YANKEETOWN, FL, USA
N3508B
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.
On June 5, 1999, about 1430 eastern daylight time, a Beech E-35, N3508B, registered to a private individual, experienced structural damage to the empennage while descending through 4,500 feet near Yankeetown, Florida. Visual meteorological conditions prevailed at the time and no flight plan was filed for the 14 CFR Part 91 personal flight. The private-rated pilot, the sole occupant, was not injured. The flight originated about 1330 from the Orlando Executive Airport, Orlando, Florida. The pilot who is a co-owner of the airplane stated that after takeoff the flight climbed to 8,500 feet then when the flight was approximately 25 miles from his destination airport, he began a descent. While descending through 4,500 feet in visual meteorological conditions at an indicated airspeed of between 160 and 170 mph in smooth air, while in a slight right descending turn, he encountered what he thought was turbulence. He then slowed the airplane, and landed uneventfully at an airport other than his destination airport. During a preflight to the airplane later during the day, he noted damage to the skin near the ruddervators. He then contacted an airframe and powerplant mechanic who inspected the airplane and confirmed the structural damage. The NTSB was notified 11 days after the event; the airplane was examined on June 30, 1999, by the NTSB investigator-in-charge, the FAA Flight Standards District Office inspector-in-charge, a representative from FAA Aircraft Certification Office located in Wichita, Kansas, and parties to the investigation consisting of one Raytheon Aircraft Company individual and one American Bonanza Society service clinic inspector. The other co-owner reported about 1 month after the accident that, "...on about three occasions I experienced some light vibrations in cruise configuration. It only lasted a matter of seconds and I do not remember what RPM or power setting it was on because it cleared itself up without requiring any immediate action." The results of the external examination of the airplane revealed that damage to the sides of the fuselage was confined between fuselage stations (FS) 233.5 and 272. A single compression wrinkle oriented at an angle of approximately 45 degrees was noted on the right side of the fuselage starting at the right hand channel Part Number (P/N) 35-410001-128 at FS 256.9 to the stringer P/N 35-41009-10 located at FS 233.5 (See Photo 1). Examination of the left side of the fuselage revealed a horizontally oriented compression wrinkle at the left hand channel P/N 35-410001-30, between fuselage stations FS 233.5 and 256.9 (See Photo 2). Vertically oriented compression wrinkles were noted on the forward and aft ends of the horizontal compression wrinkle. Compression wrinkles were also noted on the bottom fuselage skin between FS 233.5 and 256.9 oriented approximately 20 degrees from the longitudinal axis of the airplane starting at the left and right hand stringers P/N 35-410011-10, and 35-410011-11 respectively, located at FS 256.9 (See Photo 3). The lower fuselage skin was pulled from the rivets at FS 256.9 (See Photo 4). The skin of both elevator assemblies was made of magnesium and paint drips flowing towards the trailing edge were noted on both elevators. The shop manual in effect at the time of painting indicates, "when the elevator/rudder control surface is being repainted, suspend it by the trailing edge so that excess paint will drain towards the leading edge. According to the American Bonanza Society service clinic inspector, 26 rivets on the trailing edge of the right elevator adjacent to the trim tab were the wrong size, 1/8th inch instead of 3/32nd inch, and the skin aft of the trailing edge rivets was not trimmed to the skin doubler on both ruddervators. Also, paint was observed on the terminal ends of the elevator flight control cables. Both elevators were then removed for static balance check using a triple beam scale which determined that the left was found to have an underbalance of 21.47 inch pounds and the right was found to have an underbalance of 21.38 inch pounds. Specification for balance is 16.8 to 19.8 inch pounds. The left and right elevator counter balance weights and housings were weighed and found to be 1,371 and 1,378 grams, respectively. A report prepared by the FAA ACO individual describing the observed damage and discrepancies is an attachment to this report. Additionally the individual from the FAA ACO reviewed the ground vibration tests and flight flutter tests reports from Raytheon pertaining to the accident make and model airplane. The review determined that the flight test portion pertaining to flutter was conducted to 225 mph, and flutter clearance was shown by analysis to 270 mph. The flutter analysis reviewed did not show any critical frequency around 38.3 Hertz (Hz), which is associated with the propeller operating at 2,300 rpm. The analysis to the accident airplane is not applicable due to the fact that the balance of both elevators was out of the prescribed limits. Postaccident, engine vibration sensors were placed on the front and rear of the engine crankcase and the engine was started to determine vibration using Chadwick-Helmuth Co., Inc., equipment. The vibration was checked between 2,000 and 2,300 rpm in 100 rpm increments. The maximum vibration recorded was .6 inches per second velocity (IPS) at 2,300 rpm from the front sensor. The maintenance records indicate that the propeller was balanced to .15 IPS on March 16, 1998, after "rework". According to the FAA approved Publication Number AW-9511-2, titled "The Smooth Propeller" by James E. Fackler of the Chadwick-Helmuth Company, Inc., during the balance/vibration check, "WARNING - If IPS (Inches per Second Velocity) exceeds 1.2 at propeller RPM do not attempt balance...." The publication also states that acceptable balance level is .2 IPS at the rpm of the propeller. For this model airplane, Raytheon did not establish propeller/engine dynamic balance limits. The propeller was properly installed on the engine and was correct as indicated by the airplane type certificate data sheet. Examination of the propeller at an FAA approved propeller repair station revealed that the chord width at propeller blade station 42 was less than prescribed limits on both propeller blades. The airfoil thickness of propeller blade No. 1 was .022 inch below minimum specified thickness at blade station 42. Testing of the pitot static system was accomplished which revealed a static leak. Testing of the airspeed indicator was accomplished in 10 mph increments from 80-200 mph. The results with the test unit set to 160, 170, 180, and 190 mph revealed that the airplane airspeed indicator was indicating 4, 5, 5, and 3 mph higher respectively, than the test unit. The airspeed indicator had original markings. Airworthiness Directive (AD) 94-20-04 applicable to the accident airplane states in the compliance section, "Required initially within the next 100 hours time-in-service (TIS) after the effective date of this AD [November 28, 1994], unless already accomplished, and therafter as indicated in the body of this AD." The AD indicates in part to: 1) check the balance of the elevator/rudder (ruddervators) control surfaces in accordance with the shop manual 35-590096B which was reissued November 15, 1960. This is required any time the ruddervators are repaired or painted (even if stripes are added), 2) inspect the fuselage bulkheads at the specified fuselage stations with a repetitive inspection every 100-hour time in service, 3) fabricate a placard that reduces the Vne, Vno, and Va indicated airspeeds unless listed inspections and instructions are carried out. Review of the aircraft maintenance records revealed both ruddervators were reskinned and rebalanced on October 27, 1982, at an aircraft total time of 7,513 hours. The aircraft was painted last on August 18, 1986; the entry does not indicate that the control surfaces were balance checked. The shop manual in effect at the time of painting indicates, "After any repainting or repair, the finished surface should be check balanced to ensure that its static moment about the hinge line is within the manufacturers prescribed limits." The airplane had accumulated approximately 156 hours since then at the time of the accident. An entry dated March 8, 1988, indicates, "Rerigged rudders and elevator per maintenance manual." The first entry in the maintenance records indicating compliance with AD 94-20-04 associated with an annual inspection, dated October 15, 1995, indicates in part, "c/w [complied with] AD 94-20-04 v-tail inspection...." An entry also associated with an annual inspection dated July 18, 1997, signed for by the same mechanic who signed the October 1995 inspection, indicates in part, "C/W [complied with] AD 94-20-04 V-tail bulkhead insp [inspection], ck [check] ok." The October 1995 and the July 1997, entries does not indicate that the balance of the ruddervators were checked. Airworthiness Directive 94-20-04 was signed off as last being accomplished during an annual inspection on July 6, 1998; there was no record that balance check of the ruddervators was performed. The airplane had accumulated at the time of the accident approximately 79 hours since then. There was no entry in the maintenance records since AD 94-20-04 was last signed off as being completed to the date of the accident indicating work being performed to the ruddervators. According to an aerospace engineer from the FAA Wichita Aircraft Certification Office who participated in the investigation, AD 94-20-04, requires a mechanic to check the balance of the ruddervators and also repeats the requirement for balance check if the ruddervators are repaired or painted. He also stated that based on the wording of the AD in the compliance section, a mechanic does not have to check the balance of them if the maintenance records reflect previous balance check and no repairs or painting is carried out on the ruddervators after the previous balance check. According to the mechanic who signed off the annual inspection to the airplane in October 1995, which was the first inspection after the effective date of AD 94-20-04, the ruddervators were not removed for balance check. After reviewing AD 94-20-04, he indicated that the reason that the balance check of the ruddervators was not accomplished was because the compliance section of the AD states in part, "...unless already accomplished." He also stated that he believes the AD is "hard to interpret" and mainly involves checking the fuselage bulkheads every 100 hours time in service. Discussions with several FAA certificated mechanics revealed they believe AD 94-20-04 does not require balance check of the ruddervators based on the wording in the body of the AD that states required unless already accomplished. The mechanics stated that the AD requires a mechanic to check the specified bulkheads. Additionally, the owner of a fixed-base operator whose facility inspected 53 Beech Bonanzas in 1999, reported, in his opinion in reference to AD 94-20-04, many mechanics "take credit for compliance by inspecting only the two rear bulkheads. In many cases there is no inspection of the total structure that accepts and distributes the flight loads imposed on those bulkheads...." He also stated that balance check of the ruddervators is not being accomplished even though there is evidence that they have been painted or repaired. Another mechanic stated that in reference to AD 94-20-04, if the airplane does not look like it was recently painted, then he would not check the balance of the ruddervators.
The failure of maintenance personnel to balance check the ruddervators after painting the airplane and unclear information contained in the compliance section of AD 94-20-04 resulting in the failure of maintenance personnel to balance check the out-of-balance ruddervators and subsequent in-flight flutter and structural damage. Findings in the investigation were 1) improper modification/repair to both propeller blades by unknown person(s) resulting in them being below specification for chord width, and one of the propeller blades being below specification for chord thickness, 2) wrong sized rivets near the trim tab of the right elevator were noted and the skin of both ruddervators was not trimmed correctly, 3) paint drips flowing towards the trailing edge of both ruddervators contrary to shop manual.
Source: NTSB Aviation Accident Database
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