The report provides a thorough analysis of the problem, with
obvious evidence of extensive research.
The actuator is confirmed to be On-Condition by the aircraft
manufacturer. The manufacturer's recommended maintenance program
includes a system check every 200 hours, and the Cessna aging
aircraft Continuing Maintenance Program calls for a visual
inspection of the flap system every 2,500 hours. Schedule 5 does
not require a specific inspection of the flap actuator.
The failure mode of broken drive gears is to either jam the flaps
in a fixed position or to allow the flaps to instantly retract
in-flight. The jammed flaps situation is not of specific concern,
as the aircraft certification requirements include to be able to
land without flaps. However, an instantaneous flap retraction could
be more significant, especially at critical periods of flight.
Instantaneous flap retraction may have been considered an unlikely
event and not specifically tested during certification of the
aircraft
Unfortunately, if the failure was caused by mechanical overload, a
regular maintenance program would not prevent failures unless the
inspection was conducted at very frequent intervals. This is
emphasized by the variable times when failures have occurred. An
effective maintenance program could. therefore be very expensive.
There are a number of these aircraft and related aircraft models on
the Australian Register.
We are seeking further information from the pilot who encountered
this event to ascertain the effect of the flap retraction. Due to
the time elapsed since the incident, and considering that the
aircraft has since been sold, we may not be able to accurately
determine the affect on the aircraft when the flaps retracted. Any
additional information you have from the incident report would
therefore be appreciated.
The result of questions you have asked the manufacturer would also
be of value in making a final determination in regards to this
recommendation.
In summary, the recommendation has merit, but has not been
implemented pending receipt of further information. Advice of the
manufacturer's comment would be appreciated when received.
