Kick-Back!

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, different ignition 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.

 

Copyright © 2017 Hartzell Engine Technologies LLC