Incident Overview
While flying in smooth air under visual meteorological conditions (VMC) during a right descending turn at an indicated airspeed between 160 and 170 mph, the pilot encountered what he believed to be turbulence. He reduced the aircraft's speed and landed uneventfully at an alternate airport. After landing, damage to the empennage was noted.
Ruddervator Condition and Maintenance History
Inspection revealed that both elevators/rudders (ruddervators) were out of balance limits, and discrepancies pertaining to both ruddervators were documented. The ruddervators had been reskinned and rebalanced in October 1982, and the aircraft was painted in August 1986. However, no record of a balance check after painting was found in the maintenance records, contrary to the shop manual's requirement. This condition persisted through the last entry in the maintenance records.
Airworthiness Directive (AD) 94-20-04, effective November 28, 1994, required, in part, a balance check of the ruddervators and stipulated that the check must be repeated if the ruddervator(s) are repaired or painted. The compliance section of the AD stated, "...unless already accomplished." The AD was first complied with in October 1995, again in July 1997, and most recently in July 1998. However, no evidence of an actual balance check was annotated in the maintenance records during any of these AD accomplishments. At the time of the incident, the aircraft had accumulated approximately 79 flight hours since the last compliance with AD 94-20-04 and approximately 156 hours since painting.
The mechanic who first performed AD 94-20-04 in October 1995 stated that the ruddervators were not balance checked at that time due to the wording of the compliance section, which indicated that the check was not required if previously accomplished. The mechanic also noted that AD 94-20-04 was "hard to interpret." Personnel from the FAA Aircraft Certification Office (ACO) stated that because the AD is subject to interpretation in the field, it may be revised.
Propeller and Airspeed Indicator Findings
Review and analysis of the manufacturer's flight test report pertaining to flutter indicated no critical frequency around 38.3 Hertz, which corresponds to the frequency when the propeller is operating at 2,300 rpm. Post-incident, the maximum vibration recorded from the engine/propeller was 0.6 inches per second (ips). The propeller had been balanced to 0.15 ips 15 months before the incident. The maximum limit for attempting to balance the propeller, as established by the test equipment manufacturer, is 1.2 ips.
The properly installed propeller was found to be out of specification for chord width on both blades and for thickness on Blade No. 2.
Additionally, the aircraft's airspeed indicator was tested and found to indicate 3 to 5 mph higher than the test unit when tested between 160 and 170 mph.