Goshen, IN, USA
N480FX
Enstrom 480
The certified flight instructor (CFI) stated that the student smoothly lowered the collective and set the helicopter on the ground as he was rolling off the throttle. The helicopter then started to bounce (ground resonance) and the CFI reached down to make sure the collective was down and the throttle was off. The helicopter continued to "shake violently and ripped itself apart." The helicopter had a new design of elastomeric damper installed 91 flight hours before the accident. The new design dampers had been flight tested at the factory and approved by the Federal Aviation Administration prior to being put into service. The helicopter had experienced severe vibrations on two previous flights, one of which resulted in the chin bubble being broken. Inspection of the accident dampers, at the damper manufacturer's factory, determined the dampers were within manufacturer's specifications. After the accident flight the helicopter manufacturer recalled all the new design elastomeric dampers.
HISTORY OF FLIGHT On August 5, 2005, about 1100 central daylight time, an Enstrom 480, N480FX, sustained substantial damage after landing at Goshen Municipal Airport (KGSH), near Goshen, Indiana. The training flight was operating under 14 CFR Part 91. Day visual meteorological conditions prevailed at the time of the accident. No flight plan was filed. The student pilot sustained serious injuries and was hospitalized. The certified flight instructor (CFI) and rear seat passenger sustained minor injuries. The flight originated from Warsaw Airport, to Goshen Airport to pick up a passenger, then a flight to Grand Rapids to pick up the owner/pilot in training, and then back to Goshen Airport. The CFI stated that upon landing at Goshen Airport: "The winds were light and variable from the north. He [student pilot] set up with a right hand downwind for grass Runway 36. He made the appropriate radio calls and a nice normal approach to the center of the runway. He then stopped, cleared the tail and made 180 degree turn to the south. He then proceeded to hover taxi back to the ramp. [The student pilot] then made a radio call prior to crossing Runway 9-27 and also when he was cleared on the other side. He then proceeded to the ramp area. His hover taxi was very slow and controlled. On the ramp, [the student pilot] made two right hand turns to point back to the helipad, both times clearing the tail before turning the aircraft. [The student pilot] then moved the aircraft forward to the helipad. He then smoothly lowered the collective and set the aircraft on the ground as he was rolling off the throttle. The aircraft started to bounce. I reached down to make sure the collective was down and the throttle was off. Then the aircraft started shaking violently and ripped itself apart." The student pilot stated that he does not recall the accident. WITNESS STATEMENTS The rear seat passenger stated: "[The student pilot] piloted the helicopter all the way to Goshen, making all of the appropriate calls along the way. When we arrived at Goshen, the winds were light out of the North. [The student pilot] decided to use Runway 36. He had a very good approach and came to a hover over Runway 36. [The student pilot] began to hover taxi to the ramp crossing runway 9-27 and making the proper radio calls as he crossed. He hovered to the ramp and cleared his tail before each turn. [The student pilot] came to a hover over the helipad facing into the wind. He slowly and carefully lowered the collective and set the helicopter down on the helipad with the collective full down and [the student pilot] rolling off throttle the rest of the way the helicopter began to shutter and bounce. Then a split second later the helicopter started to shake very violently. I looked forward at [the CFI] and [the student pilot] and the both of them had their hands on the collective to hold it full down and I could see that [the CFI's] wrist was completely turned at a ninety degree angle like a backward 'L', throttle was completely rolled off.The aircraft began ripping itself apart." Another witness who was at the fuel pumps at the time of the accident stated: "I was actually close to the scene. I was out by the fuel pumps at the time of the accident. I took time to watch as the helicopter landed. It appeared to be a good, normal landing. The next thing I knew was that the helicopter began to shake back and forth. From the time the shaking started the intensity increased. As this happened the helicopter rose about 2 feet off the ground. It appeared as if the people in the helicopter were trying to shut it down. As quickly as this took place I saw that the helicopter tail broke off from the body of the helicopter. As soon as that took place the helicopter rose off the ground-about 30 feet." PERSONNEL INFORMATION The CFI, who was seated in the right seat. held a commercial single engine land certificate, a flight instructor helicopter certificate and an instrument helicopter rating. The CFI's last medical examination was completed May 4, 2005. He held a second class medical certificate with a limitation to wear corrective lenses. According to the CFI's logbook, he had 2,107 hours aircraft total time, 1,965 hours rotorcraft total time. He had 92 hours total in the Enstrom 480. He had flown 84 hours in the last 90 days, 29 hours in the last 30 days, and 5 hours in the last 24 hours in the make and model. The student pilot's flight times were not received by the National Transportation Safety Board (NTSB). AIRCRAFT INFORMATION N480FX, an Enstrom 480, powered by a turbo shaft, 285 horsepower Allison engine, model number 250-C20W, was a normal category helicopter with skid type landing