On 06/18/2010 06:30 AM, Tony Renshaw wrote:
> So, I'd like to
> kill them off from my aft mounted battery. I am planning a kill
> switch in the passenger headrest area, but was wondering running a
> single Rotax alternator if anyone has advice on whether I could run a
> "relay" that would isolate the large battery voltage with a small
> charge via a panel mounted switch?
What you could do:
Mount a (car-style) relay (soleonid) of 40 Amps switching capacity next
to the battery. This connects the system bus with the battery. This
relay is activated by applying 12 Volts on its coil. The 12 Volt for
this is taken separately via a small fuse (about 1 amp), fed to the
switch on the instrument panel, and then to the coil. The switch is then
used to power up/down the ship. This fuse should be mounted as closely
as possible to the battery.
This is a pretty normal way to control the system bus. So far so good.
Now you add a separate KILL switch. This switch simply shorts the 12
Volt running to the power switch to ground. Throw this switch, and the 1
Amp fuse blows.
After this has happened:
1) The power to the coil of the relay is gone. Thus the relay drops off
and cuts the power to the ship.
2) The power to the power switch, which runs via the separate fuse, is
gone too, as the fuse has melted. There is no wire left carrying any
voltage anymore in the ship.
3) There is no way to power the ship up again. Power is permanently
gone... until you replace the fuse.
Needless to say that this KILL switch needs to be of a very good
quality, and guarded by a mechanical latch. As with all protections, you
introduce a small quantity of failure points as well. If the fuse
inadvertently blows (they sometimes do), power to the ship is gone. If
the KILL switch fails in the ON position, or is inadvertently switched
ON, power is irreversibly gone too.
Also, for this system to make sense, you should have no permanently
powered items in your ship (like the manual recommends for the stall
warner and vertical trim).
Another thing you could do is to obtain an old fashioned ELT. They are
scrap now 121.5 is no longer monitored by satellites. It should be
possible to get them on eBay for scrap value. Go for the switch inside
that activates by any (de)acceleration over 6G's, and use that one to
blow your fuse automatically. Again, of course you introduce another
failure point.
Personally, I didn't go all this far. What I did is to feed the system
bus via a car-style relay of 40 Amps mounted next to the read mounted
battery. The relay is permanently connected to the +12Volt, as is its
coil. The other end of the coil is connected via the system switch to
ground.
So, if the aircraft is powered down, the only voltage carrying wire is
the wire to the switch... but it is connected to the 12 Volts with the
coil of the relay in series with it. So, if this wire is shorted to
ground, the only thing that will happen is that it activates the relay
(which then powers up the ship). This is the worst that can happen.
Reasons that I didn't go any further are:
If you want to be sure that all spark-potential power is gone, you
shouldn't bring laptops, mobile phones, camera's etc. with you. The
modern lithium-ion batteries pretty much explode when they are shorted,
which is likely to happen if they receive a dent or are ruptured,
punctured or whatever during a crash.
Even without power to the ship, it is likely that crashing will
generated some sparks by its own, due to friction of metal parts. With
the fuel tank behind your back, with its connections just 1 cm above the
airplane bottom, it is unfortunate that a fire is very likely in a
serious crash.
I have been considering to make a fuel dump. A large valve to allow the
fuel to escape while still being airborne. Without fuel, sparks are not
dangerous anymore. Of course this makes only sense if you know the crash
is going to happen (just like with the kill-switch) and you have the
ship under control enough to make a controlled crash landing. Kill
switches or fuel dumps are worthless of course if you are going to dive
above Vne into the ground anyway.
I didn't implement a fuel dump, as this would introduce a potential
hazardous failure point in the fuel system. Imagine the fuel dump to
activate inadvertently...
What you should do in my opinion in case of a Rotax 914, is to have a
one way valve in the fuel return line. If the engine breaks off the
airplane, the tank can not syphon itself empty via the broken fuel
return line. And of course, if you have to make a crash landing, but are
able to delay the event, such as in case you left your gear behind on
the runway, or your rudder cable broke, or your elevator is stuck at the
140 knots position, or whatever... you'd better fly your tank almost
empty before attempting to land.
So, in case of an anticipated crash landing, I will just close the fuel
valve, throw the master switches, and go for the best. If the crash is
severe enough that it ruptures the fuel tank, changes are that it wasn't
a survivable crash anyway.
Frans
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