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N8118L accident description

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Tail numberN8118L
Accident dateJuly 08, 2004
Aircraft typeRobinson R-22 Beta
LocationRochester, MN
Near 43.963056 N, -92.426944 W
Additional details: None

NTSB description

HISTORY OF FLIGHT

On July 8, 2004, at 1218 central daylight time, a Robinson R-22 Beta helicopter, N8118L, piloted by a student pilot and a flight instructor, was destroyed when it impacted terrain near Rochester, Minnesota. The student pilot was fatally injured and the flight instructor received serious injuries. The 14 CFR Part 91 cross-country instructional flight was operating in visual meteorological conditions without a flight plan. The last leg of the flight departed from the Austin Municipal Airport (AUM), Austin, Minnesota, about 1155.

The cross-country flight originated from a helicopter pad in front of the helicopter's hangar at a farmstead located about 5 nautical miles northeast of the Rochester International Airport (RST), Rochester, Minnesota. The first leg of the cross-country flight was from the helicopter pad to Fillmore County Airport (FKA), Preston, Minnesota, located about 20 miles to the southeast. The second leg of the flight was from FKA to AUM, located about 33 nautical miles to the west.

A witness reported that the helicopter was shut down when it arrived at AUM. The witness reported that the pilots started the helicopter in order to depart, but soon shut the helicopter down again in order to put on additional fuel. At 1146, a total of 4.8 gallons of fuel was added at AUM. The witness, who had flown about 98 hours in a Robinson R-22, reported that he fueled the left main tank. He reported the helicopter sounded fine, and that there were no indications that anything was wrong with the helicopter.

The flight departed AUM on its final leg of the cross-country flight, and was en route to the helicopter pad located about 29 miles to the northeast of AUM. The navigation log found in the helicopter after the accident indicated that the estimated time en route to the helicopter pad was 23 minutes. The navigation log indicated that the estimated fuel burn was 9 gallons per hour, and that the aircraft would burn about 3.5 gallons of fuel en route from AUM to the helicopter pad.

The owner of the helicopter witnessed the accident. He reported that he was in the process of starting a Robinson R-44 helicopter that was positioned in the grass southeast of the helicopter pad, and about 300-400 feet east of the accident site. He reported that he had started the R-44's engine and had engaged the rotor blades when he first observed the R-22 approaching from the southwest. The R-22 was about 300 feet above ground level (agl) and over flying a machine shed. He reported that the helicopter appeared to be making a normal approach toward the helicopter pad. He reported the helicopter was about 100 feet in altitude and going about 30 knots. When he looked at the helicopter again, he saw it "bobble" twice (yawing motion). Within 2-3 seconds, the helicopter rolled, "fell out of the sky" and impacted the ground. He shut down the R-44 and went to the downed helicopter to provide assistance. He turned off the R-22's ignition key and master switch.

The flight instructor reported he could not remember much about the accident. He reported that the student pilot was at the controls and they were flying a right downwind approach to the helicopter pad. On the downwind leg they observed that the R-44 was being started and that it was positioned near the hangar. The flight instructor reported that they were going to land in the field and let the R-44 helicopter depart, and then they would reposition to the helicopter pad. He stated, "I think that was our plan." He reported that he did not think there was anything mechanically wrong with the helicopter, and did not remember anything being out of the ordinary.

The helicopter impacted the terrain in a pasture about 350 feet west of the helicopter pad. The local fire department and paramedics responded to the accident and assisted with the recovery of the flight instructor. There was no post impact fire.

PERSONNEL INFORMATION

The pilot held a commercial pilot certificate with single-engine land, single-engine sea, multi-engine land, helicopter, and airplane instrument ratings. He was a certified flight instructor (CFI) with airplane single-engine land, multi-engine land, helicopter, and airplane instrument instructor ratings. He held a second class medical certificate. He had a total of about 6,300 flight hours. He had logged about 275 hours in a Robinson R-22 helicopter and about 25 hours in a Robinson R-44 helicopter.

The student pilot held a third class medical certificate. He had logged about 30 hours in Robinson R-22 helicopters. The student pilot had a non-fatal helicopter accident on April 27, 2004, when he was the pilot-in-command of a Robinson R-22, N40725, during a solo instructional flight. The National Transportation Safety Board accident number for that occurrence was CHI04CA111.

AIRCRAFT INFORMATION

The helicopter was a single-engine Robinson R-22 Beta, serial number 2409. The helicopter seated two and had a maximum gross weight of 1,370 pounds. The engine was a 160-horsepower Lycoming O-320-B2C engine, serial number L-17880-39A. The last annual inspection was conducted on March 3, 2004. The last 100-hour inspection was conducted on April 16, 2004. The helicopter had a total of 2,424 hours at the time of the accident, and had accumulated 56 hours since the last 100-hour inspection.

