Prosser, WA, USA
N7658B
Downer Bellanca 14-19-2
During cruise flight the airplane lost pitch authority after a flutter event partially disabled its elevator and resulted in structural damage to the horizontal and both outboard vertical stabilizers. Although the airplane had limited pitch authority following the event, the pilot was able to successfully land the airplane in a field. The airplane was the subject of an airworthiness directive (AD) concerning play in the pitch trim system, which required inspection of the pitch trim tab for play at 100-hour intervals. The play in the pitch trim system had the potential to cause such a flutter event if not complied with. However, the airplane had just come out of its annual inspection and the pilot also examined the airplane in accordance with the AD just before flight. Additionally, no evidence of such play was observed during a postaccident examination. The pitch trim system had failed, but it appeared to be because of overload damage, likely sustained from the high oscillatory forces induced during the flutter event. Examination of the elevator revealed that the upper lug that connected the elevator pitch control tube to the elevator bellcrank had failed. The weld joints in that area exhibited a lack of fusion and penetration, creating gaps between the filler with the control arm surfaces and the lug. These gaps allowed fatigue cracks to initiate and propagate. Once the cracks had grown large enough, the remaining intact weld material fractured from overstress, leading to the separation of the lug from the control tube. The lug failure resulted in a partial disconnection of the control surface, which likely started the flutter event. This ultimately led to the failure of the pitch trim control arm, which would have exacerbated the flutter. Corrosion and primer identified at the lug fracture area indicated the surfaces had not been properly prepared before welding. Although the lower lug had not failed, it also exhibited similar evidence of gaps caused by a lack of fusion and penetration. It could not be determined if the lugs were welded at the time of the airplane’s manufacturer, more than 66 years before the accident, or during a subsequent repair.
On March 26, 2022, about 0750 Pacific daylight time, a Downer Bellanca 14-19-2 (Cruisemaster), N7658B, sustained substantial damage when it was involved in an accident near Prosser, Washington. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that the reason for the flight was to operate the engine at high power, to break in the piston rings after a new engine cylinder had been installed. He reported performing a thorough preflight, which included a check of the pitch trim system, during which he did not find any anomalies. Shortly after departure, and before he had reached the intended maneuvering area, he felt a mild vibration in the airframe. He was initially not concerned, because it felt like prior instances when he had encountered a fouled spark plug or bad spark plug lead. He adjusted engine power and determined that the vibrations were only present at higher power settings. He also adjusted the pitch trim but did not feel any vibration in the trim handle. He began to maneuver the airplane in a shallow descending turn when the vibrations came back, but now more pronounced, such that he could feel them through the flight control system. A few seconds later he heard a loud bang coming from the tail, followed by violent shuddering and shaking and rapid forward and aft control yoke deflections. He reduced engine power and increased back pressure on the control yoke to slow the airplane down, and when he looked back, he could see the elevator and trim tab rapidly oscillating. With limited pitch control, he began to maneuver the airplane for a forced landing by modulating engine power to a speed that minimized the vibrations. He judged the descent rate and airspeed to be too high to land, and continued to maneuver the airplane, describing the experience as closer to guiding a falling object rather than flying. He extended the landing gear, and prepared the airplane for a forced landing, and about 50 to 100 ft above ground level, decided that he was still descending too fast to land. He decided to apply engine power to control pitch attitude, and the airplane appeared to respond, such that the airplane began to flare over a field, although still at a very high airspeed. The main landing gear touched the ground, and he applied full aft pressure to the control yoke, and the airplane skipped once, touched down again, and eventually came to rest after a ground roll of what he estimated to be about 1,500 ft. Examination of the airplane revealed that the right vertical stabilizer and underside of the right horizontal stabilizer fabric skin had departed the airplane, and the left vertical stabilizer was bent about 10° in the vertical plane. The horizontal stabilizer and elevator were constructed of tubular steel structural members covered in fabric. Each elevator was