NTSB Factual Report

HISTORY OF FLIGHT

On June 11, 2012, about 0720 mountain daylight time, an Ayres Corporation S2R-G6, N70WY, landed short of runway 28 at Riverton Regional Airport, Riverton, Wyoming. Sky Aviation was operating the airplane under the provisions of 14 Code of Federal Regulations (CFR) Part 137. The airline transport pilot was not injured; the airplane sustained substantial damage from impact forces. The local aerial application flight departed Hidden Valley Airport, a private dirt strip near Riverton, about 5 minutes prior to the accident. Visual meteorological conditions prevailed, and no flight plan had been filed.

The pilot reported that shortly after departure, he observed movement of the torque and temperature gauges to extremely high values. The engine did not respond to his speed or power lever inputs. He stated that the linkage and amount of travel of the controls felt normal, but the engine did not follow the control movement. He decided to divert to Riverton, which had a longer, paved runway. He lined the airplane up for landing on runway 28, and shut the engine off. During the descent, the airplane encountered a downdraft, and clipped a light, which damaged the landing gear. The airplane touched down in the dirt, and came to rest 30 feet short of the runway.

The fuel control unit (FCU) had previously been removed in the fall of 2011 when it failed to move from a midrange power setting. In the spring of 2012, the company sent the FCU to an overhaul shop for examination and repair. The unit was bench checked and returned; the operator installed it on the airplane. Again, the engine did not respond to the power or speed levers. Another mechanic with more experience on this engine checked the rigging, and determined that the rigging was correct. The FCU was removed, and sent to the overhaul shop again. Upon its return, the operator's mechanic installed the FCU; it operated properly with only a minor adjustment to the overspeed governor. No adjustments had been made to the unit since that time, and the airplane accumulated about 50 hours prior to the accident flight with no anomalies noted.

FCU Examination

The National Transportation Safety Board investigator-in-charge oversaw an examination of the FCU at the Woodward Governor Company facilities in Rockford, Illinois, on June 20, 2012. Detailed reports of the testing and metallurgical examinations are in the public docket for this accident.

Visual examination revealed that the speed setting shaft, power lever shaft, and drive shaft all rotated freely. Crush damage was observed on the housing near the Pt2 bellows. The retaining ring for the bellows was bent, and exposed the packing beneath it. There was also damage to the cover between the 12 point screws.

Woodward personnel completed an examination protocol per the production acceptance test procedures that checked standard acceleration schedules for various altitude and temperature conditions, as well as other specified conditions. Several of the results were out of limits, and indicated a Pt2 bias shift. The flow levels at the 40-degree power lever settings were at 208 pounds per hour (pph) at 60 degrees F, and 225 pph at -65 degrees F. These values were 20 percent above the nominal setting (185 at 60 F and 225 at -65 F); these would cause a slightly higher flight idle setting, but not cause the reported problem. These fuel flows with the sensor in the failed position (toward -65F) would have reduced power below the pilot's reported condition.

Disassembly of the unit revealed several anomalies within the unit.

The low temperature sensor was leaking from a hole in the tubing. Woodward personnel examined the tubing, and determined that there was corrosion pitting. However, they also determined that this would not have precipitated the observed engine anomaly.

Technicians reassembled the P2 bias shaft in the control, and calibrated it per the installation instructions. They then measured bell crank output position at various P2 camshaft positions. Shifting the calibration to meet the measurements taken during the as-received inspection established corresponding cam shaft positions with the new settings. This change in the calibration resulted in a shift of the cam shaft position, and Pt2 values that were similar to the as-tested results. This led Woodward investigators to conclude that changes in the position of the Pt2 power lever adjustment resulted from impact damage as the linkage did not have any significant loading in operation. Thus, the flow anomalies during the test reflected the damaged condition of the unit.

A reason for the reported problem could not be determined.

NTSB Probable Cause

An engine overspeed shortly after takeoff for reasons that could not be determined because postaccident examination revealed no mechanical malfunctions or failures that would have precluded normal operation.