Piper PA-31-350 N31NW accident near Baltimore, Maryland

Probable cause

An in-flight overload failure of the left elevator inboard balance weight rib for undetermined reasons.

NTSB analysis

While cruising at 7,000 feet with the autopilot activated, the pilot reported that the airplane's nose pitched down slightly, then up slightly, followed by another downward pitch that resulted in a 200-foot altitude deviation. The pilot disengaged the autopilot, re-leveled the airplane at 7,000 feet, and re-engaged the autopilot. Several minutes later, the pilot noticed a vibration in the control yoke, which he described as a "buzzing," and noticed that the airplane was flying in a right slight sideslip. The pilot trimmed the rudder and noted no further vibration. The remainder of the flight was uneventful. Upon landing and exiting the airplane, the pilot noted that the outer portion of the left elevator was bent downward, just outboard of the outermost hinge point and at an angle nearly perpendicular to its normal position. Detailed examination of the elevator showed that its forward spar had deformed but had not fractured. The inboard rib of the balance weight rib tip structure was deformed and fractured from its upper to lower chord aft of its leading edge. The fracture features of the rib were consistent with overload separation, and exhibited no evidence of a fatigue failure. No evidence of any preexisting damage was observed. The total number of flight hours the elevator had accumulated prior to being installed on the accident airplane could not be determined, although examination of the elevator and its sub-components confirmed that it complied with all relevant manufacturer's service bulletins and FAA airworthiness directives.

Text from the NTSB. Open the NTSB docket for ERA11LA141 for the full record.