Snohomish, WA, USA
N13SD
HUGHES OH-6A
The pilot reported that about five minutes after departure, in cruise flight, at 100 knots and 1,000 feet msl, the engine rpm fluctuated with a noticeable engine noise reduction. Subsequently N2 rapidly decelerated and the helicopter yawed to the left. The pilot initiated an autorotation, however, due to heavily wooded terrain, the pilot had to maneuver around obstructions to a landing site. During the maneuver, the helicopter collided with a tree branch which broke the chin bubble. At approximately 100 feet agl, the pilot started the deceleration. Approximately 30 feet agl, the forward airspeed was at or near zero. The helicopter descended faster than the pilot anticipated as he pulled in collective to slow the descent and then a significant amount of collective to cushion the landing. The helicopter landed with more force than normal and the helicopter rolled to the right after landing. The right side fore and aft skid legs collapsed at the mid point of the struts and the helicopter came to rest on its right side. The engine was inspected and subsequently removed for an engine test cell run. During the engine test cell run, the engine was found to be making more than rated horsepower at all points.
HISTORY OF FLIGHT On November 29, 2002, at 1450 Pacific standard time, a Hughes OH-6A, N13SD, registered to and operated by the Snohomish County Sheriff's Office as a public use training flight, collided with a tree near Snohomish, Washington, following a loss of engine power. The helicopter subsequently landed in a clearing and rolled over. Visual meteorological conditions prevailed at the time and a company visual flight rules flight plan was in effect. The helicopter was substantially damaged and both commercial pilots were not injured. The flight originated from a helipad in Snohomish about five minutes before the accident. The pilot-in-command (PIC) reported that about five minutes after departure, in cruise flight, at 100 knots and 1,000 feet mean sea level (msl) with the second pilot at the controls, the engine rpm fluctuated with a noticeable engine noise reduction. Subsequently N2 rapidly decelerated and the helicopter yawed to the left. The PIC announced that he had the controls and the second pilot surrendered the controls to the PIC. An autorotation was initiated and the PIC verified that the rotor and N2 needles were split. The flight was over heavily wooded terrain and the PIC maneuvered to land in a clearing. The airspeed was decelerated from 100 knots to 80 knots and the PIC announced "rotor in green, aircraft in trim." The N2 was at approximately 30 percent as they approached the intended landing site. The PIC stated that he did not believe that they could make the clearing and changed to another landing spot on the east side of a road. The PIC maneuvered between two trees, however, the helicopter struck one of the tree branches at the left side of the chin bubble which broke. At approximately 100 feet above ground level, the PIC started the deceleration. The PIC stated that approximately 30 feet agl, the forward airspeed was at or near zero. The helicopter descended faster than the PIC anticipated and he pulled in collective to slow the descent then "a significant amount of collective to cushion" the landing. The helicopter landed with "slightly more force than normal and the aircraft started to roll right after landing." The right side fore and aft skid legs collapsed at the mid point of the struts and the helicopter came to rest on its right side. The terrain was soft and slightly sloping. PERSONNEL INFORMATION At the time of the accident, the PIC held a commercial certificate for helicopters with a helicopter instrument rating. The PIC reported a total flight time of 2,612 hours in rotorcraft, with 85 hours total time in the make and model helicopter involved in the accident. Approximately 73 hours were reported as pilot-in-command. The PIC held a Class 2 medical certificate dated December 19, 2001, with a limitation to posses glasses for near and intermediate vision. At the time of the accident, the second pilot held a private certificate for single-engine land aircraft and a commercial certificate for helicopters. The second pilot reported a total flight time in all aircraft of 410 hours, with 343 hours in rotorcraft operations. A total flight time of 142 hours had been accumulated in the make and model helicopter involved in the accident. Approximately 130 hours were reported as pilot-in-command. The second pilot held a Class 2 medical certificate dated March 21, 2002, with a limitation to wear corrective lenses and posses glasses for near and intermediate vision. Both pilots accomplished a flight review on September 26, 2002, in a Bell UH-1H. Personnel training records indicated that both pilots were training and transitioning from the Bell UH-1H to the Hughes OH-6A. A