gear. The helicopter was owned by the student pilot. The aircraft's last annual was done May 23, 2005, with an aircraft total time of 1454.0 hours. Damper Kit P/N 4230030-1 was installed during this annual inspection. The aircraft had a total time of 1545.0 hours at the time of the accident. METEOROLOGICAL INFORMATION The weather at the time of accident was clear, with calm winds, no visibility restrictions and no turbulence. WRECKAGE AND IMPACT INFORMATION The helicopter's tail was sheared from the body of the helicopter and came to rest about 30 feet away across the ramp area. The front seats were found about 10 feet away. The instrument panel was separated from the fuselage and lay in pieces near the fuselage. A piece of the engine's transmission mount pylon tube was found broken and sent to the NTSB Materials Laboratory for inspection. Inspection of the flight controls and engine did not reveal any evidence of preimpact failure or malfunction. The new design dampers, that were installed during the latest annual inspection, were retained for further investigation. TESTS AND RESEARCH Inspection of the broken transmission mount pylon tube at the NTSB Metallurgical Laboratory determined that the break was an overstress fracture with no evidence of a preexisting crack or fatigue. The three Elastomeric Dampers were tested at the manufacturer's facility in Erie, Pennsylvania on October 25, 2005, under the supervision of the National Transportation Safety Board. Damper numbers LK0053, LK0058, AND LK0088 were inspected for damage and conformity to manufacturer's specifications. The following was noted during the inspections: LK0053 Elastomer: -No evidence of damage or failure in the form of bond separation except for very slight edge bond separation at the point of most severe center plate deformation. No elastomer cracks are observed, slight ozone checks are visible, but not out of the ordinary. Metal components: -Bronze bushing cracks were observed in flange (approx 12 & 6 o'clock positions on both ends) on inboard end of damper. -Clamping bolts on inboard end are in good condition and torque stripe is in-line. -Evidence of impact in outer plate lead-lag pin window. Damage is observed toward the ends and along one side of the oval window. -Both outer metals are deformed -Evidence of impact on bottom outer plate (near name plate) -Center plate deformed along its length and also has an indentation on the edge of the plate approximately 0.90" from outboard end of plate. -Evidence of center plate impact on outer member at each end (more pronounced at outboard end). -Outer members separated at outboard end (measured gap=0.107" fwd blade side end and 0.227" at other end). Bolt sheared near "nut side" on outboard end of damper. -Spherical bearing has noticeable axial and radial "play" by hand feel (see measurement below) some dirt & dust noted. Staking markings are intact and staking joint appears to be in good condition. -Rotational break away torque measured 0.14-0.22 in-Lbs -Axial play measured 0.007" -Scrapes and wear observed on outboard end of outer member plates, surface nearest grip. -Witness mark noted on edge of center plate adjacent to the Spherical bearing. LK0058 Elastomer: -No evidence of damage or failure in the form of bond separation except for very slight edge bond separation at point of most severe plate deformation. No elastomer cracks observed, slight ozone checks are visible, but not out of the ordinary. Metal components: -Bronze bushing crack observed in flange (approx 6 o'clock position on lower, 12 o'clock on upper, both ends) on inboard end of damper. -Clamping bolts on inboard end in good condition and torque stripe is in-line. -Evidence of impact in outer plate lead-lag pin window. Damage observed toward the ends and along one side of the oval window. -Outer member upper plate deformed/bowed slightly at center. Lower plate has little/no visual evidence of deformation. -Center plate deformed along its length and also has an indentation on the edge of the plate approximately 0.90" from outboard end of plate, also slight mark on opposite end (same edge) -Evidence of center plate impact on outer member at each end (more pronounced at outboard end). -Outer members separated at outboard end (measured gap=0.008" fwd blade side end and 0.043" at other end). Bolt appears to be intact, head and nut in place, nut torque stripe is aligned. -Spherical bearing has noticeable axial and radial "play" by hand feel (see measurement below), some dirt & dust noted. Staking markings are intact and staking joint appears to be in good condition. -Rotational break-away torque measured 0.12-0.22 in-Lbs -Axial play measured 0.005" -Scrapes and wear observed on outboard end of outer member plates, surface nearest grip. LK0088 Elastomer: -No evidence of damage or failure in the form of bond separation. Slight surface crack (approx 3/8" length) observed on inboard edge of upper outboard pad cracks, slight ozone checks are visible, but not out of ordinary. Metal components: -Bronze bushing cracks observed in flange (approx 12 & 6 o'clock positions on both ends) on inboard end of damper. -Clamping bolts on inboard end in good condition and torque stripe is in-line. -Evidence of impact in outer plate lead-lag pin window. Damage observed toward the ends and slight damage along one side of the oval window. -No visual evidence of deformation along length of outer plates. Deformation visible at outboard ends only (where two halves come together) -Center plate deformed along its length and also has an indentation on the edge of the plate approximately 0.90" from outboard end of plate, also slight mark on opposite end (same edge) -Evidence of center plate impact