Wilmington, NC, USA
N2215R
ROBINSON R22 Beta II
While the helicopter was en route at an estimated 200 hundred feet, a witness on the ground heard the engine "spitting and sputtering." Another witness saw parts “shedding” from the helicopter, then saw it reverse direction and descend until she lost sight of it behind a building. The helicopter subsequently impacted the ground in a vacant lot and erupted into flames. A postflight examination of the engine revealed no mechanical anomalies. The carburetor air temperature gauge was reported to be inoperative and interpolation of a carburetor icing probability chart revealed atmospheric conditions conducive to "serious icing" at glide power. However, the position of the carburetor heat could not be determined and the investigation could not establish the role that carburetor icing may have played in the accident. An examination of the two main rotor blades revealed that each had a section missing, beginning about equidistant from the rotor hub. Both sections were subsequently located away from the main wreckage. Both main rotor blades were also coned upwards from the hub, which, in conjunction with the missing blade sections, was consistent with a low rotor rpm condition. An examination of the tail rotor driveshaft, and the push-pull tube running along it, revealed that they had become entwined, consistent with a “whirl” condition. According to a representative of the manufacturer, such a whirl mode could occur if an overspeed occurs; the manufacturer's maximum operating main rotor rpm limitation is 102%. The event likely began with a partial loss of engine power, followed by the pilot’s delayed response to a low rotor rpm condition, which then resulted in the coning of the main rotor blades and a loss of main rotor blade material.
HISTORY OF FLIGHT On March 13, 2008, at 0940 eastern daylight time, a Robinson R22 Beta II, N2215R, was substantially damaged during a forced landing in Wilmington, North Carolina. The certificated commercial pilot was fatally injured. Day visual meteorological conditions prevailed, and no flight plan was filed for the local flight which departed Wilmington International Airport (ILM), Wilmington, North Carolina. The personal flight was conducted under the provisions of Title 14 Code of Federal Regulations Part 91. According to airport personnel, the pilot was planning to fly south in order to take aerial photographs of a business and then proceed to another location to pick up a student for a training lesson. A witness, located about 1/2 mile from the accident scene, observed the helicopter when it passed overhead, and noted that "the engine was spitting and sputtering." The witness also noted that the helicopter appeared to be "about 50 feet above the pine trees," which were estimated to be 150 feet in height. Another witness stated that the helicopter appeared to be "just above the top of a billboard sign" when she first noticed it. She stated that she "saw parts shedding to the south then [it] started to reverse direction." The witness then lost sight of the helicopter behind the roof of a building and observed smoke. According to another witness, he saw the helicopter as "parts began to fall off," it impacted the ground, then "blew up into flames." There was no radar track of the helicopter, and no onboard recording devices were located. PERSONNEL INFORMATION The pilot held a commercial pilot certificate with ratings for airplane single-engine land, airplane multiengine land, rotorcraft-helicopter, and instrument airplane and helicopter. He did not hold a flight instructor certificate. The pilot’s most recent second-class medical certificate was issued on February 7, 2008. According to the pilot’s logbook, he had accumulated 542 total helicopter flight hours, with 433 flight hours in the accident helicopter make and model. He completed the Robinson Helicopter Company Pilot Safety Course on November 16, 2006, with a rating of "average." AIRCRAFT INFORMATION The helicopter, serial number 3084, was registered to and operated by Maintenance Services LLC, Rocky Point, North Carolina. It was a two-seat, single main rotor, single-engine helicopter that was constructed primarily of metal, was equipped with skid type landing gear, and was manufactured in February, 2000. The helicopter was powered by a Lycoming O-360-J2A engine, serial number, L-37378-36A. The maximum gross weight of the helicopter was 1,370 pounds. The helicopter was issued a standard airworthiness certificate on May 11, 2000. The tail rotor was powered through a tail rotor drive shaft that was 141 inches in length and enclosed in a tapered tail boom. The drive shaft was attached on the forward end to a flex plate (coupling) in the rear of the fuselage just aft of the drive pulley, and at the aft end, to another flex plate just forward of the tail rotor gearbox. A single bearing was mounted on the tail rotor drive shaft about 44 inches aft of the forward flex plate, and was attached to the airframe via an arm, a clip, and bolts with washers. The purpose of the assembly was to