Incident Description
During descent at a speed of 215 miles per hour, the aircraft encountered violent buffeting. The crew initiated recovery by reducing power, and the aircraft landed without any further incident.
Examination Findings
Post-incident examination revealed that the horizontal stabilator was permanently bent downward on both sides. Inspection of the counterbalance arm assembly showed that two small weights, referred to as slugs, had been used to achieve a balanced condition when a Supplemental Type Certificate (STC) modification was completed on the stabilator.
Modification History
The aircraft had been modified with gap seals on May 25, 1982, in accordance with STC SA516GL. The STC contained a limitation that it should not be incorporated into any aircraft having other approved modifications unless it was determined that it would not introduce any adverse effect on the aircraft's airworthiness. At the time of the incident, the aircraft had eight other modifications installed.
On July 10, 1982, the owner of the STC issued a service bulletin that effectively removed the gap seals on all PA-30 and PA-39 type aircraft that had previously been modified.