De Havilland DHC-3T N3125N

The airline transport pilot was conducting an on-demand commercial taxi flight. The pilot reported that, during level cruise flight, he felt a vibration in the airplane that progressed such that the airplane exhibited a nose-down pitch. He also believed that there was a change in the elevator. The pilot reduced the airspeed and applied gradual pitch corrections to avoid excessive aerodynamic loads on the tail. Once the airplane was slowed and recovered from the pitch deviation, the vibration stopped, and the remainder of the flight was uneventful. However, after landing, the pilot and a mechanic examined the airplane and found damage to the elevator servo tab and spar structures inside the right elevator. Examination revealed that the elevator auxiliary spar and rear spar forward of the servo tab location exhibited fractures and buckling consistent with overload. The servo tab hinge was intact and attached to the elevator, and nearly all of the rivets remained installed through the tab upper skin and hinge; however, the tab lower skin was separated from the hinge along the rivet line of the C-channel such that the tab structure was splayed open. The rivet holes in the C-channel portion of the skin were intact but had some elongation and deformation consistent with the rivets pulling out. Dark staining was evident around every rivet on the upper servo tab skin. Such staining, commonly known as "smoking rivets," is typically caused by the loosening of one or more rivets, which allows relative movement between the rivet(s) and the underlying structure. However, if the joint is adequately riveted, there should be no movement between the rivet(s) and the underlying structure. About 100 flight hours before the accident, the elevator servo tab had been disassembled and its hinge replaced. The elevator servo tab uses steel blind rivets to fasten the upper skin, hinge, and upper leg of the C-channel together because there is no access to the lower side of the joint, which is needed for a typical driven rivet. Replacing the hinge required drilling out each of the rivets fastening the tab skin, C-channel, and hinge together; drilling out rivets can enlarge the holes beyond their nominal diameter. In an adequately riveted joint, the rivet expands to fill the hole and should slightly deform the hole, making each hole diameter slightly larger than the original; however, if the diameter is already larger than required, the rivet may not adequately fill the hole. Postaccident examination of the servo tab skin found that the material used to construct the tab was about 25 percent thinner than the skin material specified in the original servo tab design and that the hinge used was not in accordance with the original servo tab design. The servo tab is subject to a harsh vibratory environment due to the natural airframe vibration and the propeller wash induced on the airplane's tail. The holes in the servo tab upper skin and upper leg of the C-channel, which had been previously attached to a different hinge, were likely slightly larger than nominal when the new hinge was installed. Based on the dark staining evidence, the rivets installed on the accident servo tab likely did not adequately fill the holes in the tab structure, which allowed them to loosen over a short period of operation. The airplane was inspected less than 2 flight hours (2 days) before the accident flight as required by an airworthiness directive (AD) that specifically required checks for elevator servo tab condition, fastener security, and tab free-play. Although maintenance personnel found the tab to be compliant with the AD's maximum free-play/trailing edge deflection allowances, the dark staining should have provided maintenance personnel a visual indication that there was a problem with relative movement between the rivets and the underlying structure; however, no maintenance action was taken. The reported vibration during the accident flight and the overload damage observed on the servo tab and elevator spar structures are consistent with aerodynamic flutter of the elevator servo tab that excited to the point of imposing damaging loads on the servo tab and the elevator spar structures. Both airspeed (airflow) and inadequate structural stiffness are required for a servo tab flutter event to develop. Inadequate stiffness of the servo tab was evident by both the inadequate servo tab skin thickness and the loose rivets. Airspeed information was not obtained for the accident flight. Generally, as airspeed increases, a flutter condition is exacerbated.

Quoted verbatim from the NTSB record.