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Airworthiness Concern Sheet entitled TCM Magneto Impulse
Coupling has been released by the FAA
Date: 5/31/02
FAA Description of Airworthiness Concern (Who, What, Where, When, How?
Attachments: RA and appropriate data)
Impulse couplings are installed on the magneto to retard the spark timing and
thereby facilitate starting the engine. The original configuration of the TCM
magnetos utilized a riveted retention mechanism for the impulse coupling
flyweight. It should be noted that the “shower of sparks” magnetos are not
equipped with impulse couplings.
Impulse couplings incorporate moving parts, which are subject to wear. Service
reports indicated that the TCM magnetos were experiencing failures of the
riveted flyweight retention mechanism and, as result, the FAA issued
Airworthiness Directive (AD) 78-09-07 in 1978. The AD originally set mandatory
inspection intervals for all Bendix (now TCM) impulse couplings at 1,000 hours
time-in-service (TIS). The AD was revised three times in the next 5 years until
AD 78-09-07 R3 was issued in 1983 with a 500 hour TIS inspection interval.
In January 1992, TCM changed the impulse coupling flyweight retention
configuration from riveted to snap ring. TCM initiated a core return campaign
and replaced many early hot upset riveted couplings in the field with the snap
ring configuration. In addition to the retention technology change, the shape of
the flyweight was altered thereby changing the dynamics such that wear at the
flyweight axle is no longer compounded by wear of the tail against the body.
Other changes were made with the snap ring coupling introduction, including the
letter S stamped on both sides of the nose of each flyweight so that snap ring
couplings may be readily identified while the coupling is
still assembled to the magneto.
The current AD, 96-12-07, superseded AD 78-09-07 R3 and mandates inspection of
all TCM/Bendix impulse couplings, regardless of the flyweight retention
configuration, at 500 hour TIS intervals. TCM Mandatory Service Bulletin (MSB)
645, dated April 4, 1994, which is referenced in this AD, specifies inspection
of riveted couplings at 100 hour intervals and snap ring couplings at 500 hour
intervals.
The riveted impulse couplings are an obsolete configuration
that has been out of production since 1992 and has accounted for all the
failures since 1985. There have been no reported failures since 1997. All of the
reports of these impulse coupling failures in recent years involve the Lycoming
O-540 or IO-540 series engines with two single magnetos installed. None have
involved the dual, D-2000/D-3000 series, magneto models. However, the data base
may be small because Malfunction or Defect Reports, which comprise the data
base, are not mandatory and, therefore, may not have been submitted. The FAA
would like to determine the number of riveted configuration impulse couplings
that are still in service in order to continue to evaluate the potential safety
impact of this configuration magneto.
The FAA is using parallel channels to approach this issue. Concurrent with this
ACS, a Special Airworthiness Information Bulletin (SAIB) will be issued to all
owners of aircraft with the listed engines installed. The SAIB and this ACS
request the following information:
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The impulse coupling flyweight retention type (the letter S
stamped on both sides of the nose of each flyweight
indicates snap ring retention, the absence of the S indicates riveted)
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The model of the magneto.
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The time-in-service of the impulse coupling.
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The engine make/model.
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Any other relevant comments or information. Comments are
particularly requested relative to the current AD and whether the 500 hour
inspection interval is adequate, should the riveted couplings be inspected at
100 hour intervals,etc.
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Previously unreported in-service problems with either type
impulse couplings involving wear that caused engine failure or other problems.
* the actual pdf file
can be found here
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