Primary finding
Probable cause
The separation of the stabilator actuator from the stabilator tab, which resulted in pitch control surface flutter and structural damage.
Investigator assessment
Analysis narrative
Shortly after takeoff the airplane began to shake violently with the control yoke moving rapidly forward and aft from stop to stop. The pilot was able to perform an on-airport emergency landing after shutting down the engine, but sustained hand injuries due to the extreme control forces and oscillations. The airplane sustained substantial damage to the stabilator structure in a manner consistent with it exhibiting aerodynamic flutter. Postaccident examination revealed that the stabilator tab actuator was disconnected from the control tab, and the bolt, nut, and washer that connected them were missing. The disconnection of the actuator allowed the tab surface to uncouple, which resulted in flutter and structural damage to the stabilator. Circumferential scoring around the bolt hole within the tabs indicated that the bolt was likely loose for an undetermined period before failure. The nut had likely not been replaced since the airplane was manufactured almost 60 years before the accident, so its locking properties would have been significantly diminished.
Source record
Factual narrative
On August 8, 2023, about 1500 Pacific daylight time, a Piper PA-24-260, N503JA, was substantially damaged when it was involved in an accident near Fullerton, California. The pilot sustained minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot stated that, shortly after takeoff, the airplane began to shake violently with the control yoke moving rapidly forward and aft from stop to stop. The vibrations were so extreme that he assumed the engine had suffered a catastrophic failure, such as a detached engine cylinder. He shut down the engine and turned the airplane back to the airport for landing. The pilot stated he was able to land, but sustained hand injuries due to the extreme control forces and oscillations. The airplane sustained substantial damage to the stabilator structure and postaccident examination revealed that the stabilator tab actuator was disconnected from the control tab, and the bolt, nut, and washer that connected them were missing (figure 1). Figure 1 - Stabilator tab disconnected from the actuator. The tabs sustained extensive scratch and peening damage to both their inner and forward surfaces consistent with high frequency oscillations. The paint around the end of the tab actuator arm had chipped away, revealing similar peening damage to its steel surface. Neither the bolt nor the nut that connected the tabs to the actuator were located, and there was circumferential scoring to the tabs around the bolt hole bore. Neither bolt hole appeared to be elongated. Likewise, the bushing within the actuator did not appear to be out of round. According to the airplane’s illustrated parts catalog, the actuator retaining nut was supposed to be a self-locking type, with no provisions for a cotter pin. The mechanic who had performed an annual inspection 5.5 flight hours before the accident stated that he did not remove or replace either the bolt or the nut, and that he performed a functional check of the pitch trim system with no anomalies noted. Examination of the maintenance logbooks did not reveal any evidence that the nut had been replaced since the airplane was manufactured in 1965.