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Cessna Solenoids

 

Affected Aircraft

All/most 12V and 24V Cessna's manufactured before ??? (we suspect this issue has been rectified in the "new Cessna's" but have not been able to determine a "switchover" date as of yet)

 

Affected Starter(s)

All applicable manufacture/models.  For Sky-Tec, these include both 12V and 24V variants of models LS, PM, HT, and NL on Lycoming engines and ST3 on Continental engines.

 

Complaint/Issue

Activating starter yields no response from starter.  Sometimes just a "click" is noted.  Sometimes a high (amp) draw is also noted on the battery.

 

Course of Action

Remove starter.  Note damage or send to factory for analysis (preferred).

 

Physical Observations/Damage

Burned metal smell often accompanies the experience.  Internal starter inspection reveals damaged/discolored and often burned components including armature (commutator), brushes, brush insulator(s), motor housings (field) and other internal components.  Also often accompanied by damaged and/or deteriorated drive pinion and/or aircraft ring gear.

 

What Happened?


The aircraft's starter solenoid stuck closed.  When the start switch was released, the starter relay failed to open usually due to arcing/welding across the starter contactor's plunger/contacts.   

 

What about the Bendix?  Maybe it stuck.


Since Sky-Tec starters do not use mechanical Bendix drives to actuate the starter, this is actually nearly impossible for a Sky-Tec starter to keep itself engaged with the aircraft ring gear.  Sky-Tec starters are electromechanically engaged therefore requiring voltage to engage the starter's drive pinion gear with the ring gear.  Without voltage, the pinion simply cannot remain in the flywheel.  A spring and a helical return will both force the drive pinion back out of the ring gear and into the rest position.

 

If utilizing a Bendix starter, then yes, this very well may have caused the problem (and likely did - it is a very common failure mode of starter Bendix drives).

 

Why pick on Cessna?  Can't this happen to any similarly configured aircraft?


This absolutely could (and occasionally does) happen to any/all brands of aircraft.  However, we see this occur far more often with Cessna aircraft because of the type (architecture) of relay Cessna specified for use as a starter contactor in production of their aircraft.

 

Wrong Part - By Design?


Observing the parts manual for most Cessna aircraft, you will note the same part number used to describe both the starter contactor as well as the master switch relay (master contactor).  

 

For instance, one of our customers, Willie Zeiger who flies a beautiful Cessna 185 out of Anchorage Alaska, notes in a letter to the factory, "Both relays are rated for continuous duty and are good for both the master relay and starter relay."

 

However, the duty of each of these functions (starter contactor vs. master relay) are quite different and, as such, should (and in other makes of aircraft DO) require different types of relays.  

 

Relays usually incorporate two coils - a "pull" and a "hold" coil - and a system of springs (to return the solenoid when power is released) to best function for a particular task.  A master contactor pulls quite slowly/hard but holds quite easily.  This is because the master solenoid remains closed throughout the duration of the trip (i.e. "continuous duty").  A starter contactor, on the other hand, "pulls" quite easily (to close the "gap" quickly to minimize arching from the larger current loads created by the starter motor) and is only designed to "hold" for only short periods of time. (i.e. "intermittent duty").

 

Whatever their reason (or lack thereof), when Cessna specified relays for their aircraft, they often specified a continuous duty solenoid to be used as the starter contactor.  As a result, due to the excess current trying to "jump the gap" on the slowly closing continuous duty solenoid, the contacts become pitted and ultimately, in time, are prone to becoming welded shut altogether, thus causing the run-on problem and resultant starter  (and/or ring gear) damage.

 

Compounding the issue seems to be the overall poor quality of the solenoids used by Cessna.  Those solenoids, some manufactured by White Rogers, are built of very poor quality components, poor manufacturing and an overall weak design that easily compound their being prone to failure.

 

The Fix


The logical resolution would be to install the "right" component (an intermittent solenoid) for the starter contactor.  However, the issue of certification arises.  If Cessna's parts manual specifies the wrong part to be the legally certified replacement part, are we not obligated to install the wrong part?  We have not been able to determine a certified replacement for these incorrectly specified parts.

 

Being in Alaska, our friend Mr. Zeiger found it much easier to deal with his local ACO who informed him that even asking the question was a waste of their time as they encouraged him to proceed with his "minor modification".  He filed a 337 anyway and was very kind to leave us a sample starter solenoid 337 should you desire to use it as a template for your own.  Mr. Zeiger also provided us the following files you may also find useful:

 

Sample 337:  Starter Solenoid Conversion to Eaton PN: 6041H202, (MS24171D2)
* Note:  There may be a typo in the description of this solenoid on the sample 337
Starter solenoids should be "intermittent duty" - not "continuous duty"

Sample 337:  Master Solenoid Conversion to Eaton PN: 6041H215, (MS24171D1)

Eaton Catalog Page Showing Solenoids Above

Photo of Installed Eaton Starter Contactor

Starter Run-On Light Modification - Wiring Diagram

Starter Run-On Light Modification - Photo Installed

 

We are not sure how your local ACO works, but we do not think this would be the same reaction we should expect from the Ft. Worth office (though we haven't exactly tried, yet either).  However, if you should find luck achieving a certified and legal replacement for the Cessna starter contactor, please send us a note explaining how you did it.  We'd be happy to share the information here for others to gain from your pioneering, like Mr. Zeiger did.  Thank you, Willy!

 

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