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.
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.
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."
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, but were unable to get them into balance. The pilot reported that he called repair facilities to get information on how to get the ruddervators into balance.
The pilot reported that he was informed that it was necessary to add an additional pound of lead weight to the counterweight horn. He reported that he was told that the aircraft manufacturer had produced a new counterweight that was about a pound heavier than the original counterweight.
The pilot reported that he installed an additional pound of lead weight to each counterweight. The right and left ruddervators were balanced to 18.67 and 18.66 inch pounds, respectively. The total weights of the right and left counterweights were 3.63 and 3.67 pounds, respectively.
On August 1, 1996, the pilot flew a maintenance test flight. He flew the airplane in straight and level flight at 160 mph. The pilot reported that the airplane experienced a violent vibration. The vibration lasted about 3 to 5 seconds and stopped when the pilot pulled back on the throttle. An uneventful landing was made.
The pilot reported that he thought the vibration was related to the engine or propeller, and not the empennage. An inspection of the airplane revealed that the 256.9 bulkhead was cracked near the four corners of the bulkhead. A crease about one inch in length was found on the outside skin at the 256.9 bulkhead. No other damage was found. The ruddervators were inspected for balance and were found to be within the proper balance range. The only anomaly found on the airplane was the pound of lead that had been added to the counterweights.
The pilot reported that he contacted the aircraft manufacturer and sought advise on how to get the ruddervators into balance and how to fix his airplane. He reported that he was told to install a serviceable B35 model 256.9 bulkhead, and to use the F35 model counterweight. The pilot reported that a representative from the aircraft manufacturer indicated that a new 256.9 bulkhead was not available, but that a serviceable B35 bulkhead was certified for use with an F35 model counterweight.
The pilot obtained a serviceable B35 model bulkhead from an aircraft salvage facility. The B35 bulkhead was stronger than the original bulkhead because it had a U-shaped doubler added to the bulkhead. The original bulkhead was formed from a single piece of aluminum. The B35 bulkhead was installed on the airplane.
The F35 model counterweight differed from the original counterweight, although they shared the same part number. The original counterweight weighed 2.13 pounds. The F35 counterweight weighed between 2.73 and 2.83 pounds. The pilot used the right and left 2.13 counterweights to fashion plaster of paris molds. He melted down lead and poured it into the molds. He shaved down the lead weights so that the right and left counterweights weighed 2.95 and 2.84 pounds, respectively. The lead counterweights formed were about the same size and weight of the F35 counterweights. No additional lead washers were added to the counterweights. Using the newly fashioned counterweights, the right and left ruddervators were balanced to 18.54 and 18.51 inch pounds, respectively.
The B35 bulkhead and the re-balanced ruddervators were installed on the airplane. On April 28, 1997, the pilot performed a test flight. He reported that the airplane experienced the same vibration that had occurred on August 1, 1996. He reported that he had anticipated that the vibration might occur, and pulled back on the throttle within three seconds. The pilot landed the airplane. The inspection of the airplane did not reveal any damage to the airplane.
Tests and Research
In September, 1993, the aircraft manufacturer issued a Beechcraft Safety Cummunique on the subject of flutter. Much of the safety communique is reproduced below:
"Beechcraft has recently received reports of three aircraft sustaining structural damage to the empennage as a result of flutter. None of the occurrences resulted in an accident. Flutter is a phenomenon that can occur when an aerodynamic surface begins vibrating. The energy to sustain the vibration is derived from air flow over the surface. The amplitude on the vibration can (1) decrease, if airspeed is reduced; (2) remain constant, if airspeed is held constant, and no failures occur; or (3) increase to the point of self destruction, especially is airspeed is high and/or is allowed to increase. Failure or destruction of control surfaces can lead to an in-flight breakup of the airplane.
Aircraft are designed so that flutter will not occur in the normal operating envelope of the airplane as long as the airplane is properly maintained. Owners and operators of aircraft have the primary responsibility for maintaining their aircraft. To fulfill that responsibility, it is imperative that all aircraft receive a thorough pre-flight inspection. Further, owners should take their aircraft to mechanics who have access to current technical publications and prior experience in properly maintaining that make and model aircraft.
In the case of any aircraft, decreasing the damping and stiffness of the structure or increasing the trailing edge weight of control surfaces will tend to cause flutter. If a combination of those factors is sufficient, flutter can occur within the normal operating envelope. Examination of the three aircraft following these incidents revealed that in all three cases the empennage control surfaces were out of balance as a result of repainting, substitution of materials, or both.
Improper tension on control cables or any other loose condition in the empennage can also cause or contribute to flutter. In one case, an upper stabilizer bolt was loose, In another case, spanwise movement of a control surface was approximately 1/8 inch. In another, trim tab hinge wires were improperly installed. In each of these three occurrences, the empennage sustained structural damage which precluded accurate determination of control cable tension.
Although these three occurrences involved the Bonanza model 35 type aircraft, any make and model of aircraft can experience flutter if not properly maintained. Proper flight control surface maintenance is imperative to the continued safe operation of all airplanes. Pilots should pay particular attention to control surface attachment hardware during pre-flight inspection. Looseness of fixed surfaces or movement of control surfaces other than in the normal direction of travel should be rectified before flight."
The safety communique stated that "increasing the trailing edge weight of control surfaces will tend to cause flutter." N2839V was inspected thoroughly after both vibration occurrences. Each time the ruddervators were inspected for balance, and in both cases the ruddervator balance was within the operating range of 16.8 and 19.8 inch pounds.
The safety communique stated that "Improper tension on control cables or any other loose condition in the empennage can also cause or contribute to flutter." The pilot of N2839V reported that prior to the first vibration occurrence, the empennage of the aircraft had received a thorough inspection and new attaching hardware had been installed while the ruddervators were being reskinned. After the second vibration, the cable tensions were checked and found to be on the low side of the allowed cable tension range.