METEOROLOGICAL CONDITIONS

At 1233, the weather conditions reported at RST were: Winds 050 at 6 knots; sky clear; visibility 10 statute miles; temperature 22 degrees C, dew point 11 degrees C, altimeter 29.99 inches of mercury.

WRECKAGE AND IMPACT INFORMATION

The helicopter impacted the terrain at coordinates 43 degrees 57.466 minutes north, 092 degrees 24.923 minutes west. The nose of the helicopter was oriented on a 160 magnetic heading. There were no ground impact marks other than the ground scars below the main wreckage of the helicopter. The cockpit, cabin, fuselage, landing gear, engine, main transmission, main rotor mast, main rotor hub, main rotor blades, and the forward two bays of the tailcone made up the helicopter's main wreckage. The distribution of wreckage parts consisted of parts associated with the tailcone, empennage, tail rotor gearbox and tail rotor of the helicopter.

The helicopter came to rest on its right side in a nose down attitude with the aft end of the fuselage being held up by the two forward bays of the tailcone, which were bent 60-70 degrees to the right. The lower right side and nose of the cabin received crush damage. The windshields were broken out. The windshield bow and the two forward door posts were disconnected from the roof and the left door post was also disconnected from the chin. The aft door jam and bulkhead around the upper seat belt mount on the left side were collapsed inward. The lower left frame was bent inward at the battery box mount. The lower right frame was broken at the landing gear attachment point and bent in several places. The right skid tube of the landing gear was broken fore and aft of the forward strut attachment. The right aft strut attachment was partially separated from the skid tube. Both struts on the right side were angled aft. The left skid tube and struts were undamaged. The forward crosstube was bent aft, and the aft crosstube was straight.

The inspection of the cockpit revealed that the mixture was in the full rich position and the mixture guard was in place. The throttle was about two-thirds open. The governor switch was in the "ON" position. The carburetor heat was in the full up (on) position and the carburetor heat valve was found in the open position. The fuel valve was in the open position. All circuit breakers were found down or in. The warning lights were inspected and no light bulb coils were found stretched.

The inspection of the fuel system revealed that the main fuel tank and the auxiliary fuel tank were not compromised and the fuel caps were secure. There was no fuel in the main tank or auxiliary fuel tank. The rubber vent hose was disconnected from the main tank vent tube consistent with impact damage. The rubber crossover vent hose was disconnected from the auxiliary tank vent tube consistent with impact damage. The fuel lines were intact and showed no signs of leakage. The fuel finger strainers had no blockages. The vent lines were clear. The gascolator was intact and had no fuel in it. The flex fuel hose to the carburetor was undamaged and about one ounce of fuel was found in the line. The throttle of the carburetor operated smoothly and the accelerator nozzle squirted a stream of fuel. The carburetor float bowl was drained of about 3-4 ounces of fuel. The carburetor inlet screen was clear. The low fuel light system was functionally tested with no discrepancies.

An inspection of the accident site two days after the accident occurred revealed a blighted area of grass that was about 1 square foot in size that had an appearance that was consistent with "fuel burn." The location of the blighted grass correlated to the area near where the rubber vent hose was found disconnected from the main fuel tank.

The inspection of the flight controls revealed discontinuities. The breaks were consistent with overload fractures. There was no evidence of pre-impact anomalies of the flight control system.

The inspection of the drivetrain revealed that there was no visible damage to the V-belts. The actuator was fully extended with the limit switch activated. The upper sheave rotated on the clutch shaft which turned clockwise and locked up when turned counter-clockwise.

The tail rotor drive shaft was separated from the tailcone and was located about 138 feet west of the main wreckage. It was in one piece with most of the intermediate and aft flexplates still attached. It was found bowed and bent with about a 90-degree angle bend.

The tail rotor gearbox was broken in half with the input section remaining attached to the tailcone bulkhead and empennage. It was found about 40 feet south-southwest of the main wreckage. The output shaft section that retained the pitch control and tail rotor blade assembly was found about 20 feet west of the main wreckage. One tail rotor blade was found broken about 6 inches from the inboard attachment bolt. The other blade was bent outward about 7-9 inches from the inboard attachment bolt. Both input and output gears were undamaged and their associated bearings rotated.

The main rotor drive system was inspected. The main rotor drive shaft was rotated one full revolution with no abnormalities in the main rotor gearbox or the clutch driveshaft. There was no visible damage to the hub or spindles, and there were no indications that the spindle tusks made contact with the hub. The droop stops were in place and undamaged. The elastomeric teeter stops were in place. The stop for blade labeled "Blade #1" was undamaged, and the stop for the blade labeled "Blade #2" was split horizontally across the center and its bracket was bent.

