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1. 09:43 PM - Rotax 912 UL alternator drop-out (Paul Kuntz)
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| Subject: | Rotax 912 UL alternator drop-out |
I have a homebuilt Pipistrel Sinus with a Rotax 912 UL that I flew to
Oshkosh and back last week. The panel has a dual Dynon Skyview display with
two-axis autopilot, plus radio and transponder. The total electrical load
with everything running is 7 to 8 amps. Occasionally it can go to 10 amps
if I'm charging cell phone and iPad en route. I'm pretty confident that I
have the electrical system wired so that the ammeter is measuring the total
electrical load.
The alternator dropped out once on the outbound trip, but came back after
removing the iPad charging load. The symptom was a voltage drop from 13.8
volts to 12.5 volts. On preflight the voltage may read that high for a
minute or so on a freshly-charged battery, but it quickly drops to 12.2 or
12.1 volts. In flight, the voltage would drop to 12.5 volts and stay there
for several minutes. I would think that if the alternator had failed, I'd
see the voltage drop to the same 12.1 volts that I see on preflight, and
keep dropping slowly. I can't see why it would drop to 12.5 volts and stay
there, unless the voltage regulator is failing in a way that I'm not
familiar with.
The alternator dropped out (same symptoms) a few times on the trip back,
but by reducing the load to 3 amps (one Skyview display, radio and
transponder), it stayed on line except for awhile when it seemed that the
mid-day sun shining on the dashboard was adding extra heat behind the
panel. That time we brought it back by directing cool air from a fresh air
vent forward toward the spot where the voltage regulator is mounted inside
the cabin on the back side of the firewall.
So the problem seems to be heat-related. Does this make sense? The Rotax
alternator is supposed to be rated for 18 amps, and able to handle a steady
load of 14 amps easily. Does the Ducati regulator have a thermal
self-protect cutout? Do I perhaps have a regulator that is ready to fail
completely, or should I just put an avionics cooling fan behind the panel
to get the air moving?Any other ideas?
Regards,
Paul Kuntz
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