The AP’s job is only to ensure you have same weight of approved paperwork as your MTOW. They cannot refuse you a CoA if the paperwork is correct, but they can limit the area of your phase 1 to a rediculously small area if they think your a risk to others.
Bill Keenher is my AP and I found him extremly helpful as he did not suggest changes as part of his AP role, but rather as a fellow builder & aviator that has seen enough to know whats safe & works. So I think it prudent you listen but ultimately its up to you to build the plane your happy with.
I only have 1 battey & 1 alternator, but battery is sized to provide more time on the essential buss than I have time in fuel, so an alternator failure should not cause me too much concern. Alternator is an 80 amp from a subaru, modified to external regulation and I use a B&C linear regulator which has the overvoltage protection and gives complete control over alternator operation.
The reasoning behind using a 80 amp alternater was not that I required so much power but that the larger alternators have much better bearings, and according to my sparkie, bearings seem to be the most popular failure mode, the rpm of the rotary is really going to punish a small alternators bearings unless it is really geared down, then that could punish the windings if power demand is close to alternator rating.
Andrew
Hi there people
We are in the process of getting the Certificate of
Airworthiness for our Glasair powered by Renesis 4 port.
Our AP is questioning the electrical system and wondering if
we should have redundant supplies in case of electrical
system failure.
Can any of you make comments on risk or preferred systems.
Our setup has one automotive 40amp alternator and two HC20
(PC680 equivalent) paralleled (two batteries are used due to
physical location) and mounted in tail for W&B and to
keep cool.
My planning was:
1. Keep battery cool
2. Replace every two years
3. Battery Failure modes:
Open circuit cell - Alternator and second battery do
the job (No indication that one battery has failed)
Short Circuit cell - Battery voltage drops and
overcharges ??
4. Alternator failure modes:
Open Circuit - Low current and Low voltage alarm
with 30min electrical reserve capacity
Short Circuit - Charge circuit fuse blows then same
as open circuit
I understood that a shorted cell was very low risk in AGM’s
if not overheated.
I thought creating redundancy may introduce other failure
modes.
Appreciate any feedback.
Steve
Hi
Steve,
Are
you signed up for the Matronics Aeroelectric List, and do
you have a copy of the Aeroelectric Connection book? If
not, then stop, do not pass go, and do both. Those issues
are pretty well hashed out there.
Now
to specifics. Have you done a load analysis for your
plane? If not, you might be a bit surprised at how much
current can get sucked up by modern homebuilts, especially
running automotive style injection. FWIW, 55-60 amp
alternators are available for very little money (at least
here in the USA) and likely will weigh no more than your
40A model. The higher output models will be running a lot
farther from their max output (think heat).
Failure
modes: I'd agree that properly maintained, the battery is
one of the least likely things to have a catastrophic
failure. Alternators, not so much, though modern ones are
pretty tough. You left out one failure mode: overvoltage
due to regulator failure. That is the one that presents
the biggest risk, because it can take out everything
electrical in the a/c. There are simple protections for
it, which result in the same plan B as your other failure
modes: get on the ground within 30 minutes. Some have
tested battery life in VFR a/c using Tracy's engine
controller & minimal airframe electrical loads, &
saw around 35-40 minutes life with a PC-680 size SLA
battery.
More
later; time for breakfast.
Charlie
Ok, refueled.
The 'simple protections' mentioned earlier are an overvoltage
detection module that monitors bus voltage and if it gets too high,
has the power to shut down the alternator. B&C Specialty used to
sell a little 'crowbar' module that did this job. It's fairly simple
to fabricate your own, or you can buy something 'off the shelf' that
can work in a similar manner. ex:
https://www.tomtop.com/p-e1021.html
Here in the USA, inspectors have accepted single alt, single
battery, but most don't even question it; it's 'experimental'. But
if you accept the premise that odds of multiple failures in a single
flight *extremely* unlikely, and battery failures are almost always
'graceful' (meaning gradual and detectable), then the battery
becomes the backup for the alternator. It's our job to decide how
long we need that backup to last in operation, and to do our due
diligence in maintaining the battery in good condition. That does
mean doing capacity testing, because battery power and battery
capacity are not the same thing. A very unhealthy battery can start
an engine, but might have very reduced capacity to keep the
electronics alive for the expected time. In my mind, the biggest
risk is the battery's variable capacity as it ages.
I chose to add a 2nd identical alternator (~10 lb weight penalty),
and my wiring architecture has my engine functions on one bus and
the rest of the plane on a separate bus. Either bus can feed the
other if there's a power feed issue to a bus (bad switch or
contactor), and the engine will continue to run if the master must
be turned off ('smoke in the cockpit' scenario).
Charlie
Regards
Andrew Martin
Martin Ag
|