My first in-flight mechanical failure

Peter Duniho wrote:

The capacitor is in the circuit in parallel with the normal path of the
current. If it fails in the closed circuit state, this provides an
alternate route for the current coming from the magneto, shorting it out
and
preventing a spark from being generated. But when it fails in the open
state, there is still the normal path of the current available for
shorting out the magneto when the ignition switch is in the off position.

Make sense?


Yes, thanks.

- Andrew

An open mag capacitor would make the spark very weak, and starting
would probably be impossible on such a mag. However, at higher RPMs
there might be enough spark to make the engine run. It'd raise hell
with the points in short order though.

I'm always surprised that there isn't more emphasis on what to expect
when doing a mag check at different speeds - like at idle which will
show bad points or that bad condenser. At cruise you could find a bad
coil or a harness problem - but you better do that near an airport
just in case. I wouldn't ever do a mag check at full power except on
the ground.

There are some recent ADs involving mag capacitors.

On 27 Sep 2004 13:34:51 -0700,
(Michael) wrote:

Peter R. wrote
I just received a call from my trusted mechanic early this morning.
He discovered that a capacitor failed in-flight, which in turn grounded
the mag.

This is a bit beyond my limited knowledge of aircraft mechanics and I
cannot correctly explain what a capacitor does, but the good news is
that it is much cheaper to replace a capacitor than a mag.

When I was an auto mechanic, we still had points type ignition
systems. Each set of points included a condenser, which is another
name for a capacitor.

The points provided the make and break component that caused the coil
to build voltage and discharge it creating the spark that is sent to
the distributer where it was distributed to the spark plugs.

The points sometimes saw as much as 12 volts but most had a ballast
resistor to reduce the amount of voltage the points had to deal with.

Electrons have inertia, once they are in motion, they like to keep on
moving. That's why when you unplug a light, or fan or something that
is drawing current, you see a spark from the plug. The moving
electricity wants to keep moving and attempts to keep the connection
as you pull the plug out.

The same thing happens at the points. When they are bumped open by
the cam that opens and closes the points inside the distributer, the
electricity tries to continue to flow through the points. Without the
capacitor/condensor, they'd arc every time and burn out quickly, or
build up a cap of burned metal, or literally weld themselves together.

The capacitor provides a momentary alternative (but dead end) path for
the electricity when the points open. The capacitors I'm familiar
with consist of thin aluminum foil insulated on both sides by a thin
paper, wrapped up in a roll and encased in a metal cylinder. The
cylinder is usually capped with some ceramic or plastic cover out of
which extends a wire.

Here is an "official" and more complete explanation of a capacitor I
found on the web: "Capacitor (Condenser) - The capacitor performs
several functions. It prevents the points from arcing and prevents
coil insulation breakdown by limiting the rate of voltage rise at the
points. It's primary function is to provide for a rapid decay of the
primary coil current. The capacitor also "third-harmonic" tunes the
coil, raising the peak output voltage and increasing the secondary
voltage rise time. This increases the efficiency and the amount of
energy transferred to the spark plugs. If the coil secondary voltage
rises too quickly, excessive high frequency energy is produced. This
energy is then lost into the air-waves by electro-magnetic radiation
from the ignition wiring instead of going to the spark plugs where we
would like it to go. Voltage rise time should be more than 10
microseconds; a 50-microsecond rise time is OK. Conventional systems
have a typical rise time of about 100 microseconds."

The capacitor is tested by checking to see if the wire is grounded to
the body of the capacitor. If it is, the capacitor has failed. If it
is not, the capacitor is theoretically ok.

The capacitor provides the afore mentioned dead end path for the
electricity when the points open. Remember, the electrons WANT to
keep on moving. They want to keep flowing through the points and can
and will arc to attempt to do this. But the when the points open, the
electrons see the path into the capacitor and jump inside and circle
all the way to the end of the foil, finds it's a dead end and races
all the way back out to the points. But they are open too far now for
the electricity to arc across them so they race back to the end of the
capacitor then back out again in ever diminshing cycles as it looses
energy.

You can see this on the oscilloscope, it's the up and down oscillating
lines that continue in diminishing amplitude after the spark fires.

The person who invented the capacitor was pretty smart. Magnetos
still use points, so they need to have capacitors or the points would
fail quickly. If the capacitor shorts to ground, it grounds out the
the points, which of course fails the magneto.

Corky Scott

Corky Scott ) wrote:

snip
The person who invented the capacitor was pretty smart. Magnetos
still use points, so they need to have capacitors or the points would
fail quickly. If the capacitor shorts to ground, it grounds out the
the points, which of course fails the magneto.

Thank you for taking the time to expand on the explanation. It is now
coming together for me.

--
Peter

In article , Peter R.
writes:

Not knowing what to
expect, I was pleasantly surprised to discover that the engine continued
to deliver with no roughness.

Landing was uneventful...

Good show. That's why they give you two of those.

Years ago, had an emergency in a SuperCub when a hydraulic valve lifter
assembly went fubar at 6000 agl. Performance was slowly deteriorating, and
streaks of oil started appearing on the windscreen. We declared an emergency
and arrived over FUL with over 3000 feet to spare. The clowns in the tower
wanted to vector us off towards Disneyland to decend so that they wouldn't have
to mess up the traffic in the pattern. Still not knowing the exact cause of
the problem and being concerned that the engine might quit at any moment, we
didn't think that flying away from a made airport was a good idea, and in no
uncertain terms declined the instruction. The traffic was cleared and after a
few tight circles we landed with no problem. We cleared the runway and shut
down.

Later we found out that the tower guys had a bet going on wether or not we had
run low on fuel. After landing, they had expected to see us sneak over to the
fuel pit. They didn't expect to see us imediately shut down off the taxiway.
When one of the ground workers came over and saw oil dripping out of the
engine, their contest was resolved.

John

JohnMcGrew wrote:

Later we found out that the tower guys had a bet going on wether or not we had
run low on fuel.

Nice to see someone casually betting on what could be a life or death
situation. Unlike a coach betting on his own team, though, at least the
controller gave you what you needed to have a successful outcome,
regardless of the side they chose in the bet.

--
Peter

In article , Peter R.
writes:

at least the
controller gave you what you needed to have a successful outcome,
regardless of the side they chose in the bet.

Only after we disregarded his "instructions" and told him what we were going to
do.

John