N101CDCirrus Design SR22 · 2001

On October 5, 2008, about 1230 eastern daylight time, a Cirrus SR22, N101CD, experienced an asymmetric flap extension while approaching Lakeland Linder Regional Airport (LAL), Lakeland, Florida. The certificated private pilot was not injured. Visual meteorological conditions prevailed, and no flight plan was filed for the personal flight, which originated from Daytona Beach International Airport (DAB), Daytona Beach, Florida, and was conducted under the provisions of 14 Code of Federal Regulations Part 91.

According to a written statement submitted by the pilot, after entering the traffic pattern at LAL, he slowed the airplane to the flap extension speed, and set the flap selector to the 50-percent position. After moving the flap selector, the pilot heard a "bang" and the airplane rolled to the right. The pilot then responded with a "full roll input," and contacted air traffic control to advise them of the problem. The pilot looked outside and noticed that the left flap was fully extended, but could not see the right flap’s position.

The pilot subsequently landed the airplane uneventfully using the "full roll input." After exiting the airplane, the pilot observed the left flap in the fully extended position, and the right flap in the fully retracted position.

The pilot reported that he possessed 1,450 total hours of flight experience, with 350 hours in the incident airplane make and model. His most recent FAA third-class medical certificate was issued on February 6, 2008.

The reported weather at LAL, at 1250, included winds from 080 degrees at 10 knots, visibility 12 statute miles, a broken ceiling at 2,900 feet, and a broken ceiling at 25,000 feet.

Following the incident, the airplane was examined and repaired by local maintenance personnel. During the repair process, a mechanic found that the flap actuator rod end was bent 90 degrees and broken. After removing and replacing the broken part, operational checks revealed that the flap actuator motor, flap position sensors, and other electrical components operated normally, with no anomalies noted.

The fractured portions of the flap actuator rod end were sent to the Safety Board Materials Laboratory for further examination. According to the Materials Laboratory factual report, the rod end exhibited a bend in the threaded portion and a fracture consistent with an overstress event located adjacent to the bend. Plastic deformation and crack formation at the roots of the threads along the outer bend of the rod end were also consistent with having occurred during the initial bending event. The thread crests were deformed, and exhibited a scalloped appearance. Several threads along the inner radius of the bend were also deformed. Features on the inner bend of the rod end fracture surface were consistent with low cycle fatigue-like crack propagation under high stress.

Scanning electron microscopy of the fatigue crack fracture surface was consistent with a high stress low cycle fatigue event, with orthogonal cracks emanating from the fatigue fracture surface. The fatigue crack accounted for 7% of the rod end fracture surface, while the remainder of the fracture surface had a ductile dimple appearance consistent with fracture by ductile overload.

The hardness of the rod end was measured to be 89.9 HRB, corresponding to an approximate tensile strength of 89 ksi. The measured tensile strength of the rod end was near to the upper end of the tensile strength range for material of its type. The mating portion of the fractured rod end remained straight, with the exception of some deformation to the thread closest to the fracture surface.

Following the incident, the airframe manufacturer published a service bulletin, which in part described the circumstances that could have allowed the asymmetric flap deployment. The bulletin stated, "Recently, Cirrus discovered a sequential dependent condition where the 0% flap actuator sensor fails and the actuator extends past the 0% point, the actuator rod end could bend. In the event of deploying the flaps to 100% with a bent rod end, the flaps could over-deploy and allow the flap actuator linkages to over-center. If the rod end were to fail while in this position an asymmetric flap condition could result." The bulletin went on to state, "To prevent this condition from occurring, this Service Bulletin installs a collar to the flap actuator to prevent the actuator from over-extending and replaces the actuator end fitting and rod end with versions of a new design. These combined changes reduce actuator travel in the event of a sensor failure."