HISTORY OF FLIGHT
On April 13, 1998, at 1000 central daylight time, a Beech B35, N5246C, experienced tail flutter during cruise flight near Bentonville, Arkansas. At 180 mph the aircraft began shaking violently. When the pilot reduced power the airplane stopped shaking, and the pilot made an uneventful landing. The pilot and one passenger were not injured. The 14 CFR Part 91 flight departed Springdale, Arkansas, and was en route to Bentonville Municipal Airport, Bentonville, Arkansas. Visual meteorological conditions prevailed and no flight plan had been filed.
The pilot reported that the airplane was just leveling off after a descent of 700 feet. The wind was light in smooth air. The airplane was in a 15 degree angle of bank right turn, 1 to 2 degrees nose down attitude, and at 180 mph. He reported the airspeed was in the middle of the yellow arc. The pilot reported that the airplane began to shake violently. He described the vibration like a rivet gun hitting against the fuselage. He said the vibrations reverberated throughout his body. He did not feel any vibrations in the controls. He pushed over on the control column after about three seconds of vibration. The vibration stopped. The pilot and passenger thought it was turbulence and continued flying for about five minutes. The pilot made a 2G turn in order to take some photographs out the left side of the airplane. An uneventful landing was made at Bentonville, Arkansas.
The pilot and passenger did not notice any damage to the airplane when they put it in the hangar. Later as they were talking about the event, a friend told them they experienced tail flutter. They returned to the hangar and found the damage to the right stabilizer and ruddervator. They notified a repair facility which specialized in repairing V-tail Bonanza's. The pilot and owner were informed to not disturb the aircraft in any manner until it could be inspected by the proper authorities.
PERSONNEL INFORMATION
The pilot held a commercial certificate with single and multi-engine land ratings. He held a Second Class medical certificate. He had a total of about 2,000 hours of flight time, with 50 hours in the same make and model of aircraft. He was a Certified Flight Instructor with about 300 hours of instructional time given.
The owner of the airplane did not have a pilot's certificate. He had owned the airplane for approximately four years. He had hired the pilot to fly the airplane.
AIRCRAFT INFORMATION
The airplane was a single engine Beech B35 Bonanza, serial number D-2638. The airplane seated four and had a maximum gross weight of 2,650 pounds. The engine was a 196 horsepower Continental E-185-8 engine. The last annual inspection was conducted on March 10, 1998. The airplane had flown one hour since the last inspection and had a total time of 6,466 hours.
The following Airworthiness Directives were complied with during the annual inspection:
AD 58-18-01 Aft fuselage bulkhead cracks.
AD 75-05-04 Stabilizer attach fittings.
AD 89-05-02 Ruddervator torque fittings.
AD 94-20-04 Ruddervator balance.
AD 97-06-11 Ruddervator push rods.
The airplane had been painted on January 28, 1998, and both ruddervators were checked for static balance.
The propeller was dynamically balanced to 0.10 IPS in July 1997.
The airframe logbooks indicated that the ailerons and ruddervators were reskinned, painted, and balanced on March 20, 1996. The airplane had flown approximately 153 hours since the ruddervators had been reskinned.
Aircraft Inspection
The aircraft inspection revealed the following:
Flight control cables:
No excessive looseness was exhibited.
Ruddervator Balance:
Both left and right ruddervators were in the proper balance range of 16.8 to 19.8 inch pounds. The left and right ruddervators balanced at 17.68 and 18.2 inch pounds, respectively.
Ruddervator Counterweights:
The ruddervator counterweights and the aluminum shells to which the counterweights are attached were weighed. They weighed 3.62 and 3.61 pounds. The aluminum shell and attaching hardware weighed about .6 pounds. The two counterweights weighed about 3 pounds each. (The original counterweight installed on the B35 Bonanza weighed 2.13 pounds)
FS 256.9 Bulkhead:
The 256.9 bulkhead had cracks in the upper two corners of the bulkhead.
Right Ruddervator:
The right ruddervator had a skin tear on the top skin from the front spar at the center hinge point, halfway back (8.5 inches) to the trailing edge. The bottom skin was buckled in this area at the forward spar. The forward spar was broken, just outboard of the center hinge point. The top spar flange and the vertical part of the spar were broken in two. The crack had proceeded from the top of the flange and worked its way down to the bottom flange. The bottom flange had not yet cracked in two. The center hinge had broken.
Right Stabilizer:
The stabilizer was securely attached to the stabilizer attach fitting and the FS 256.9 bulkhead. The top of the right stabilizer was deformed, starting at the rib just outboard of the center hinge and stabilizer rear spar running forward and outboard to the forward spar at the stabilizer tip. The aft end of the stabilizer was twisted upward from the center hinge to the outboard hinge and tip.
The inboard ends of the stabilizers, at the fuselage, had moved. The right stabilizer at the leading edge moved down about 1/8 inch. The left stabilizer at the leading edge moved up about 1/16 inch.
Left Stabilizer:
The stabilizer was securely attached to the stabilizer attach fitting and the FS 256.9 bulkhead. The left stabilizer had a slight bend and buckle in the aft spar on the top of the spar flange about 3/4 of the way out to the tip. There was no other wrinkling on the top stabilizer. The bottom of the left stabilizer was wrinkled at the outboard hinge area. The wrinkle was about 7 inches long, running forward and downward from the outboard hinge.
