Incident Overview
On October 5, 2008, at approximately 1230 eastern daylight time, a Cirrus SR22 (registration N101CD) experienced an asymmetric flap extension while on approach to Lakeland Linder Regional Airport (LAL) in Lakeland, Florida. The airplane was being operated under 14 Code of Federal Regulations Part 91 as a personal flight, and had departed from Daytona Beach International Airport (DAB). The certificated private pilot, who was the sole occupant, was not injured. Visual meteorological conditions prevailed, and no flight plan was filed.
Pilot's Account
According to a written statement from the pilot, after entering the traffic pattern at LAL, he reduced the airplane's speed to the flap extension speed and moved the flap selector to the 50-percent position. Immediately after moving the selector, he heard a "bang" and the airplane rolled to the right. The pilot applied full roll input to counter the roll and notified air traffic control of the problem. Looking outside, he observed that the left flap was fully extended but could not see the right flap. He subsequently landed the airplane without further incident using full roll input. After exiting, he noted that the left flap was fully extended and the right flap was fully retracted.
The pilot reported 1,450 total flight hours, with 350 hours in the accident airplane make and model. His most recent FAA third-class medical certificate was issued on February 6, 2008.
Weather Conditions
The weather at LAL around the time of the incident, reported at 1250, included winds from 080 degrees at 10 knots, visibility of 12 statute miles, a broken ceiling at 2,900 feet, and a broken ceiling at 25,000 feet.
Examination and Findings
Following the incident, local maintenance personnel examined and repaired the airplane. During the repair, a mechanic discovered that the flap actuator rod end was bent 90 degrees and broken. After replacing the broken part, operational checks of the flap actuator motor, flap position sensors, and other electrical components showed normal operation with no anomalies.
The fractured rod end was sent to the Safety Board Materials Laboratory for examination. The laboratory report indicated that the rod end exhibited a bend in the threaded portion and a fracture consistent with an overstress event adjacent to the bend. Plastic deformation and crack formation at the thread roots along the outer bend were consistent with the initial bending. Thread crests were deformed and had a scalloped appearance. Several threads along the inner radius of the bend were also deformed. Features on the inner bend fracture surface were consistent with low cycle fatigue-like crack propagation under high stress. Scanning electron microscopy showed the fatigue crack accounted for 7% of the fracture surface, while the remainder had a ductile dimple appearance consistent with overload. The rod end's hardness measured 89.9 HRB, corresponding to an approximate tensile strength of 89 ksi, near the upper end of the material's tensile strength range.
Manufacturer Response
After the incident, the airframe manufacturer issued a service bulletin. The bulletin described a sequential dependent condition where the 0% flap actuator sensor could fail, allowing the actuator to extend past the 0% point and potentially bend the rod end. If flaps were then deployed to 100% with a bent rod end, they could over-deploy and cause the actuator linkages to over-center. If the rod end failed in that position, an asymmetric flap condition could result. The bulletin introduced a collar to prevent over-extension and replaced the actuator end fitting and rod end with redesigned parts to reduce actuator travel in the event of a sensor failure.