After the first vibration occurrence, the trim tab hinge wires were found to be properly installed. After the second vibration occurrence, the trim tab hinge wires were found to be improperly installed. However, there were no indications of looseness, binding, or free play between the trim tab and ruddervator.
The magnesium ruddervators fot N2839V had been sent to a repair facility to be reskinned. The facility that reskinned the ruddervators specialized in overhauling flight control surfaces. The operator of the facility reported that she had sent the pilot a set of ruddervators that had already been reskinned. She reported that overhauled ruddervators did not weight the same as original ruddervators produced by the factory. She gave the following reasons for the increased ruddervator weight:
1. The new magnesium skins obtained from the manufacturer were primed (painted with a protective coating) on both sides of the skins. Originally, the skins were primed on only the outside of the skin.
2. Rivets vice spot welds were used.
3. A 6 inch aluminum doubler was added to the overhauled ruddervators.
4. An aluminum edge strip was added.
5. Aluminum torque fittings were used instead of the originally manufactured magnesium torque fittings.
6. Too much paint applied to the ruddervators instead of a light coat of paint.
7. The thickness of magnesium skins varied. Three magnesium skins used for reskinning ruddervators were checked for total weight. The three skins were primed on both sides. The weights of the skins were 2 lbs, 10.5 oz., 2 lbs. 10.1 oz., and 2 lbs. 9.9 oz, respectively. The operator of the repair facility reported that she had weighed an original skin which had been primed on only one side, and still had a coat of paint on it. She reported that the total weight of that skin was 2 lbs. 6 oz.
The part number for the counterweight that was installed in the ruddervator horn was P/N35-660040-40. It was determined that the same part number for the counterweight applied to four different counterweights. The part number applied to the following counterweights:
1. Model 35, A35, B35 2.13 lbs.
2. Model C35 2.38 lbs.
3. Model D35 2.48 lbs.
4. Model F35 2.73 to 2.83 lbs.
In 1990, the aircraft manufacturer produced another counterweight with the part number, P/N 35-660040-67. It weighed 3.8 to 4.0 pounds. It was authorized by the aircraft manufacturer to be used in Beech models 35 through G35 Bonanza's.
The aircraft manufacturer's maintenance manual and shop manual did not indicate that the "-40" counterweight had varying weights. Nor did the manuals indicate that a "-67" counterweight existed. The manuals did not indicate that that a counterweight of differing weights could be used to balance the ruddervators.
The aircraft manufacturer's shop manual specified the procedures for adding lead washers when checking for and obtaining proper ruddervator balance. The lead washers weighed about 0.05 pounds each. The shop manual stated the following:
"To obtain the correct elevator/rudder balance, on airplane serials D-1 thru D-7309, lead washers are added or removed as required."
"On serials D-1 through D-3350, a maximum of 11 lead washers (Beechcraft Part No. 183809) can be installed to obtain correct balance. A brazier lead screw may be installed between the two existing screws at the forward, outboard tip of the elevator/rudder if more that 6 washers are required. Distribution of the washers is as follows:
When the maximum of 11 washers is required, install 3 on the outboard screw, 4 on the center screw and 4 on the inboard screw. If less than the maximum is needed, distribute the washers accordingly. If 6 or less washers are needed to obtain correct balance, the center screw in not used; install 4 washers on the inboard screw and 2 on the outboard screw.
On airplane serials D-3351 thru D-7309 a maximum of six 183809 or three 35-660042-1 and three 35-660062-3 lead washers may be used to balance the elevator/rudder. If the 183809 washers are used, install 3 on the outboard screw and 3 on the inboard screw. If less than the maximum number is needed, distribute the washers accordingly. When using the 35-660042 washers, install three 35-660042-1 washers on the inboard screw and three 35-660042-3 washers on the outboard screw. Distribute the washers accordingly if fewer than the maximum are needed."
A representative from the aircraft manufacturer reported that -40 and -67 counterweights were authorized for use on Beech models 35 through G35. However, since no records were found that substantiated the authorization for the counterweights use, the aircraft manufacturer conducted a flutter analysis of the counterweights in 1997. The manufacturer reported that the heavier counterweights provided better flutter dampening than the original 2.13 lb. counterweight.
Since April 16, 1994, there have been 14 reported accidents involving flutter in Beech model 35 through G35 Bonanza's. The accidents are listed below:
Location Model Date Ruddervators CW* in Balance
1. Spring Hill, Florida 35 4/16/94 No NA
2. Winterset, Iowa 35 5/17/94 Yes 3/3 lbs.
3. Anniston, Alabama A35 5/7/95 Yes NA
4. LaVerne, California 35 5/10/95 Yes NA
5. Jacksonville, Illinois A35 6/4/95 No NA
6. Portale, New Mexico B35 6/28/96 No NA
7. Leesburg, Florida B35 6/29/96 Yes NA
8. Spokane, Washington 35 8/2/96 Yes 3.6/3.7 lbs.
9. LaFayette, Georgia NA 1/27/96 No NA
10. Griffen, Georgia 35 12/96 No NA
11. Spokane, Washington 35 4/28/97 Yes 2.8/2.9 lbs.
12. Lexington, Kentucky D35 4/29/97 Yes 2.2/2.4 lbs.
13. Poplar Grove, Illinois G35 10/6/97 Yes 2.5/2.5 lbs.
14. Bentonville, Arkansas B35 4/13/98 Yes 3.2/3.2 lbs.
CW: Counterweight NA: Information not available
Parties to the investigation included the Federal Aviation Administration, Raytheon Aircraft Company, and the American Bonanza Society.
The airplane has been undergoing repair, and it is intended to be back in service in 1998.