Blade #1 was found positioned aft and laying flat at the accident site. No leading edge gouges, nicks, or chordwise scratches were noted.

Blade #2 was found positioned forward and standing on the trailing edge at the accident site. It was bent upward starting about 36 inches from the pitch horn flange and bent backwards. There were dents and scratches on the lower skin, outboard of the trim tab, that ran chordwise and included gray paint marks that matched the color of the tail rotor driveshaft and push/pull tube. Red marks were present on the leading edge of the blade that matched the color of the "DANGER" decal found on the tailcone.

The inspection of the tailcone revealed that the third bay of the tailcone was separated from the second bay at the bulkhead. It was found about 150 feet north of the main wreckage. It was crushed on the left side and folded over. Inside the fold were black and yellow paint marks that were similar in color to the yellow paint found on the main rotor blades. The fourth tailcone bay was attached to the third bay by a few rivets on the right side.

The fifth tailcone bay was separated from the fourth bay at the bulkhead and was found about 100 feet south-southwest of the main wreckage. It was ripped in many places and had a crease down the left side similar to the shape of the leading edge of the main rotor blade. Pieces of the red and white "DANGER" decal were found in the crease.

The sixth bay retained the empennage and the input section of the tail rotor gearbox. The horizontal stabilizer had white and black marks on the lower skin that matched the color and pattern of the paint on the tail rotor blades. The lower vertical stabilizer was damaged just below the "DANGER" decal. There were black and white paint marks on the left side that matched the color and pattern of the tail rotor blades.

The inspection of the engine revealed that it had received almost no impact damage. Engine drivetrain continuity was confirmed by rotating the crankshaft. Thumb compression and suction were confirmed to all cylinders. The engine timing was confirmed. The engine was sent to Textron-Lycoming for inspection. The engine was run on a production test stand. The engine met all the parameters for a new or overhauled engine.

MEDICAL AND PATHOLOGICAL INFORMATION

An autopsy was performed on the student pilot at the Mayo Clinic, Rochester, Minnesota.

A Forensic Toxicology Fatal Accident Report was prepared by the FAA Civil Aeromedical Institute. The results were negative for all substances tested.

TESTS AND RESEARCH

Robinson Helicopter Company issued Safety Notice SN-24 in September 1986 with a revision in June 1994. The safety notice stated the following warning:

LOW RPM ROTOR STALL CAN BE FATAL

The safety notice stated, "Rotor stall ... can occur at any airspeed and when it does, the rotor stops producing the lift required to support the helicopter and the aircraft literally falls out of the sky." It continued, "Rotor stalls also occurs at higher altitudes and when it happens at heights above 40 to 50 feet AGL it is most likely to be fatal."

The safety notice stated that rotor stall occurs due to low rotor RPM instead of low airspeed. It explained:

"As the RPM of the rotor gets lower, the angle-of-attack of the rotor blades must be higher to generate the lift required to support the weight of the helicopter. Even if the collective is not raised by the pilot to provide the higher blade angle, the helicopter will start to descend until the upward movement of air to the rotor provides the necessary increase in blade angle-of-attack. As with the airplane wing, the blade airfoil will stall at a critical angle, resulting in a sudden loss of lift and a large increase in drag. The increased drag on the blades acts like a huge rotor brake causing the rotor RPM to rapidly decrease, further increasing the rotor stall. As the helicopter begins to fall, the upward rushing air continues to increase the angle-of-attack on the slowly rotating blades, making recovery virtually impossible, even with down collective.

When the rotor stalls, it does not do so symmetrically because any forward airspeed of the helicopter will produce a higher airflow on the advancing blade than on the retreating blade. This causes the retreating blade to stall first, allowing it to dive as it goes aft while the advancing blade is still climbing as it goes forward. The resulting low aft blade and high forward blade become a rapid aft tilting of the rotor disc sometimes referred to as 'rotor blow-back'. Also, as the helicopter begins to fall, the upward flow of air under the tail surfaces tends to pitch the aircraft nose-down. These two effects, combined with the aft cyclic by the pilot attempting to keep the nose from dropping, will frequently allow the rotor blades to blow back and chop off the tailboom as the stalled helicopter falls. Due to the magnitude of the forces involved and the flexibility of rotor blades, rotor teeter stops will not prevent the boom chop. The resulting boom chop, however, is academic, as the aircraft and its occupants are already doomed by the stall

(c) 2009-2011 Lee C. Baker. For informational purposes only.