Alternator B Failure Diagnosis

Peter, 

My original post two years ago was in reference to a friend's aircraft that I was helping with. More recently the stator in my 912iS also failed in the exact same way. Both of our aircraft had moderate loads on the airframe, with two flight displays, two radios, and equipped with autopilots. So, too light or two heavy power loading should not have been a factor.  Anyway, that argument is hearsay, there is nothing in the Rotax manual stating that you need a specific load so that your stator won't burn out. Only a maximum load is specified. 

From what I can tell, this is not uncommon on the 912iS engines that were produced before the new oil spray nozzle was added to the sprag clutch housing. To do this upgrade, Rotax says you must also change the ignition housing, which we did in both cases. However, I have been told by a recognized Rotax expert that the only difference in the new ignition housing is the elimination of one very small oiling hole, and I know there have been folks who have upgraded the sprag clutch housing and oil spray nozzle without changing the ignition housing.  In my case we were working on SLSA, so we followed the Rotax recommendation without question.  However, I would like to know from Rotax exactly why it's important to change the ignition housing during this process. I would also like to know why they are not helping with the upgrade, but that's a separate subject.  The three 912iS aircraft in my immediate circle have all experienced this failure at 320 hours, 530 hours and 740 hours.  All three sent in CSIR's with requests for financial help, and none ever received a response. 

In my opinion, you need to do the upgrade for a better chance of longevity.  Also, make sure you install the larger connector at the stator B to regulator B connection.  This newer connector looks oversized, but the bigger size and the fact that it has a metal housing allows it to act as a heat sink to remove some heat from the wiring. And if you have not already, I would replace the regulator at the same time. 

There is a rumor of a new regulator in the works that is based on MOSFET rather than IGBT switching and should reduce the heat load on the regulator and stator.  This is being developed by a third party, not Rotax.

Hi Jeff, Hi Peter

Go online to flyrotax.com and fill in a CSIR and ask your Rotax agent (I believe you are in New Zealand from what I can tell} to submit a goodwill warranty request.  If this was a factory built aircraft push a bit on them for help.  I know of a lot of Sling aircraft where they have done this and gotten parts for the change..  You will need to supply a full copy of your logbook to show that the maintenance and checks have been done.  

Give it a try, worst case is they will not respond, push for reply.  I agree with Jeff, do the connector upgrade as it is a problem area.  If the heat is getting high on the regulator you need to add some cooling airflow to the base of the regulator as it bleeds off the heat between it and the plate that it mounts on.  They do have a special heat sink that can be added to the regulator but that is telling me you do not have enough cooling air for the regulator as it dumps energy.  Jeff pointed out that you should try to use some of that energy, dont be afraid to run those bright strobes for example, you have about 30 amps on the B side alone.  A is not an issue as it runs a lot less energy.  

Cheers

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