Shady Cove, OR, USA
N1577
GARLICK HELICOPTERS INC UH-1H
The pilot reported that he was in straight and level flight over mountainous, tree-covered terrain when he heard a loud noise followed by an abrupt jolt and right yaw. The pilot initiated a descent into a valley when he heard a second loud noise followed by severe vibrations and activation of the low rotor RPM horn. The helicopter continued to descend before it impacted the trees and ground below. During an on-scene examination, control continuity was established throughout the helicopter with the exception of the 90-degree gearbox and tail rotor assembly, which were separated from the helicopter and not found. The gear box is positioned in between the tail rotor drive shaft and tail rotor assembly; it takes the input from the tail rotor drive shaft (through a quill assembly), and adjusts and transfers the energy to the tail rotor assembly. Because the gearbox and tail rotor separated in flight, the investigation was not able to examine them to determine their condition or their proper installation. However, the tail rotor gearbox input quill assembly, vertical driveshaft clamp, and two remaining gearbox attachment studs were removed for additional examination. Fatigue damage was noted along the fracture surface of the two gearbox attachment studs that were still attached to the vertical fin side of the mounting flange. The remaining four attachment holes were empty; however, helical rubbing marks were found around their perimeter along with an adjacent lip of deformed material, which is indicative of the studs being present at one point. Postaccident examination and a test run of the helicopter's engine revealed no anomalies that would have precluded normal operation. The helicopter's most recent tail rotor gearbox inspection occurred about one month prior to the accident; however, the investigation could not determine if the fatigued area would have been included in the inspection, or whether any evidence of fatigue would have been detectable at the time of the inspection.
HISTORY OF FLIGHTOn October 7, 2012 about 1513 Pacific daylight time, a Garlic Helicopters UH-1H, N1577, made a forced landing following separation of the tail rotor gearbox and tail rotor assembly approximately 4 miles southwest of Shady Cove, Oregon. The commercial pilot sustained minor injuries and the helicopter was substantially damaged. The helicopter was registered to a private individual and operated by Columbia Basin Helicopters under the provisions of Title 14 Code of Federal Regulations Part 133. Visual meteorological conditions prevailed and no flight plan was filed for the local flight that departed Merlin (Grants Pass), Oregon at 1400. The pilot reported that the purpose of the flight was to long line buckets of water onto a wildfire. After dropping three loads of water, he started his flight back to base. During straight and level flight, he heard a loud noise and felt an abrupt jolt and right yaw. He descended into a valley when he heard another loud noise followed by severe vibrations. During this time, he also heard the low rotor RPM horn activate in the cockpit; he lowered the collective to maintain RPM and proceeded to an area of small trees. Slightly above tree level, he raised the collective and descended into the trees; the helicopter impacted the ground and rolled to its right side before it came to rest. PERSONNEL INFORMATIONThe pilot, age 30, held a commercial pilot certificate for helicopters issued on February 9, 2004, and a second-class airman medical certificate issued on December 8, 2011 with the limitation that he must wear corrective lenses. The pilot reported that as of the accident date, he had 2,959 total hours, 744 of which were in the accident helicopter make and model. He flew 243 hours within the 90 days preceding the accident, and 24 hours within 30 days preceding the accident date. AIRCRAFT INFORMATIONThe two-seat configured helicopter, serial number 65-09577, was manufactured in 1965. It was powered by a Honeywell T53-L-13B turboshaft engine, serial number LE-18674. Review of the helicopter's logbook records showed a 25 hour tail rotor gearbox inspection completed on September 4, 2012, at a recorded tachometer reading of 6,284.2 hours. A 50 hour inspection, which included an inspection and cleaning of the tail rotor blades, was completed on August 24, 2012, at a recorded tachometer reading of 6,258.2 hours. On August 17, 2012, skin on the left side of the vertical fin was replaced due to "cracking at lower rivets," and a