet another backup battery) for limited navigation to get you to a safe landing. So (except for the crash you mentioned before the system was redesigned years ago) there is no single point of electrical system failure.
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Re: DA 42 electrical circuit structure / redundancy

Post by salim »

Thank your for your answers Dave !

Do you have an idea of how the system would react in case on bus is shorted ? Which breaker(s) would pop ?

Also, looking at the electrical diagram I am under the impression that the ECU busses remain fed by their respective alternators if you turn the electrical master on off. Do you agree ?
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Re: DA 42 electrical circuit structure / redundancy

Post by CFIDave »

If either of the main LH or RH busses were to be shorted, it would likely pop either (or both) the BAT or ALT breakers (located on the bottom of the panel) for that bus, as too much current would rush into that bus from either the battery being shorted, or the alternator tried to generate too much current for that bus.

And yes, it appears that the ECU busses would continue to be fed by their respective alternators if the ELECTRICAL MASTER is turned OFF.
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Re: DA 42 electrical circuit structure / redundancy

Post by jast »

I‘m pretty sure the single red master switch is a certification requirement that you can remove power from the cockpit with a single switch in case of a fire/smoke. This does not include the ecus since they are not located in the cockpit. So you have a full redundancy on every part of the system to stay airborne except the fuel itself. (Btw Rudder and ailerons are considered „redundant“ and elevator and elevator trim are.)

On the Lycoming DA40s you have a combined Bat/Alt switch but they are mechanically couple so that one of them (BAT?) makes the cockpit dark.
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Re: DA 42 electrical circuit structure / redundancy

Post by salim »

On the SR22, you have one switch for each of the two batteries, and one for each of the two alternators.
So it seems more reliable than our single red switch.

But I don’t know how likely is a failure of a simple mechanical switch. It might be extremely remote....
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