Left Ruddervator:
The left ruddervator was undamaged.
Trim Tabs:
The left and right trim tabs were undamaged.
Fuselage Damage:
Damage to the fuselage included wrinkles about one inch long located on the left and right side of the FS 256.9 bulkhead.
The airplane was inspected for items installed that were not original to the aircraft as manufactured. The installed items included:
1. Fuselage 10 gallon auxiliary fuel tank.
2. Stall warning removed from trailing edge (Flying vane type) and new style installed on leading edge of left wing.
3. P35 style square wingtips.
4. Cleveland wheels and brakes.
5. Beech long third window kit with extended hat shelf. Beech Model Kit 337.
6. New style pilot's windows on left and right.
7. Beryl long sloped windshield. 3/8 inch thick.
8. Cowl gill panels have "Cat whiskers."
9. Reskinned ruddervators and ailerons.
10. Stall cuffs on inboard leading edges of both wings. These came installed on C35 and later models.
11. Battery box forward of firewall.
12. New style instrument panel from keyboard switches up.
13. Individual front seats from late model Bonanza.
14. S35 overhead air console.
15. Pointed fiberglass tail cone.
16. Beryl air/oil separator.
17. Oil filter kit.
TESTS AND RESEARCH
The magnesium ruddervators of the airplane had been sent to a repair facility to be reskinned about three years prior to the accident. The facility that reskinned the ruddervators specialized in overhauling flight control surfaces.
The right ruddervator and right stabilizer were taken to a repair facility for disassembly and inspection. The disassembly of the ruddervator revealed the parts weighed the following:
Counterweight and shell with one washer added 3 lbs. 8.9 oz. (3.56 lbs.) (0.05 lbs. difference noted from on site inspection.)
Horn hardware 0.9 oz.
Magnesium skin 2 lbs. 10.5 oz.
Ruddervator spar 12.6 oz.
Trim tab hinge 2.0 oz.
Trim tab doubler 0.8 oz.
Aluminum strip outside edge 0.9 oz.
The inspection of the right ruddervator revealed that the magnesium skin had been primed on both sides. The top coat paint on the outside surface appeared consistent without runs or streaks.
The inspection of the right stabilizer revealed a 45 degree diagonal buckle that started from the outboard spar and continued through the ruddervator trim tab hinge. The diagonal buckle intersected the trim tab hinge at approximately seven inches from the outboard edge of the trim tab. An aluminum trim tab hinge doubler had been added to the trim tab hinge when the ruddervators were overhauled.
The operator of the repair facility who had 16 years of experience reported that, "During our experience of re-skinning ruddervators for straight, A & B-35 Bonanza's, the originals did not have a doubler in the hinge. However, approximately 90 per cent of these ruddervators were all cracked about 7 & 7.5 inches back on the hinge." The operator also reported that the manufacturer installed the aluminum trim tab hinge doubler as original equipment on all C through G model Bonanza's.
It was learned through the course of investigating similar cases of tail vibration, that overhauled ruddervators did not weigh the same as original ruddervators produced by the factory. The following are 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 six inch aluminum hinge doubler used inside the ruddervator trim tab hinge was added to the overhauled ruddervators.
4. An aluminum edge strip was added.
5. Too much paint applied to the ruddervators instead of a light coat of paint.
The part number for the counterweight that was originally 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."
The shop manual stated the following:
"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 than 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."
The -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.
ADDITIONAL INFORMATION
The Federal Aviation Administration issued Airworthiness Directive (AD) 98-13-02 on July 7, 1998, which placed a speed restriction on Models 35, A35, B35, and 35R airplanes, all serial numbers, certificated in any category. Part of the compliance section of the AD stated:
"To prevent in-flight vibrations caused by the affected airplanes operating at excessive speeds, which could result in airplane damage and possible loss of control of the airplane, accomplish the following:
(a) Fabricate a placard that restricts the never exceed speed (Vne) to no more that 144 miles per hour (MPH) or 125 knots (KTS) indicated airspeed (IAS) and install this placard on the instrument panel with in the pilot's clear view. The placard should utilize letters of at least 0.10-inch in height and contain the following words:
"Never exceed speed, Vne, 144 MPH (125 KTS) IAS"
The aircraft manufacturer conducted a ground vibration test (GVT) on a Model 35 airplane in August 1999. The data from the GVT test was used to prepare a flutter analysis for the Model 35 with original ruddervators and re-skinned ruddervators with heavier balance weights.
The aircraft manufacturer issued a Service Bulletin (SB) SB 27-3358 in February 2000. The reason for the SB was stated as:
"The airplanes listed in EFFECTIVITY are approximately 50 years old. The FAA issued an airworthiness directive to restrict the speed of those airplanes in an effort to increase their safety. This Service Bulletin is being issued as a result of an investigation performed by the Federal Aviation Administration and Raytheon Aircraft Company to develop procedures to remove the speed restriction while maintaining the airplane in a safe condition.
This Service Bulletin provides inspecting procedures for the empennage and related portions on the airplanes listed in EFFECTIVITY conforming with Type Certificate No. A777. The procedures do reduce the upper ruddervator balance limit from 19.8 to 18 inch-pounds to increase the margin of safety for these airplanes."
Parties to the investigation were the Federal Aviation Administration, Raytheon Aircraft Company, and the American Bonanza Society.