certificated flight instructor reviewed Snohomish County's flight program in 2002 and submitted a report dated November 21, 2002, reported that the program had recently made significant safety improvements however, a formal Hughes OH-6A transition, and formal mountain qualification courses were recommended. AIRCRAFT INFORMATION A review of maintenance records indicated that during the month of October 2002, the helicopter was undergoing routine maintenance and airframe modifications. The modifications included the installation of a removable Nightsun SX-16 and a Flir Imager System. A 100-hour inspection was completed on November 22, 2002. On November 23, 2002, the helicopter was test flown then flown back to Snohomish. No discrepancies were noted during the flights. The next flight was flown on November 25, for three separate flights totaling approximately 6.5 hours in duration. During this time, the pilot reported that there were brief N2 rpm fluctuations that quickly stopped, however, no write-ups or maintenance troubleshooting were accomplished as the pilot thought that the fluctuations might have been caused by the newly installed Nightsun and Flir Imager. The pilot also reported that there were no aural cautions or lights. The next flight was on November 26, a night training flight. During this flight, another brief N2 fluctuation was noted which quickly stopped. No follow-up maintenance action was taken. At the completion of this flight, 55 gallons of fuel was added from a storage tank. The next flight was the accident flight on November 29, 2002, approximately 14 hours since the last maintenance inspection. The pilot reported that the Nightsun and the Flir had been removed prior to the flight. At the time of the accident, the airframe had accumulated a total flight time of approximately 3,057 hours. Total engine time was approximately 1,962 hours. On December 4, 2002, investigators from the National Transportation Safety Board, Federal Aviation Administration, Snohomish County Sheriff's Office, Classic Helicopter Corporation, Rolls-Royce and The Boeing Company inspected the airframe and engine at the Snohomish County Search and Rescue facility in Snohomish, Washington. During the inspection, there was no apparent impact damage to the engine compartment or to the engine. The engine mounts were intact. Both the lower airframe sumps on the left and right side were drained. The samples were clear of contaminants and appeared to be Jet A fuel. The engine fuel filter was full of fuel. The sample contained a small amount of debris that appeared to be wood particles. The fire shield to fuel nozzle supply line was full of fuel. No contaminants were noted. All pneumatic B nuts were tight and torque painted. The fuel control unit was properly rigged, as was the power turbine governor. The compressor turned freely and smooth. The compressor inlet was unobstructed. Continuity was established to the starter generator. The fourth stage power turbine wheel was intact and rotation showed continuity to the PTO and main transmission. The fuel nozzle screen appeared intact when viewed through the inlet opening. The annunciator panel warning lights were tested and all illuminated. The audio engine out tested satisfactory. The engine was removed from the airframe and prepared for shipment to Rolls-Royce, Indianapolis, Indiana. ADDITIONAL INFORMATION Snohomish County personnel reported that fuel for the helicopter was obtained directly from a fuel depot located at Harvey Field, Snohomish. The fuel was put into a fuel storage tank. The filter at the storage tank is changed every six months. The last filter change was on August 9, 2002. The filter was inspected and found to be clean. The sump drain contained contaminants of wood particles. Fuel from the nozzle was clean. No additives were used in the fuel. A "run as received" engine test was conducted at Rolls Royce on December 12, 2002. A Federal Aviation Administration Inspector from the Indianapolis Flight Standards District Office was present for the engine run. The inspector reported that during the preparation process, it was found that the power turbine governor arm was difficult to move. The engine was placed in a test cell. The inspector reported, "After start and warm up, the engine was put through a power calibration run and the data recorded. Three wave off (large and rapid power changes) were accomplished and no discrepancies were noted during the run. The Power Turbine Governor worked normally." The data was analyzed and the engine was determined to be making more than rated horsepower at all points. All wreckage and associated maintenance documentation was released to Snohomish County by January 22, 2003.
Loss of engine power for undetermined reasons while in cruise flight. A tree was a factor.
Source: NTSB Aviation Accident Database
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