A
Sky-Tec Tech Letter posted by Les Staples, President, Sky-Tec Partners LTD.
“What
is a kickback?”you may ask.A
kickback is a case in which a cylinder of your engine fires
before the piston reaches TDC (top-dead-center) and the piston
tries to go backward.A
single cylinder can produce as much as 25-50+ HP and is bound
to win in setting the direction of rotation over a 2HP
starter. Kickbacks can be momentary: Cylinders 1,
3 & 4 do the right thing and keep the prop going the right
direction, but that #2 cylinder trying to go the wrong way
sends a shocking impact to the whole engine and especially
accessories - most namely, the STARTER.
“Does
the prop go backward?”
No. Not usually. In fact, here's a video one of
our customers captured of a kickback in their Cessna
150.
See the prop briefly pause at the 0:03
second mark (third compression stroke maybe)? Yep.
That's it. A kickback. One cylinder tried to go
backward. Didn't look like much, did it? But the
damage that could have caused could be catastrophic to starter
or other engine components.
“Well,
what causes kickbacks?”
The overall answer is, “The engine’s ignition system”.That is to broad an answer to be really useful, so
let’s narrow that down.If a magneto has just been installed and was not
static-timed correctly, a kickback may occur.Other than that simple cause, differentignition systems can have different types of problems
that are the root cause of the problem.Let’s see which ones your aircraft may have:
If
you have a“shower
of sparks” starting system (also called “retard point
mag”), go to #1, below.
If
you have impulse couplers in both mags, go to #2, below.
If
you have an impulse coupler in only one mag, go to #2,
below.
#1
Problems with “shower of sparks” starting systems.
This
type of system relies on a set of points on one of the
magnetos that is set to fire at or after TDC and a vibrator
system to generate multiple high-voltage sparks when this set
of points is enabled.The
normal, advanced, set of points in both magnetos must be
disabled during cranking or a kickback may occur.
A
complex key switch for magneto selection and starting is used
to perform this function.When the key is turned to the START position, several
things must happen at once:the normal P-Leads of both magnetos are grounded to
prevent the magnetos from firing too early, the P-Lead to the
retarded points in one of the magnetos is un-grounded to
enable firing at TDC, the “shower of sparks” vibrator is
turned on and the starter relay is activated.If any one of the following problems is present, a
kickback may occur:
The
key-switch is bad and one of the normal P-Leads remains
ungrounded during cranking.This allows the normal, advanced points to fire
early and cause a kickback.
One
of the P-Leads is broken or loose.This allows the normal, advanced points to fire
early and cause a kickback.
One
or both of the magnetos are defective.
#2
Problems with impulse-coupler starting systems.
This type of system
relies on a spring-loaded mechanism in one or both magnetos
which, when turned slowly, will “snap” forward at the
correct time and accelerate the armature in the magnetic field
to produce a stronger spark.At the same time, the firing is mechanically retarded
to fire at TDC or after.
If
you have impulse couplers in BOTH magnetos, go to #3 below.If you have an impulse coupler in only one (the left)
magneto, one or more of the following problems can cause a
kickback:
You
have a separate push-button for the starter and you are
cranking with the magneto switch in the BOTH position.This allows the magneto that does not have a
coupler to fire early and cause a kickback.You may get away with this starting procedure for
years and hundreds of starts as the spark will be weak -
but, one day, the mixture, temperature and fate will be
just right an you will have a kickback.
You
have a separate push-button for the starter and you are
cranking with the magneto switch in the LEFT position.A defective switch or broken or loose P-Lead to the
right magneto, allowing it to fire while cranking and
cause a kickback.
You
have a key-start switch and the switch is defective or
there is a broken or loose P-Lead to the right magneto,
allowing it to fire while cranking and cause a kickback.
The
impulse coupler in the Left magneto is defective and fires
early.
#3
– impulse couplers in both magnetos.
An
impulse coupler in one of the magnetos is defective and
fires early.
Troubleshooting
the Ignition System
Some
simple tests can be made with an accurate mili-ohmmeter which will find many
of the potential problems.Measuring resistance between the aircraft frame
(ground) and the P-Lead terminal on each magneto while trying
to crank (with the battery disconnected, of course) should
show low
resistance to ground while attempting to crank the engine
(for impulse coupled mags).
Put a timing light on the magneto.
Is it firing before top dead center?
Remember
that, in the case of a key-start switch going bad, it may be
intermittent.Therefore,
watching the meter while wiggling the key in the “crank”
position may show the resistance to ground varying.This indicates a switch problem.
The
good news.
Many
Sky-Tec starters feature kickback protection. Internal
mechanisms are designed absorb the shock of the kicbkack
before it can do much damage to the starter or other engine
components. If kickbacks are a concern for your
aircraft, please look closely at each Sky-Tec starter model
and select a model that offers the best protection for your
particilar aircraft.
