Hamid,

 

Thanks for your sensible approach to this issue.  I’ll try and address your questions as best I can.  See below

 

Shannon Knoepflein   <--->   kycshann@kyol.net

 

If you KNOW FOR SURE that the only thing that has gone wrong is a dead alternator and nothing else is affected, then continuing the flight, in fact continuing operations till you get back to the home base several flight segments later is not a problem.  However, the reality is that you don't know what has gone wrong and why. 

 

Why would you not know what has went wrong?  I guess I’m missing something here.  I guess we should evaluate the failure modes of the alternator.  If it is overloaded or shorted, the ANL pops and its disconnected.  This occurs at about 80A on an ANL limiter correct?  If the internals of the alternator fails, the circuit breaker pops the field and again, its dead.  If an over/under/runaway voltage condition occurs, the regulator gets it and shuts it down.  If the regulator has failed, the alternator is dead anyway because of not field.  If the belt to the alternator breaks, no output. 

 

Did the alternator suffer a catastrophic failure and has damaged other things as well? 

 

What do you mean by this?  Do you mean internals to the engine, belts, or electrical components?  If you are talking about a pad driven alternator, if it messes something up internal to the engine, I don’t think you’d have even 30 minutes.  If an alternator belt breaks, its dead, no power production…not sure what else this damage.  If you mean electrically, how could an alternator failure damage anything?  I’m not at all saying it couldn’t…I’m just asking how and what, as I have little experience (read none) with this failure mode.

 

 Did the alternator fail because of excessive load (a resistive short in the system) and the other alternator and the batteries are about to follow in short order? 

 

Wouldn’t an ANL limiter pop first?  Wouldn’t the failing device that is going to a resistive short’s protection (fuse or cb) kick in?  What devices fail in this mode?  I would think that few modern electrical devices, especially devices I would have on an Ess Buss would have this sort of failure mode (ie Chelton EFIS, Garmin 430, JET attitude indicator, Trutrak auto pilot, Garmin transponder, PSE audiopanel, etc)

 

What evidence do you have of this sort of resistive failure?  Being a EE myself, I understand the concept, but have never seen it in practice, especially in a solid state device.  What is your experience here?  This sounds really interesting, and perhaps my experience is extremely lacking in this area.  I’ve had a few solid state devices act strange, but never as I think you are describing.

 

Unless the device was on the Ess buss, I don’t see this as a concern.  (ie flaps, lights, hyd pump which would seem to have this resistive failure mode, but wouldn’t reside on the Ess bus)

 

Are there any other secondary failure modes that have compromised the system? 

 

Explain what you have in mind here.

 

Is the apparent failure of the alternator in reality some other failure of the electrical system? 

 

Example?

 

Did you learn about the failure when it happened, or are you becoming aware of it when the battery started to get almost drained?

 

This is one of the big drawbacks of most sytems, unless you have a way to monitor alternator current (which fortunately I do).  ScottK and I exchanged emails on this very subject a week or so ago on the LML.  If you recall that exchange, this is one of the exact reasons that I stressed a system should be overdesigned to have enough runtime on an Ess buss to outlast your fuel.  This will almost surely ensure that you have enough battery reserves to reach the ground in the case of a dual alternator failure.  You likely wouldn’t know the failure occurred until your battery had dipped below 13 volts where a warning pops up.  At this point, you could have already drained a considerable amount of your reserve.  Having the system capacity designed to compensate for this is a very desirable feature and was one of the main reasons I stressed a good measure would be to be able to continue flight past your fuel endurance.

In a perfect world you will know the answers to all those questions and will make an informed decision about whether to continue the flight or to land soon.  In the real world, you will have to guess, which brings me to the next point.

 

I’m really not sure where you feel the guessing would occur.  If you have a warning light, you know it isn’t producing.  If a CB or fuse pops on a component, you know it has a problem and you should leave it off.  If the field breaker pops, the alternator has failed and you leave it off.  If you have some sort of current monitor, you can see how much current the alternator is producing, or how much the battery is supplying.  What are we having to guess about?

Sorry, you calling me a "punk" certainly doesn't prove anything.

The choice of the phrase from a classic movie, while amusing, could be considered inappropriate.  However, the gist of the message was on point: You are counting on your luck when you press on after an APPARENT failure of an alternator by ASSUMING that everything else is fine.

 

I wouldn’t assume anything.  However, I could check the different parameters I have available to me (alt light, volt gauge at alt, volt gauge at batt, amps from alt, amps from bat, circuit breakers popped, compenents failed, smoke, smell, etc) and make a reasonable decision whether to continue flight.  I’ve said this before, and I’ll say it again so you are clear.  I’m not at all suggesting continued flight when a failure occurs is the best choice in all situations.  My original point was a system battery capacity should be designed so it could continue flight if both alternators failed, as this would give you a nice margin of SAFETY (that’s what this is all about) to get to the ground if you need to.  If the parameters you are looking at are telling you something has failed very badly, ie smoke or a bad smell in the cockpit, then by all means get to the ground.  On the other hand, if the parameters are telling you the alternator just quit outputting current, ie the alt switch is on, voltage is dropping and light is on, yet the breaker hasn’t popped, nor any other breaker, no smells or smoke (brushes stuck in alternator, for example, would produce this result), then by all means continue the flight.  If you ever are unsure about whats going on or have any questions, don’t continue, its that simple.

On a slightly unrelated note, let me expand on the term "resistive short" used above.  I came up with that term to differentiate from a "dead short" which is a short between two wires with a very low resistance (almost zero).  I have come up with the non-standard term "resistive short" to describe a short where there is some small but consequential resistance between the two wires.  This could be a 0.5 ohm short between the 12V and ground.  It will conduct 24A and generate 288W of heating.  In some instances a resistive short can be a worse condition than a dead short, especially when we have not thought about it and included it in our design.  The good news is that a resistive short is fairly hard to create and hence rarely encountered in real life.  The bad news is that it is most likely to happen as a secondary failure after something else (like and alternator) fails.

 

See above and explain when and how this would happen, as you have my curiousity.

 

Thanks for the great discussion.  I hope to learn a lot from this.  This list has so much knowledge that I hope we can tap into more of it.