dampen shaft oscillations. According to the helicopter maintenance logbook, the helicopter’s most recent annual inspection was completed in accordance with manufacturer's maintenance manual checklist on December 14, 2007, at 1,389 hours of operation. The engine’s most recent inspection, a 100-hour inspection, was completed on October 13, 2007. The last maintenance accomplished on the accident helicopter was on March 6, 2008, and consisted of an engine oil and oil filter replacement by the accident pilot at 1,461 total hours. METEOROLOGICAL INFORMATION The 0953 recorded weather at ILM, located approximately 8 miles north of the accident site, included winds from 230 degrees at 4 knots, visibility 10 miles, clear skies, temperature 17 degrees C, dew point 4 degrees C, and an altimeter setting of 30.03 inches of mercury. WRECKAGE AND IMPACT INFORMATION The helicopter impacted the ground in an open lot approximately 3 feet from several storage trailers. The helicopter exhibited impact damage and post-impact fire damage. The debris path was on a heading of 342 degrees and the helicopter came to rest heading about 140 degrees. The main wreckage consisted of the fuselage, main rotor assembly, tail boom, and tail rotor. The tail cone was separated from the fuselage at a right angle and no leading edge damage was found on the tail rotor blades. Several pieces of the main rotor blade, the empennage, and the helicopter anti-collision beacon were collocated approximately 350 feet south of the main wreckage. The wreckage was examined at the accident site and then recovered to a secure facility at ILM for further examination. Examination of the helicopter revealed spanwise coning of the two main rotor blades. One of the main rotor blades was missing a section of the trailing edge, approximately 6 inches chordwise by 40 inches spanwise, starting about 26 inches from the blade hub. The other main rotor blade was missing a section of the trailing edge, approximately 6 inches chordwise by 61 inches spanwise, starting about 24 inches from the blade hub. One of the separated main rotor blade sections exhibited an impression, consistent in shape, to the leading edge of the vertical stabilizer. The main rotor system was rotated through the main gear box without binding. The drive belts between the engine and transmission were not identified in the burnt wreckage. The empennage, consisting of the vertical and horizontal stabilizer, was detached from the aft bulkhead of the tailcone. The tail rotor drive shaft forward flex plate exhibited rotational distortion and crumpling, and was separated from both connecting yokes. The aft flex plate was not damaged. The tail rotor drive shaft damper bearing assembly, which consisted of a housing, an inner and outer race, and bearings, was damaged. The damper bearing inner race was in place on the drive shaft and was able to rotate in place. The outer race and bearings were separated and could not be located. The bearing housing remained attached to the arm. It could not be determined if the damper assembly operated as intended prior to the accident sequence. The interior of the tail cone, in the vicinity of the forward flex plate, displayed rotational scoring around the entire circumference. The electrical wires had been pulled through the grommets and the push/pull tube was wrapped around the drive shaft. The tail rotor gear box rotated smoothly, and the pitch control bell crank support was separated from the gearbox. The engine crankshaft was rotated using the cooling fan, and continuity of the crankshaft to the rear gears and to the valve train was confirmed. Thumb compression and suction was produced on cylinders 1, 3, and 4. Cylinder 2 produced minimal compression and further examination revealed the intake valve springs were compressed to a height of 1.4 to 1.5 inches but were unbroken. Both the inner and outer springs were discolored consistent with thermal damage. Cylinder 2 was removed from the crankcase and no damage was noted to the intake or exhaust valves, cylinder wall, piston, or connecting rod. The interior of all cylinders were inspected using a lighted boroscope; no anomalies were found. The carburetor bowl was separated from its upper housing and was approximately 2 feet from the front of the engine. The main fuel nozzle was broken off and the fuel lines were separated. The mixture control cable was detached but was in the full rich mixture position. The throttle control rod remained attached to the throttle control arm but was separated from the throttle control forward of the firewall. The throttle plate was found about 3/4 fully open. The spark plugs were dark gray in color and the electrodes were intact. The carburetor fuel inlet screen exhibited thermal damage, but was absent of debris. The oil cooler and hoses were damaged, but oil was visible throughout the engine. The engine oil filter paper element was charred. No metallic debris was noted on the oil filter element or the oil suction screen. The magnetos had thermal damage and were unable to be tested. Although