Aero-Tips!
A good pilot is always learning -- how many times have you heard
this old standard throughout your flying career? There is no truer
statement in all of flying (well, with the possible exception of
"there are no old, bold pilots.")
Aero-News has called upon the expertise of Thomas P. Turner,
master CFI and all-around-good-guy, to bring our readers -- and us
-- daily tips to improve our skills as aviators. Some of them, you
may have heard before... but for each of us, there will also be
something we might never have considered before, or something that
didn't "stick" the way it should have the first time we memorized
it for the practical test.
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Aero-Tips 10.11.06
With two spark plugs in each cylinder, and four to six (or more)
cylinders in each engine, it may be inevitable that you'll see
evidence of spark plug failure. The trick is to catch (and correct)
the situation before it becomes catastrophic.
Two for the show
Piston engines have two spark plugs per cylinder for a couple of
reasons:
- Redundancy. If one plug quits the other can
continue to support combustion.
- Efficiency. Two ignition sources in a cylinder
create a quicker, more complete fuel burn, transferring potential
into power while the cylinder's valves are closed and explosive
force acts directly on the piston.
Note: You know two plugs are more efficient
than one, because you see a power loss (RPM drop) when switching to
a single magneto (turning off one plug per cylinder) during your
engine run-up.
Interesting indications
If you have the capability of monitoring all cylinders you'd see
an interesting indication when switching to a single magneto-each
cylinder's Exhaust Gas Temperature (EGT) will increase. But doesn't
single-ignition operation reduce power output? Sure. But when a
single spark plug is firing in a cylinder the fuel/air mixture does
not burn as fast; the fuel/charge charge is still on fire when the
exhaust valve opens, and the hot flame soars over the EGT probe in
the exhaust manifold. Although there is less power being developed
and the temperature inside the cylinder itself is indeed cooler, it
looks like the EGT is hotter because of the indicating error
created when active fire passes over the EGT probe. From where you
sit in the cockpit, turning off (or losing) a spark plug causes
that cylinder's indicated EGT to rise.
In-flight mag check
This means that, in the event of an in-flight spark plug
failure, the EGT on the affected cylinder will increase. If you
have a good engine monitor-and actively monitor indications-you'll
detect the rise and can pretty easily distinguish between an
ignition problem and others issues (like a stuck exhaust valve).
How can you do this? Perform an in-flight magneto check. There's
nothing that says you can't momentarily switch to a single magneto
in flight, just like you do before takeoff. In fact, some spark
plug failure modes will only show up at altitude, under cruise
running conditions. In cruise (and, if equipped, on autopilot as
your attention will be diverted), switch to a single magneto. All
EGTs should rise. Switch back to BOTH and the EGTs should go back
down. If an EGT does not rise on a single magneto, one of two
things will happen when you switch to the other magneto:
- That cylinder will "go dark", or die completely, indicating a
failed spark plug.
- That cylinder's EGT will remain high but constant on the other
magneto, possibly indicative of a sticking exhaust valve or a
disconnected magneto p-lead.
If you don't have an engine monitor it's harder to point
directly to an individual cylinder or even a single spark plug, but
you can still use RPM (with a fixed-pitch propeller or below the
governing range of a controllable-pitch prop) and a single-point
EGT to detect an anomaly.
Find a problem? Get the spark plug cleaned or replaced before
conditions worsen and you have an in-flight engine failure.
Aero-tip of the day: Save money on
troubleshooting and head off an engine failure by checking spark
plug indications before and during flight.