My first in-flight mechanical failure

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.

In case you ca

Most mag installations in modern (read - designed for electrical
systems and radios) aircraft have a capacitor between P-lead and
ground. This absorbs some of the electrical noise, and generally
reduces static on the radios.

These capacitors fail with depressing regularity, but they usually
fail 'open' - that is, they stop doing their job and radio noise
increases, but the mag keeps working. This is the first time I've
heard of one failing 'shorted' - but if one does fail shorted, you
ground the P-lead and it's as if you turned the mag off.

Glad that worked out for you - those things ARE cheap - should be
about $30 for the part and 30 minutes to change it.

Michael

Michael wrote:

In case you ca

Of course I care. Hence the secretly placed bait, which you
thankfully took!

Most mag installations in modern (read - designed for electrical
systems and radios) aircraft have a capacitor between P-lead and
ground. This absorbs some of the electrical noise, and generally
reduces static on the radios.

Very interesting. Over the last three months I have been having mag
noise problems (pop-pop-pop on frequency) whenever I used the Garmin
G430 com radio and full power, almost exclusively when tuned to a higher
frequency (130.00 or above). This interference did not occur with the
older B/K comm radio I use as my second radio, so I would end up using
the B/K radio during cruise.

When I asked a trusted avionics shop about this, they explained that the
mag itself was causing the interference and that the Garmin circuitry
was more sensitive to the interference than the older B/K radio. Based
on your explanation, I wonder now if it was really this capacitor
showing signs of imminent failure?

These capacitors fail with depressing regularity, but they usually
fail 'open' - that is, they stop doing their job and radio noise
increases, but the mag keeps working.

Good to know.

This is the first time I've
heard of one failing 'shorted' - but if one does fail shorted, you
ground the P-lead and it's as if you turned the mag off.

That is almost word-for-word how my mechanic explained the mag failure.

Thank you for your explanation.

--
Peter

Michael wrote:

These capacitors fail with depressing regularity, but they usually
fail 'open' - that is, they stop doing their job and radio noise
increases, but the mag keeps working.

Which would yield an unexpectedly "hot" prop with the switch in the "off"
position, yes?

- Andrew

Andrew Gideon wrote

Which would yield an unexpectedly "hot" prop with the switch in the
"off" position, yes?

No!

Bob Moore

Bob Moore wrote:

Andrew Gideon wrote

Which would yield an unexpectedly "hot" prop with the switch in the
"off" position, yes?

No!

Sigh

So that would only occur if the capacitor failed "closed"?

- Andrew

"Andrew Gideon" wrote in message
online.com...
Which would yield an unexpectedly "hot" prop with the switch in the
"off" position, yes?

No!

Sigh

So that would only occur if the capacitor failed "closed"?

No. If the capacitor failed closed, that would short out the magneto, and
prevent it from working. In fact, that's exactly what happened to the
original poster here. It would not result in a "hot" prop.

Andrew Gideon wrote

So that would only occur if the capacitor failed "closed"?

No!
Just can't win, can you Andrew.....:-)

Bob Moore

Bob Moore wrote:

Andrew Gideon wrote

So that would only occur if the capacitor failed "closed"?

No!
Just can't win, can you Andrew.....:-)

So that would only occur if the capacitor failed sideways?

- Andrew

"Andrew Gideon" wrote in message
online.com...
These capacitors fail with depressing regularity, but they usually
fail 'open' - that is, they stop doing their job and radio noise
increases, but the mag keeps working.

Which would yield an unexpectedly "hot" prop with the switch in the "off"
position, yes?

To elaborate on Bob's correct reply...

A broken p-lead would result in a "hot" prop even with the ignition off.
But the capacitor, not being the normal component used to short out the mag,
would have no such effect if it failed open.

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?

Pete

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