fire-damaged, examination of the instrument panel revealed that the engine rpm needle was in the approximate upper limit position and the rotor rpm needle was in the approximate lower limit position; however, it was unable to be determined if those were the indications at the time of the accident. The ignition key position was unable to be determined due to thermal and impact damage. MEDICAL AND PATHOLOGICAL INFORMATION A postmortem examination of the pilot was performed on March 14, 2008 at Coastal Pathology Associates, P.A., Jacksonville, North Carolina, with the cause of death determined to be "massive blunt trauma." Toxicological testing was performed post mortem at the Federal Aviation Administration (FAA) Bioaeronautical Sciences Research Laboratory, Oklahoma City, Oklahoma with no anomalies noted. TESTS AND RESEARCH Push-Pull Tube and Clutch Assembly Materials Examination The NTSB Materials Laboratory examined the intermediate flex plate, a portion of the push-pull tube, a portion of the main tail rotor drive shaft, and the bearing assembly housing. All fractured surfaces were consistent with overstress separation as a result of the accident. There were no signs of any mechanical malfunctions. Robinson Helicopter Company “Safety Notice (SN-34), Aerial Survey and Photo Flights – Very High Risk,” states, in part, "Aerial survey and photo flights should only be conducted by well trained, experienced pilots who: 1) Have at least 500 hours pilot-in-command in helicopters and over 100 hours in the model flown; 2) Have extensive training in both low RPM and settling-with-power recovery techniques; 3) Are willing to say no to the observer or photographer and only fly the aircraft at speeds, altitude, and wind angles that are safe and allow good escape routes." Robinson Helicopter Company “Safety Notice (SN-36), Overspeeds During Liftoff,” states, in part, "…overspeeds caused a tail rotor drive shaft vibration which led to immediate failure of shaft and tailcone… damper is not effective above 120% RPM." According to testing documentation provided by a representative of the helicopter manufacturer, “whirl” modes (which can result in harmonic oscillation of the tail rotor drive shaft and can lead to shaft distortion) on the R22 occur at the following engine rpm: 1st - 15.2 % main rotor rpm 2nd - 60.6% main rotor rpm 3rd – 132.0 % main rotor rpm According to a representative of Robinson Helicopter Company, the first two whirl modes would not occur in flight as they are below the normal main rotor rpm operating range. The third whirl mode could occur if an overspeed occurs; the manufacturer's maximum operating main rotor rpm limitation is 102%. FAA publication P-8740-24, "Tips on Winter Flying," discusses carburetor icing and some of the hazards associated with it. The publication states, in part, "In general, carburetor ice will form in temperatures between 32-80 degrees F when the relative humidity is 50 percent or more. If visible moisture is present, it will form at temperatures between 15 -32 degrees F." In order to counteract carburetor icing, the pilot should apply full carburetor heat immediately. The publication also noted that the engine "may run rough initially for a short time while ice melts." Interpolation of a carburetor icing probability chart contained within the publication revealed that the atmospheric conditions were conducive to "serious icing" at glide power. The helicopter was configured with a carburetor heat assist device. The device was designed to adjust the amount of carburetor heat applied based upon movement of the collective. Lowering the collective would increase the amount of carburetor heat from the level preselected by the pilot, and raising the collective would reduce the amount of carburetor heat. The carburetor heat knob could be locked in the off position, rendering the carburetor heat assist device inoperative. Prior to takeoff, with the rotor at an rpm of 100 percent the pilot would select an amount of carburetor heat application that would place the indicator needle above the upper limit of the yellow arc of the Carburetor Air Temperature (CAT) gauge. It would still be the responsibility of the pilot to monitor the CAT gauge and to select an amount of carburetor heat that would keep the CAT gauge needle out of the yellow arc. The CAT gauge had been reported as inoperative prior to the accident flight. Published Emergency Procedures The FAA-approved flight manual, in the emergency procedures section, "Power Failure-General," states, in part, "Pilots Required Emergency Action is: 1. Lower collective immediately to maintain rotor rpm, and enter normal autorotation. 2. Establish a steady glide at approximately 65 knots. 3. Adjust collective to keep rotor rpm in the green arc, or, full down if lightweight prevents attaining 97% rotor rpm. 4. Select landing spot and if altitude permits, maneuver so landing into wind."
The pilot's failure to maintain proper rotor rpm after a partial loss of engine power.
Source: NTSB Aviation Accident Database
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