held in place by a series of hinges made of metal lugs welded to the leading edge of the control tube and held in place with bolts (pins) to corresponding lugs on the horizontal stabilizer. The inboard section of the elevator was connected by a pair of similar lugs bolted to a control arm (bellcrank) in the center section of the tail (figure 1). Figure 1 – Exemplar elevator control Postaccident examination revealed that the right elevator remained attached to the horizontal stabilizer and bellcrank, and although the left elevator remained attached to the stabilizer by its hinges, the upper lug of its control tube had separated from the bellcrank (figures 2, 3). Figure 2 – Accident elevator control Figure 3 – Lug detachment from control tube The trim tab remained attached to the left elevator, but its control torque rod had separated within the support tube assembly, such that movement of the trim pushrod did not result in movement of the trim surface. The remaining components of the pitch control and trim system were intact and undamaged, and there was no looseness between the control surfaces and the flight controls in the cabin. The airplane was the subject of AD 53-16-01, which required inspection of the pitch trim tab for play at 100-hour intervals. Excessive free play had the potential to cause flutter in the pitch control system. The pilot stated that he always checked the trim tab in accordance with the AD as part of the preflight inspection, and he did not observe any play before the accident flight. Flutter is an aeroelastic phenomenon that can occur when an airplane’s natural mode of structural vibration couples with the aerodynamic forces to produce a rapid periodic motion, oscillation, or vibration. Flutter can be somewhat stable if the natural damping of the structure prevents an increase in the forces and motions. Flutter can become dynamically unstable with inadequate damping or greater speed, resulting in increasing self-excited destructive forces being applied to the structure. Flutter can range from an annoying buzz of a flight control or aerodynamic surface to a violent destructive failure of the structure in a very short period of time. Due to the high frequency of oscillation, even when flutter is on the verge of becoming catastrophic, it can still be very hard to detect. Aircraft speed, structural stiffness, and mass distribution are three inputs that govern flutter. An increase in airspeed, a reduction in structural stiffness, or a change in mass distribution can increase the susceptibility to flutter. The elevator and trim tab assembly were evaluated at the National Transportation Safety Board (NTSB) Office of Research and Engineering Materials Laboratory. Examination of the detached upper lug revealed that its weld fillet to the control tube lacked fusion and penetration, such that there was a gap between the filler and the control tube. Figure 4 – Area of detached lug on control tube Closer examination of the gaps on the control tube underneath the failed weld revealed green and light orange deposits, consistent with underlying primer and iron oxide surface rust (figure 4). The control tube also exhibited evidence of grey deposits around the failed weld, which when examined using energy-dispersive X-ray spectroscopy, revealed they were composed of aluminum oxide. The corresponding lower lug was examined, and although it had not failed, similar weld defects were observed and less than half of its welded fillet area had fused to the control tube. Examination of the trim tab control tube revealed that it had fractured at its locking bolt hole within the control arm. The fracture surfaces exhibited damage signatures consistent with overload, including rough texture and dull luster, along with chevron marks and dimpling. There was no evidence of preexisting fatigue. The airplane was manufactured in 1957. Review of Federal Aviation Administration (FAA) 337 (Major Repair and Alteration) records did not reveal any evidence of repair performed to the empennage section, and review of the available logbooks did not show any evidence of repairs to the elevators. The failed lugs appeared to be like those installed on an exemplar airplane. About 200 Cruisemaster series airplanes were made in total, all during the 1950’s. According to representatives from Bellanca Aircraft, Inc., about 45 are still flying. As a result of the NTSB Materials Laboratory findings, Bellanca performed visual examination, and dye penetrant inspections, of the elevator lug welds on 4 14-19-2 and 14-19-3 series airplanes, all of which have a similar elevator control system. Although surface corrosion commensurate with age was noted, the overall weld quality was reported as nominal, with no cracks or obvious defects observed.
An inadequately welded elevator attachment lug, which caused the elevator to partially detach from its bellcrank, resulting in flutter and structural damage.
Source: NTSB Aviation Accident Database
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