CHI97IA122

The pilot reported that he was conducting a maintenance test flight. He was flying straight and level at 160 mph. The sky was 6,500 feet broken with smooth air. The pilot reported that the airplane started to vibrate. He reported that the vibration started as a minor shake and intensified 'like an earthquake.' The pilot pulled the throttle immediately (less than 3 seconds) and the vibration stopped. An uneventful landing was made. The inspection to the airplane revealed no visible damage. The pilot reported that the vibration was the same kind of vibration that he experienced in the same airplane on August 1, 1996. The only changes made to the airplane since the first vibration had been the installation of a B35 model bulkhead, and the installation of F35 model equivalent counterweights. The airplane exhibited no anomalies. The ruddervators were properly balanced.

History of Flight

On April 28, 1997, at 1830 pacific daylight time, a Beech 35, N2839V, experienced tail flutter during cruise on a maintenance flight near Felts Field, Spokane, Washington. The pilot was conducting a test flight after maintenance had been performed on his aircraft. At 160 mph in level flight the aircraft began shaking violently. When the pilot reduced power the airplane stopped shaking, and the pilot made an uneventful landing. The pilot was not injured. The 14 CFR Part 91 flight departed Felts Field, on a local maintenance test flight. Visual meteorological conditions prevailed and a VFR flight plan had been filed.

The pilot reported he conducted a maintenance test flight to determine if the repairs he had made to his airplane had fixed the problem that had caused the airplane to be damaged in a previous accident. (See NTSB Report SEA96LA181) He flew the airplane at 160 mph in straight and level flight. He reported that at about 160 mph the airplane started to shake violently. He pulled back on the throttle after about three seconds of shaking. He made an uneventful landing.

He reported the shaking started as a minor shake and intensified "like an earthquake." He reported that the shaking intensified in amplitude very rapidly. He said the shake started as a "pure shake" and nothing else. He reported that he could hear the skin shake; it was not a buzz or hum. He said he could not feel it in the controls.

Damage to Aircraft

The airplane was inspected for damage by the National Transportation Safety Board's Investigator-in-Charge, a representative from the airplane manufacturer, Raytheon Aircraft Company (RAC), and a maintenance representative from the American Bonanza Society (ABS).

The inspection of the airplane revealed that it had not received any observable damage.

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 256.9 and 272.0 bulkheads showed no visible damage.

3. The stabilizer spar attachments showed no visible damage.

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 balance moments of 18.54 and 18.51 inch-pounds, respectively. The required static balance range is 16.80 to 19.80 inch-pounds.

6. The right and left ruddervator counterweights weighed 2.95 and 2.84 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, 0.016, and 0.016, respectively.

9. The cable tensions were checked and were on the low side of the allowable tension range.

10. The right and left ruddervator trim tabs were not installed properly. The trim tab hinge wire had been mis-threaded. There was no indications that the trim tab was either loose or binding. There was no indication of up and down play between the trim tab and the ruddervator.

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

1. The propeller was checked for dynamic balance. The normal acceptable frequency range was 0.2 ips or less. The dynamic balance was 0.1 ips at 2,250 to 2,300 RPM's.

2. The pitot static system was checked and no anomalies were found.

3. The Lord engine mounts were in good condition.

4. The aircraft was not bent or twisted.

The airplane was inspected for items installed that were not original to the aircraft as manufactured. The installed items included:

1. Ten gallon fuselage fuel tank.

2. One piece windshield.

3. "S" model baggage door.

4. Modified instrument panel and seats.

5. Late model tailcone.

6. Air/oil separator.

7. Lew Gage oil filter.

8. V-35B wingtips.

9. Cleveland wheels and brakes.

10. A B35 256.9 bulkhead was installed.

11. Unframed vented pilot side window.

12. Aztec 60-2 autopilot.

13. Air Skeg modification to the aft fuselage.

Personnel Information

The pilot held a private certificate with a single engine land rating. He had about 700 hours total flight time with 652 hours in make and model.

Aircraft Information

The pilot had owned the Beech 35, serial number D-244, for about nineteen years. The last annual inspection was done on July 31, 1996. The airframe had a total of 6,213 hours. The engine was a Continental E-185-11. The engine was the same model engine that was originally installed on the aircraft when new. The engine had about 259 hours since a major overhaul. The propeller was an original Beech propeller with electric pitch control.

The Pilot's Operating Handbook listed the Vne, Never Exceed speed, as 203 mph. The Maximum Structural Cruising speed, Vno, was 160 mph.

Under the FLUTTER section of the Beechcraft Single Engine (Piston) Safety Information guide, the following information is provided in case excessive vibration in the controls is encountered:

"If an excessive vibration, particularly in the control column and rudder pedals, is encountered in flight, this may be the onset of flutter and the procedure to follow is:

1. IMMEDIATELY REDUCE AIRSPEED (lower the landing gear if necessary).

2. Restrain the controls of the airplane until the vibration ceases.

3. Fly at the reduced airspeed and land at the nearest suitable airport.

4. Have the airplane inspected for airframe damage, control surface attaching hardware condition/security, trim tab free play, proper control cable tension, and control surface balance by another mechanic who is fully qualified."

Additional Information

The pilot reported that the vibration he experienced during the test flight on April 28, 1997, was the same kind of vibration he had experienced on August 1, 1996, when his aircraft received substantial damage during another test flight.

The pilot reported that he had owned the airplane for nineteen years and had never had a problem with the airplane until he had the ruddervators reskinned. He reported that in 1996 the tail of the airplane hit the back of the hangar when he was pushing the airplane back into the hangar. He inspected the tail of the airplane and determined that there was no visible damage. He reported that he flew the airplane numerous times between the time the tail hit the hangar and the flight on August 1, 1996. He decided to have the left and right ruddervators reskinned in 1996 since the ruddervators were original to the airplane and were showing signs of corrosion.

The pilot sent the original magnesium ruddervators to a repair station to be reskinned. The repair station sent a set of overhauled reskinned magnesium ruddervators back to the pilot. The reskinned ruddervators were not painted or balanced by the repair facility.

The pilot and an A&P mechanic repainted the ruddervators. The ruddervators were painted with the trailing edge down. The maintenance manual required that the trailing edge of the ruddervator be positioned up when painting, so any excess paint traveled toward the forward edge of the ruddervator. The pilot and mechanic tried to balance the ruddervators, but were unable to bring them into the proper balance range of 16.8 to 19.8 inch pounds.

The pilot and mechanic stripped the paint and primer from the ruddervators and re-applied a thinner coat of paint and primer to the ruddervators. They tried to balance the ruddervators again,