Accident
On October 30, 2008, at approximately 1045 eastern daylight time, a Hughes 369D helicopter, registration N606AS, sustained substantial damage during a forced landing near Carroll Township, Pennsylvania. The helicopter was conducting a local rotorcraft external load flight under 14 Code of Federal Regulations Part 133, trimming trees near a power transmission line easement using an array of saw blades suspended below. The certificated commercial pilot was seriously injured. Visual meteorological conditions prevailed, and no flight plan was filed.
According to the pilot, the flight departed Washington County Airport (AFJ) in Washington, Pennsylvania, around 0800. After landing at a zone near the accident site, the pilot reviewed maps with power company representatives, then refueled and proceeded to trim trees approximately 8 miles northwest. Returning to the landing zone, the pilot cut a line of trees on the south side of the power lines, then worked westward on the north side.
While making a bottom pass cutting, the pilot noted a vibration but found no instrument anomalies. He then heard a loud bang, and the helicopter entered a left spin. After about 270 degrees of rotation, the pilot saw a large tree, maneuvered right, then turned left as the helicopter descended rapidly. He lowered the collective but did not release the saw. At about 50 feet above ground, he increased collective to slow descent, but the last 30 feet were a freefall. The pilot reported no caution lights or other warnings before the bang.
A witness saw fluid coming from the helicopter, initially thinking it was water for firefighting, but saw no smoke. Shortly after, the helicopter spun counterclockwise and descended straight out of view, the entire event lasting about 30 seconds.
Pilot Information
The pilot reported 4,198 total flight hours, with 4,000 hours in the accident make and model. His most recent FAA third-class medical certificate was issued on January 23, 2008.
Aircraft Information
The helicopter was equipped with an Allison Engine Company (Rolls-Royce) model 250-C20B engine. At the time of the accident, the airframe had 14,020 total hours, and the engine had 8,279 hours. The most recent 100-hour inspection was completed on October 5, 2008, at 13,970 aircraft hours. The engine compressor case-half inspection and gas generator turbine overhaul were performed 589 hours prior, and the fuel nozzle flow check was done 153 hours prior.
Weather
Weather reported at Capitol City Airport (CXY), Harrisburg, Pennsylvania, about 9 nautical miles northeast, at 1056 included winds from 330 degrees at 12 knots gusting to 18 knots, clear skies, 10 miles visibility, temperature 7°C, dewpoint -4°C, and an altimeter setting of 30.43 inches of mercury.
Wreckage and Examination
The helicopter came to rest upright with extensive damage to the airframe and drivetrain consistent with a vertical impact. The engine had little visible external damage. Main rotor control linkages showed continuity from cockpit to rotor head. Engine mount struts on the right side and bottom were fractured. All fuel, lubrication, and pneumatic lines were checked and found secure. The exhaust collector tunnel had a 2-inch tear. The inlet plenum and compressor showed no foreign material or damage. Borescope inspection revealed no anomalies to the third-stage compressor vanes, and minor foreign object damage was found on the fourth-stage turbine wheel. The N1 and N2 systems could not be rotated by hand.
The two aft compressor discharge tube (CDT) snap rings were not engaged, with fretting near the CDT bases and the tubes not fully seated into their couplings. An orange room temperature vulcanizing (RTV) sealant was found around the sealing surfaces of one CDT and the outer combustion chamber.
Engine Examination
The aftermarket Turbine Outlet Temperature (TOT) instrument recorded an over-temperature event peaking at 960°C (1,760°F) lasting 20 seconds. Examination at the Rolls-Royce facility revealed no foreign object damage or compressor failure. The turbine section was not free to rotate. The first-stage nozzle was missing its rope seal, and the first-stage turbine blades showed extensive thermal damage. A single blade on the second-stage wheel had failed due to high cycle fatigue, with the fatigue crack initiating at the suction side near mid-chord root. Third- and fourth-stage turbines exhibited thermal and impact damage from out-of-balance operation. Metallurgical analysis indicated incipient melting of first-stage airfoils above 2,100°F, and non-uniform combustion or burner profile in the second-stage nozzle. The combustion liner-CDT mating surfaces showed uneven sealing and fretting.
Additional Information
Review of Rolls-Royce maintenance manuals found no provisions for using RTV sealant at the CDT interface.