doubler was added to the area; the tachometer reading was 6,231.2 hours. The pilot reported that the helicopter was due for a 25 hour inspection in about 2-5 flight hours. METEOROLOGICAL INFORMATIONThe nearest weather reporting station, Rogue Valley International – Medford Airport (MFR), Medford, Oregon was located about 12 miles south of the accident site. At 1353, MFD reported clear skies, wind from 300 degrees at 4 knots, visibility 10 statute miles, temperature 28 degrees C, dewpoint -3 degrees C, and an altimeter setting of 29.85 inches of mercury. AIRPORT INFORMATIONThe two-seat configured helicopter, serial number 65-09577, was manufactured in 1965. It was powered by a Honeywell T53-L-13B turboshaft engine, serial number LE-18674. Review of the helicopter's logbook records showed a 25 hour tail rotor gearbox inspection completed on September 4, 2012, at a recorded tachometer reading of 6,284.2 hours. A 50 hour inspection, which included an inspection and cleaning of the tail rotor blades, was completed on August 24, 2012, at a recorded tachometer reading of 6,258.2 hours. On August 17, 2012, skin on the left side of the vertical fin was replaced due to "cracking at lower rivets," and a doubler was added to the area; the tachometer reading was 6,231.2 hours. The pilot reported that the helicopter was due for a 25 hour inspection in about 2-5 flight hours. WRECKAGE AND IMPACT INFORMATIONOn scene examination by a Federal Aviation Administration inspector revealed the helicopter came to rest on its right side in mountainous, tree-covered terrain. The helicopter remained mostly intact and flight control continuity was established throughout the helicopter with the exception of the 90 degree gearbox and tail rotor assembly. The 90 degree gearbox is positioned in between the tail rotor drive shaft and tail rotor assembly; it takes the input from the tail rotor drive shaft (through a quill assembly), and adjusts and transfers the energy to the tail rotor assembly. The 90 degree tail rotor gearbox and tail rotor assembly were separated from the tail rotor gearbox input quill assembly and were unable to be located at the accident site. Two of the tail rotor gearbox attachment studs and their nuts were still attached to the input quill mounting flange on the vertical fin side; however, they were fracture separated on the gearbox side of the mounting flange. The cargo hook was found in the open position with the long line and bambi bucket still attached. The long line extended loosely aft of the helicopter into the trees where the bambi bucket came to rest. TESTS AND RESEARCHA postaccident inspection of the engine revealed no visual anomalies. The chip detector was removed and no debris was noted. The engine oil filter was also removed and it contained some small debris, however, not enough to block the filter. The engine was installed into a test cell and was started and idled temporarily before it was operated at various power settings. There were no noted mechanical failures or malfunctions with the helicopter's engine. The tail rotor gearbox input quill assembly, the vertical driveshaft clamp, and the two remaining gearbox attachment studs were removed from the airframe and sent to the National Transportation Safety Board's Materials Laboratory for additional examination. The NTSB Materials Lab reported that the two studs, which were still attached to the input quill mounting flange, were removed and the fracture surfaces were examined. Both of the studs exhibited a fatigue crack that started at the root of a thread on one side of the bolt and extended across the bolt until it met a second smaller crack that had initiated on the other side of the bolt. The relative size and location of the fatigue cracks indicated that the cracks grew initially by uniaxial bending fatigue and later by reverse bending fatigue. On one stud, the two fatigue cracks met along a parting line with no overstress region. The second stud had a rough region in between the two cracks indicative of a final overstress fracture. The input quill mounting flange was cleaned and examined. Of the six stud holes, five of them exhibited helical rubbing marks around the perimeter of the hole and a lip of deformed material adjacent to the hole; indicative of contact by studs.
The separation of the tail rotor gearbox and tail rotor assembly as a result of fatigue to the tail rotor gearbox input quill assembly attachment studs.
Source: NTSB Aviation Accident Database
Aviation Accidents App
In-Depth Access to Aviation Accident Reports
