History of Flight
On January 29, 2014, about 1515 central standard time, an Enstrom F-28A helicopter, registration N24RB, was substantially damaged during landing at General Dewitt Spain Airport (M01), Memphis, Tennessee. The flight instructor and private pilot on board were not injured. The helicopter sustained substantial damage to a main rotor blade and the tail rotor driveshaft. The helicopter was operated by a flight school under 14 Code of Federal Regulations Part 91 as an instructional flight. Visual meteorological conditions prevailed, and no flight plan was filed for the local flight that departed M01 about 1340.
According to the flight instructor's written statement, they had practiced several running landings with simulated stuck anti-torque pedal approaches; each approach varied between the right pedal, left pedal, and neutral position. While the student completed some landings, the flight instructor guarded the cyclic and collective flight controls and, at times, took control to demonstrate the maneuver. The flight instructor reported that during a low power steep approach, the profile and alignment with the landing zone on final approach appeared "correct" and the speed before touchdown was approximately 10 mph. During touchdown, the landing skids collapsed, and the helicopter came to rest in a nose-down attitude.
Personnel Information
The flight instructor held a commercial pilot certificate with ratings for single-engine land, multi-engine land, helicopter, instrument airplane, and instrument helicopter, as well as a flight instructor certificate with ratings for airplane single-engine land, instrument airplane, and helicopter. He reported 5,260 total hours of flight experience, including about 2,000 hours in the accident helicopter make and model. The student held a private pilot certificate with ratings for helicopter and instrument helicopter, reporting 225 total hours of flight experience, of which 142 hours were in the accident helicopter make and model.
Aircraft Information
The helicopter was a single-engine, three-place Enstrom F-28A with skid-type landing gear, manufactured in 1971. It had accrued 10,000 total aircraft hours and was powered by a 205-horsepower Lycoming HIO-360-CIA engine. The most recent 100-hour inspection was completed on November 22, 2013, at 9,930 total aircraft hours. According to maintenance records, the landing gear was inspected per the 100-hour inspection checklist, which stated: "Check landing gear for cracks in weld areas, bolts at all attach and pivot points for excessive wear." The maintenance manual did not require removal and examination or replacement of the skid tube clamps during the helicopter's lifetime. According to the maintainer's recounted history, the helicopter had three prior hard landing events that resulted in repairs, but maintenance records contained no entries related to examination, repair, or replacement of the skid tube clamp in its 44-year history.
Wreckage and Impact Information
Post-accident examination by a Federal Aviation Administration (FAA) inspector revealed substantial damage to the tail boom and tail rotor driveshaft. Photographs showed the tail boom damaged in multiple sections with evidence of bending. The tail rotor driveshaft was found 100 feet away; one end displayed torsional damage, and the other end displayed evidence consistent with shearing. Two of the three main rotor blades were undamaged; the inboard trailing section of the third main rotor blade was split open, and the blade exhibited compression wrinkles throughout. Examination of the landing skid revealed a broken skid tube clamp, which was recovered for further inspection. The FAA inspector reported no mechanical malfunctions or anomalies with the control system that would have precluded normal operation.
Materials Laboratory Examination
The skid tube clamp consisted of two halves affixed on one side by a bolt. The upper clamp half was bent and twisted outward, and its fracture surface exhibited features consistent with overstress. The fracture surface of the upper clamp half, opposite a weld, contained numerous ratchet marks consistent with progressive cracking from multiple initiation sites. The lower clamp half was fractured near the welded ring, with no indications of plastic deformation. The mating fracture surface adjacent to the weld displayed fatigue striations indicative of fatigue crack propagation. Multiple cracks had initiated at the weld where the ring joined the lower clamp and propagated inward through the thickness of the clamp. Chemical examination revealed the lower clamp was consistent with alloy steel; the ring was also alloy steel but with lower chromium content. The weld material was consistent with common weld filler steels. Hardness testing revealed the weld material was soft compared to the clamp and ring, with increasing hardness closer to the weld. The highest hardness was between the heat-affected zone near the weld and the weld, corresponding to the fatigue crack initiation areas. The weld cracks were not visible and could only be detected by magnetic particle or dye penetrant inspection.
Additional Information
According to a representative of the airframe manufacturer, the broken skid tube clamp was identified as part #34 in Enstrom IPC 7-51, connecting the landing gear leg to the cross tube. The landing gear leg, oleo strut, and outboard portion of the cross tube form a triangle. During landing, forces push up on the skid at the bottom corner of the triangle, causing the triangle to pivot around the upper end of the oleo strut, placing the skid tube clamp in tension. The representative reported multiple landing gear failures over 20 years; clamps were either broken or cracked, but damage was always attributed to overstress due to hard landings and never examined for evidence of fatigue.