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:
