CHI97LA119

The pilot reported that he made a climb to 7,500 feet and set his power to cruise at 160 mph. To remain clear of clouds, he started a descent to 6,500 feet. He experienced light turbulence, and the airplane began to vibrate and shake. After 10 to 15 seconds, the pilot pulled back on the throttle, and the vibration stopped. The pilot thought the vibration was engine related. He re-applied power for cruise, then the airplane started to shake again, but much more violently than the first occurrence. He reduced power and landed the airplane. The airplane's never exceed airspeed (Vne) was 203 mph, and the maximum cruise airspeed (Vno) was 160 mph. Inspection of the airplane revealed that the aft fuselage received extensive damage between fuselage station (FS) 256.9 and the 233.5 bulkhead. The longerons were bent downward, and two stringers were cracked. The left and right side fuselage panels below the stabilizers exhibited 45 degree buckling from the bottom of the FS 256.9 bulkhead to forward of the stabilizer cuffs. The bottom fuselage panel had popped rivets and torn skin, and there were two 2-inch creases in both stabilizers. No pre-existing cracks were found on the longerons. The ruddervators were properly balanced.

On April 29, 1997, at 1330 eastern daylight time (EDT), a Beech D-35, N5985C, sustained substantial damage during cruise flight when it experienced tail flutter near Mt. Sterling, Kentucky. When the pilot reduced power the airplane stopped shaking, and the pilot elected to land at Lexington, Kentucky. Neither the pilot nor the passenger received injuries. The 14 CFR Part 91 flight departed Chapel Hill, North Carolina, at 1130 EDT en route to Des Moines, Iowa. A fuel stop at Seymour, Indiana, had been planned. Visual meteorological conditions prevailed and a VFR flight plan had been filed.

The pilot reported that after departing Chapel Hill he climbed to 7,500 feet. He reported that he set the power for cruise flight which produced about 160 mph. He reported that the sky was initially clear, and then turned to partly cloudy at his flight altitude. In order to keep clear of clouds he started a descent to 6,500 feet.

He reported that he experienced some light turbulence and the airplane began to vibrate and shake. After 10 to 15 seconds, the pilot pulled back on the throttle and the vibration stopped. The pilot thought the vibration was an engine related problem and suspected the engine might fail. After checking the engine gauges, he recognized the engine was operating normally and he applied power for cruise flight. He reported that before reaching cruise airspeed, the airplane started to shake again, only much more violently than the first occurrence. After 5 to 10 seconds, the pilot reduced power and the shaking stopped. He reported the shaking, "...reminded me of an old car with wheels out of alignment and balance. I thought the engine was coming out of [the] plane... ."

The pilot continued flying at 140 mph to Lexington, Kentucky. After an uneventful landing the pilot taxied to a ramp where he could do an engine run-up. The pilot applied full power and the engine and propeller operated normally with no vibrations. After the pilot deplaned, he observed that the inspection access panel door on the left side of the empennage was being held on by one screw. He then observed the substantial damage to the tail of the airplane.

Damage to Aircraft

The airplane was inspected for damage by the National Transportation Safety Board's Investigator-in-Charge, Airworthiness Inspectors of the Federal Aviation Administration (FAA), representatives from the airplane manufacturer, Raytheon Aircraft Company (RAC), and maintenance representatives from the American Bonanza Society (ABS).

The aft fuselage damage was concentrated between the fuselage station (FS) 233.5 and 256.9 bulkheads. The aft fuselage exhibited extensive skin wrinkling and tearing between the FS 256.9 (front stabilizer spar attachment) and the 233.5 bulkhead. The rest of the aft fuselage exhibited no apparent deformation or cracks.

The damage to the fuselage included:

1. Both lower longerons were bowed inward forward of FS 256.9.

2. Both sides of the fuselage were buckled with permanent wrinkles that extended across the stringers and across the doubler around the access hole on the left side. The primary wrinkle on the right side extended diagonally across the fuselage from upper forward to lower rear. On the left side, it appeared that the primary wrinkle was also extended diagonally across the fuselage from upper forward to lower rear. However, the doubler around the opening for the access hole affected the wrinkling and it was not a straight diagonal wrinkle. The wrinkles extended a short distance into the curved top skin. There was no evidence that the access door scratched the surrounding skin.

3. The lower two stringers on the right, inner side of the aft fuselage were crippled and cracked approximately 5 inches aft of the FS 256.9 frame. The lower stringer on the left, inner side of the aft fuselage was crippled inward, forward, and adjacent to the access door reinforcement.

4. The aft fuselage, lower skin, forward of the FS 256.9 frame was wrinkled and deformed down as if it pulled away from the longerons and frame. The rivets attaching the skin to the frame had pulled through the skin. The skin was separated from the longerons for approximately the downward deformation distance. The wrinkles in the skin were not symmetrical. They were approximately parallel to the longerons, but one side was buckled inward and the other side was buckled outward.

5. There was no observable damage to the top skin, except where the ends of the wrinkles in the side panels extended to the top skin.

6. Indentations, approximately 2 inches in length, running aft from the leading edge were found in the upper and lower surfaces of both the left and right stabilizers. The indentation's were between the second and third ribs from the inboard end of the stabilizer.

The examination of the empennage revealed the following:

1. Both stabilizers were securely attached to the FS 256.9 bulkhead. Both ruddervators were securely attached to the stabilizers, and exhibited no excessive hinge wear or looseness.

2. The stabilizer spar attachment holes in the bulkheads at FS 256.9 and 272.0 indicated no bushing hole elongation. Some uniform bushing wear was detected. The bulkheads showed no visible damage.

3. The stabilizer spar attachment holes appeared round and uniform.

4. The ruddervator travel stops on the ruddervator inboard hinge fitting and the contact point on the ruddervator torque fittings showed no evidence of excessive impact loading.

5. Both ruddervators were checked for balance using the force method per the Bonanza 35 Series Shop Manual. The right and left ruddervators exhibited static underbalance moments of 19.67 and 19.27 inch-pounds, respectively. The required static balance range is 16.80 to 19.80 inch-pounds.

6. The right and left ruddervator counterweight assemblies (including the attaching skin) weighed 2.86 and 2.93 pounds, respectively. Both ruddervator counterbalance weight assemblies had approved lead washers bolted to each of the factory installed counterbalance lead weights. There were four washers on the right and six washers on the left. All of the ruddervator counterbalance lead weights were attached securely. The right and left factory installed counterbalance lead weights weighed 2.41 and 2.20 pounds, respectively.

7. The empennage control systems exhibited no excessive looseness or improper installations.

8. The aluminum skin thicknesses for the top, side, and bottom skins were the required thicknesses, 0.032, 0.020, and 0.016, respectively.

The entire airplane was inspected for any discrepancies that might have contributed to the accident. The inspection revealed the following information:

1. The propeller was checked for dynamic balance. The frequencies of the forward and aft accelerometers were about 2.0 and 0.7 inches per second (ips), respectively. The normal acceptable frequency range was 0.2 ips or less.

2. The airspeed indicator was accurate within 3%.

3. The left outboard wing near the tip had been damaged in the past. Neither wing contained any buckling, tearing, shearing, or other signs of overload.

4. The number 5 rivet on the right hand, aft engine mount was sheared.

5. There was a crack in the skin of the nose gear "dog house."

6. A large mouse nest was in the left hand outboard wing just forward of the rear spar.

7. The number 6 cylinder had been hitting the cowl door and had worn a hole into the door structure.

The airplane manufacturer's structural integrity specialist reported that, "The symmetry of the failure about the vertical centerline (except for the buckling of the bottom skin) indicates failure due to a vertical load down on the tail. The side panels probably buckled first and caused the longerons to fail by forced crippling. The damage to the bottom skin and stringers then occurred."

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