on outer member at each end (more pronounced at outboard end). -Outer members separated at outboard end (measured gap=0.290" fwd blade side end and 0.288" at other end). Bolt sheared near "nut side" on outboard end of damper -Spherical bearing has slight axial "play", no radial "play" noted by hand feel (see measurement below), some dirt & dust noted. Staking markings are in tact and staking joint appears to be in good condition. -Rotational break away torque measured 0.10-0.14 in-Lbs -Axial play measured 0.006" On October 25, 2005, at the Lord Corporation facilities in Erie, Pennsylvania, the testing of the aircraft's oleo damper's was conducted and the following findings were reported: Visual inspection of landing gear oleo dampers: -Damper box was unopened and in "as received condition" from FAA South Bend Flight Standards District Office. -Inspection notes start from "top of strut" as installed in aircraft and works down to end that is connected to skid. Right Front -Note on part that strut has been deflated by an Enstrom employee to facilitate moving helicopter to storage. -Upper lug is missing. -Gouge in center of upper clevis at aft end and impact marks on edge of clevis ears (aft edge). Inboard hole in clevis elongated. Outboard hole elongated to lesser degree. Hole surfaces are worn. Outboard clevis leg bent slightly outboard. -Valve cover is missing. -Evidence of leakage around seal -1 fairing tab is broken, clamp in place. Upper fairing tab is missing -Step bolts missing -Mark on inboard and outboard aft edge of lower clevis. Also mark on fwd edge of inboard clevis. -Lower lug tight in clevis (can not move by hand). Left Front -Upper lug tight in clevis (cannot move by hand). -Broken clevis section from cross tube still attached to bolt. -Gouge in forward oleo clevis appears to be from bolt. Clevis ears have marks on forward edges. -Evidence of leakage around seal -Clamp and fairing tab missing -Upper fairing bracket has taped marking "275" -Mark on inboard and outboard aft edge of lower clevis -Lower lug tight in clevis (cannot move by hand). -Slight forward bow in oleo strut Right Rear -Upper lug can be moved by hand. -Broken clevis section from cross tube still attached to bolt. -Gouge in forward oleo clevis appears to be from bolt. Clevis ears have marks on forward edges. -No evidence of leakage. -2 fairing tab is broken, clamp in place -Upper fairing bracket has taped marking "550" -Lower lug is snug but can move slightly by hand -Mark on centerline aft edge of lower clevis. -Some wear and slight elongation in lower lug hole. Left Rear -Upper lug snug in clevis but can be moved by hand -Small mark on fwd edge of outboard clevis. -Some wetness around seal -Clamp and fairing tab missing -Upper fairing bracket has taped marking "550" -Lower lug is snug but can moved by hand -Marks on all edges of clevis, slight mark on aft centerline of clevis Oleo Testing: Procedure -Place part in MTS machine with appropriate stroke capability -Generate load deflection curve, 0, +2.5, 0 at 1 cpm. -Repeat above with pressure transducer installed. Results -Right Front no pressure indicated (see visual inspection notes above) -Left Front 250 PSI - 406 PSI when stroked to 2.5" -Right Rear 474 PSI - 746 PSI when stroked to 2.5" -Left Rear 444 PSI - 604 PSI when stroked to 2.5" (leakage noted during test) -Load deflection and pressure deflection plots were taken for each part. One L/D plot with and one without pressure gauge installed. Visual inspection of transmission mount pylon tube -Photos taken of fractured surfaces. Agreed fracture surface needs to be evaluated by metallurgist. -Review of accident site photos of this area showed tube fracture; however, tube was in alignment and surrounding structure was intact. The FAA Rotorcraft Flying Handbook defines ground resonance as an: "aerodynamic phenomenon associated with fully-articulated rotor systems. It develops when the rotor blades move out of phase with each other and cause the rotor disc to become unbalanced. This condition can cause a helicopter to self-destruct in a matter of seconds. However, for the condition to occur, the helicopter must be in contact with the ground. If you allow your helicopter to touch firmly down on one corner (wheel type landing gear is most conducive for this) the shock is transmitted to the main rotor system. This may cause the blades to move out of their normal relationship with each other. This movement occurs along the drag hinge. If the r.p.m. is low, the corrective action to stop ground resonance is to close the throttle immediately and fully lower the collective to place the blades in low pitch. If the r.p.m. is in the normal operating range, you should fly the helicopter off the ground, and allow the blades to automatically realign themselves. You can then make a normal touchdown. If you lift off and allow the helicopter to firmly re-contact the surface before the blades are realigned, a second shock could move the blades again and aggravate the already unbalanced condition. This would lead to a violent, uncontrollable oscillation. This situation does not occur in rigid or semirigid rotor systems, because there is no drag hinge. In addition, skid type landing gear are not as prone to ground resonance as wheel type gear." ADDITIONAL DATA The CFI and student pilot were involved in a similar incident on July 10, 2005, and according to the flight instructor: "When [the student pilot] started to lift the collective the aircraft began to shake. I then reached down for the collective and [the student pilot] had already lowered i
Ground resonance experienced by the pilots.
Source: NTSB Aviation Accident Database
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