Cessna Aircraft Company 402B, VH-BYN

The aircraft had just commenced descent, and the pilot was
trimming the elevator for the descent, when the pilot heard a loud
noise and the elevator control pitched down. The pilot was able to
arrest the aircraft's nose-down pitch, but was unable to trim out
the nose-down forces. He managed to control the aircraft
sufficiently to carry out a straight-in approach and landing. After
landing, it was found that the elevator trim tab was jammed in the
nose-down position. The trim tab was removed for investigation.
X-ray examination showed that the threads in both the male and
female screw assemblies were severely worn. The shaft was bent over
its entire length and had jammed in a position that corresponded to
the nose-down position of the tab. It is suspected that the male
threads had disconnected from the female threads when the pilot
turned the elevator trim wheel. The elevator trim tab was then
unrestrained and entered a flutter mode that only ceased when the
flutter forces bent and jammed the male end of the rod. The
actuator assembly is required to be inspected for condition and
security at each 100-hour inspection, and overhauled at either
1,000 hours or 3 years, whichever is sooner. The investigation
disclosed that the elevator trim actuator had last been overhauled
at 738 hours and 2 years 4 months prior to the incident. The last
periodic inspection was accomplished 3 weeks and approximately 40
hours prior to the incident. When the actuator was last overhauled,
the repair agency replaced the male shaft due to "wear in the
shaft". The manufacturer's maintenance manual states that, should
one thread require replacing, both the male and female threads must
be replaced as a pair. The local spare parts supplier advised that
they sell the male and female threads as separate components and
their recent stock records showed that they had sold 27 male ends
and three female ends. Another supplier confirmed this purchasing
imbalance. There is a large cost differential between the male and
female threads. SAFETY ACTION Local safety action As a result of
this occurrence, the Civil Aviation Safety Authority has published
Airworthiness Advisory Circular AAC1-111, dated 8/99, detailing the
flutter problems that can be associated with inadequate maintenance
of trim tab systems. The information has been published in the CASA
Flight Safety magazine in order to give it a wider distribution
than is possible through the Airworthiness Advisory Circular. ATSB
safety action As a result of the investigation into this
occurrence, the Australian Transport Safety Bureau (formerly BASI)
issues interim recommendation IR19990187 to the Cessna Aircraft
Corporation on December 16,1999. It is recommended that Cessna
Aircraft Corporation take appropriate action to ensure that Cessna
elevator trim-tab actuator component parts affected by this safety
deficiency are recoded so that they cannot be purchased separately.
In addition, it is recommended that Cessna Aircraft Corporation
review comparable designed trim-tab actuators to ensure that they
are not affected by this safety deficiency. Additionally, the ATSB
identified the following safety deficiency: A number of incidents
have occurred where the elevator trim-tab actuator jackscrew thread
contact area of Cessna 300/400 series aircraft has deteriorated to
a point where the threads either disengaged or "jumped" threads.
This event appears to be the result of non-adherence by operators
to the instructions in the manufacturer's technical manuals. This
condition could result in control surface flutter or a jammed
control surface, with a resultant loss of flight control. The ATSB
has issued the following safety advisory notices on December 16,
1999: SAN19990188 The Civil Aviation Safety Authority should note
the safety deficiency identified in this document and take
appropriate action. SAN19990189 The US Federal Aviation
Administration should note the safety deficiency identified in this
document and take appropriate action. SAN19990218 The US National
Transportation Safety Board is advised of the recommendation made
to the aircraft manufacturer and safety advisory notices issued to
the Civil Aviation Safety Authority and the US Federal Aviation
Administration.