Ocala, FL, USA
N911AF
BELL OH-58A
The pilot stated that while on final approach for landing, at about 20 knots and 120 feet above ground level, he felt a "vibration" and the helicopter experienced a loss of engine and rotor rpm. The pilot stated that he lowered the collective for an immediate landing and heard the low-rotor warning horn during the descent. The helicopter experienced a hard landing and rotated to the left after touchdown. Once the helicopter settled, the pilot closed the throttle and exited the helicopter. Initial examination of the helicopter revealed no preimpact mechanical anomalies. The engine was then removed from the helicopter and the turbine governor was removed from the engine. As the turbine governor was removed from the engine, several rolling elements (balls) spilled from the governor and were not recovered. Disassembled portions of the governor were forwarded to the NTSB Materials laboratory and examination of the components revealed the lubricant used was consistent with what was prescribed by the manufacturer. Further examination revealed displacement of the governor driveshaft, deformation of a bearing seal, rotational damage on bearing lands and spacers, as well as spalling on rolling components of the governor. The damage was consistent with wear over an extended period, and neither the cause nor the sequence of wear and failure of the examined components could be determined. Examination of maintenance records revealed that the governor was in service 30 hours beyond its 2,000-hour time-before-overhaul (TBO) interval, but that under the military maintenance program by which the helicopter was maintained, TBO intervals could be adjusted plus or minus 75 hours to align with other scheduled maintenance. Examination of the manufacturer's Operator's Manual revealed that the loss of engine power occurred at an altitude that fell in the "avoid area" of the Height Velocity diagram. According to the manual, “Flight conducted within the avoid area of the chart (figure 9-3) exposes the helicopter to a high probability of damage despite the best efforts of the pilot."
HISTORY OF FLIGHT On October 10, 2009, at 2045 eastern daylight time, a Bell OH-58A, N911AF, operated by the Marion County Sheriff's Office, was substantially damaged following a hard landing at Ocala International Airport (OCF), Ocala, Florida. The certificated commercial pilot and the private-rated copilot were not injured. Visual meteorological conditions prevailed, and no flight plan was filed for the flight which departed OCF, about 1854. The public-use flight was conducted under the provisions of Title 14 Code of Federal Regulations Part 91. In a written statement, the pilot stated that while on final approach for landing, about 120 feet altitude, he felt a "vibration" and the helicopter experienced a loss of engine and rotor rpm. The pilot stated that he lowered the collective for an immediate landing and heard the low-rotor rpm warning horn during the descent. The helicopter experienced a hard landing, and rotated to the left after touchdown. Once the helicopter "settled," the pilot closed the throttle and exited the helicopter. The copilot submitted a written statement, and his account was consistent with the pilot's. PERSONNEL INFORMATION The pilot held a commercial pilot certificate with ratings for rotorcraft-helicopter, airplane single engine land, and instrument airplane. His most recent Federal Aviation Administration (FAA) second class medical certificate was issued in September 17, 2009. The pilot reported 1,945 total hours of flight experience, 1,680 hours of which were in helicopters. The copilot held a private pilot certificate with ratings for airplane single engine land and rotorcraft-helicopter. His most recent FAA third class medical certificate was issued December 27, 2007. The copilot reported 1,390 total hours of flight experience, 1,213 hours of which were in helicopters. AIRCRAFT INFORMATION According to FAA and operator records, the helicopter was manufactured in 1971, and had accrued 8,043 total aircraft hours. The helicopter was originally delivered to the United States Army, and was maintained under a military maintenance program. Its most recent inspection was completed on August 27, 2009. The engine turbine governor was manufactured new on April 28, 2000, and its time-before-overhaul (TBO) was 2,000 hours. The governor was overhauled November 22, 2005 at 1,242.9 hours, and installed in the helicopter on April 28, 2006 at 6,012.7 total aircraft hours. At the time of the accident, the governor had accrued 2,030.2 total hours, which was 30.2 hours past the TBO. METEOROLOGICAL INFORMATION At 2035, the weather conditions reported at OCF included clear skies, calm winds, visibility 10 miles, temperature 27 degrees Celsius (C), dew point 21 degrees C, and an altimeter setting of 30.02 inches of mercury. WRECKAGE AND IMPACT INFORMATION The helicopter was examined by an FAA aviation safety inspector. The examination revealed substantial damage to the fuselage and tailboom. Additionally, the main rotor blades were damaged and the tail rotor blades were "destroyed." The helicopter was examined by representatives of the airframe and engine manufacturers under the supervision of the FAA inspector on October 13, 2009. Initial examination of the helicopter revealed no pre-impact mechanical anomalies. The engine was then removed from the helicopter, and examined in Coatesville, Pennsylvania on November 9, 2009, where the turbine governor was removed from the engine. As the turbine governor was removed from the engine, several rolling elements (balls) spilled from the governor and were not recovered. Disassembled portions of the governor were forwarded to the NTSB Materials laboratory, and were examined on April 10, 2010. Examination of the harvested components revealed the lubricant used was consistent with that prescribed by the manufacturer. Further examination revealed displacement of the governor driveshaft, deformation of a bearing seal, rotational damage on bearing lands and spacers, as well as spalling on rolling components of the governor. The damage was consistent with wear over an extended period, and neither the cause nor the sequence of wear and failure of the examined components could be determined. After the governor was removed, a slave governor was installed, and the engine ran to rated power plus 11 percent in a test cell. ADDITIONAL INFORMATION Examination of the manufacturer's Operator's Manual revealed that the loss of engine power occurred at an altitude that fell in the "avoid area" of the Height Velocity diagram. According to the manual, “Flight conducted within the avoid area of the chart (figure 9-3) exposes the helicopter to a high probability of damage despite the best efforts of the pilot." According to the U.S. Army Technical Manual AERONAUTICAL EQUIPMENT MAINTENANCE MANAGEMENT POLICIES AND PROCEDURES (TM 1500-328-23), "For components with a TBO of 400 hours or more, the removal time may be varied above or below the assigned TBO by 75 hours." The purpose of the "early/late" replacement tolerance window time was to align component changes with "other scheduled maintenance actions."
The excessive wear and failure of engine governor components for undetermined reasons, which resulted in a loss of engine power.
Source: NTSB Aviation Accident Database
Aviation Accidents App
In-Depth Access to Aviation Accident Reports
