MIA99LA185 · Beech E-35

While in smooth air in a right descending turn in VMC conditions between 160-170 mph IAS, the pilot encountered what he thought was turbulence. He slowed the airplane and landed uneventfully at an alternate airport. Damage to the empennage was noted. Both elevators/rudders (ruddervators) were out of balance limits and discrepancies pertaining to both ruddervators were noted. The ruddervators were reskinned and rebalanced in October 1982 and the airplane was painted August 1986; no record of balance check was noted after painting (contrary to the shop manual) through the last entry in the maintenance records. Airworthiness Directive (AD) 94-20-04, effective November 28, 1994, requiring in part balance check of the ruddervators, and a stipulation to repeat the balance check if the ruddervators(s) are repaired or painted. The compliance section of the AD states, '...unless already accomplished'. The AD was completed first in October 1995, in July 1997, and last July 1998; no evidence of balance check was annotated in the maintenance records during accomplishment with the AD. The airplane had accumulated at the time of the accident approximately 79 hours since 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 then due to the wording of the compliance section that states not required if previously accomplished. The mechanic also stated that AD 94-20-04 is 'hard to interpret.' Personnel from the FAA ACO stated that because the AD is subject to interpretation in the field, it may be revised. Review and analysis of the manufacturers flight test report pertaining to flutter indicated no critical frequency around 38.3 Hertz, which is the frequency when the propeller is operating at 2,300 rpm. Postaccident, the maximum vibration recorded from the engine/propeller was .6 ips; the propeller had been balanced to .15 IPS, 15 months before the accident. The maximum limit pertaining to attempting to balance the propeller established by the test equipment manufacturer is 1.2 IPS. The properly installed propeller was out of specification for chord width on both blades and for thickness for Blade No. 2. The aircraft's airspeed indicator was indicating 3-5 mph higher than the test unit when tested between 160 and 170 mph.

Verbatim NTSB analysis and probable cause from the NTSB dataset