GROVE HILL, AL, USA
N68HJ
Bell 205A-1
During an external lift operation to pick up a fertilizer bucket, the vertical fin separated from the tail boom. The helicopter rotated to the right, and the pilot tried to move away from obstacles. The tail boom struck the ground and separated from the fuselage, while the helicopter was still rotating to the right. The main rotor blades hit a tree and the helicopter came to rest on the ground on its left side. Investigation revealed a fatigue fracture through a rivet hole of the vertical fin forward spar. An inspection of this area was signed off the day prior to the accident. However, examination of the wreckage, and a sister ship, revealed that the fracture location could not be observed during normal inspections because of other structure. One airworthiness directive (AD), and service bulletins issued by the manufacturer, addressed vertical fin spar cracks, but none applied to this ship. The AD required removal of the rivet, where the fatigue fracture initiated, to facilitate inspection of the rivet hole. The inspection was no longer required after compliance with a service bulletin that modified the spar. This helicopter was modified during its original manufacture.
On March 21, 1996, about 1410 central standard time, a Bell 205A-1, N68HJ, collided with the ground, following a loss of control while in a hover, near Grove Hill, Alabama. The helicopter was operated by Heli Jet Corporation under the provisions of Title 14 CFR Part 133. Visual meteorological conditions prevailed. A flight plan was not filed for the local fertilizer application flight. The commercial pilot was not injured and the helicopter was destroyed. The flight departed a local landing site about 1330, on the same day, with several take-off and landings occurring at the site before the accident. According to the pilot and the loader, the helicopter was in a hover, lifting the fertilizer bucket off the ground, when there was a loud bang and the helicopter began to rotate to the right, about the yaw axis. The pilot stated that a full left anti-torque control application was made with no effect. After one 360 degree rotation of the nose, the helicopter was nose high. He applied cyclic to move away from the bucket and other obstacles. The tail contacted the ground breaking off the tail boom. The main rotor then struck a tree and the helicopter struck the ground. At the accident site, the helicopter was laying on it's left side. The tail boom was separated from the fuselage and was broken into two pieces. The forward piece of the tail boom was directly behind the fuselage of the helicopter. The aft portion of the tail boom was found in front of the fuselage. The vertical fin was about half the distance between the aft section of the tail boom and the nose of the fuselage. At its separation point at the tail boom to fuselage attachment point, the tail rotor drive shaft had 360 degree rotational score marks. The vertical fin forward spar was fractured at its juncture with the tail boom assembly, aft of the 42 degree gearbox. The fracture surface on the tail boom side was smooth, and dull gray in color about 85 percent of the width of the spar. The remaining section of the crack was sharp and shiny. The fastener holes on the tail boom assembly aft of the 42 degree gear box were elongated and torn. The forward holes were gray and barely torn. As the damage progressed rearward, the holes were torn more and became shiny rather than dull gray. The damaged holes on the aft right section had the most damage, with a tear progressing through the last five holes. The damage around the fracture surface on the tail fin was consistent with that of the fracture surface on the tail boom. A metallurgical examination of the failed vertical fin spar was conducted at the Safety Board Materials Laboratory in Washington, D.C. The front spar of the vertical fin was composed of an aluminum sheet in a "C" channel shape. The left and right caps of the spar were reinforced by a series of four (4) "L"-shaped metal pieces on the forward side of the web. The metal pieces were bonded to each other and to the web by an epoxy type bonding material. Visual examination revealed that portions of the fracture propagating through the metal pieces forming the left spar cap were on a flat plane and contained crack arrest positions indicative of progressive (fatigue) cracking. Separation of the left cap occurred through the hole for the first rivet above the tail boom. Examination revealed that the fatigue fracture in each metal piece originated at the surface of the rivet hole. Scanning electron microscope examination disclosed that some areas of the fractures contained the closely spaced fine striations indicative of high cycle fatigue cracking. The examination also revealed that the laminate lay out and chemical composition of the metal pieces in the left cap of the helicopter were consistent with those specified by the manufacturer for helicopters made after May 11, 1971. According to Bell personnel, the vertical fin spar for the Bell 205A has been redesigned three times since initial production. The first design change was in 1971 and consisted of installing doublers to the spar channels. This modification was included in production models after May 11, 1971. The next upgrade used a one piece spar assembly. In 1986 doublers were again added to the one piece spar assembly. Helijet used an FAA approved progressive maintenance inspection program. Within the program, they had inspections, called "checks", identified by the letters A, B, C, D, and E. Maintenance records for the helicopter showed an "A check" inspection was signed off, on March 18, 1996. The inspection requirements for the completion of this inspection include a section on the aft fuselage/tail boom area. Step number eight of this section reads "Forward spar of the vertical fin for cracks, loose or working rivets.". A "C check" was started on March 16, 1996. Item number two in section six of this inspection reads "Check internal and external structure of tail boom for cracks, distortion, corrosion and security.". This item was signed off as being completed on March 20, 1996. Visual examination of the crashed helicopter, and discussions with the operator, indicated that prior to the final separation of the vertical fin, the location of the spar fracture was hidden from view by other tail boom structure. Bell Helicopter's 100 hour inspection and structural maintenance manual inspection (attached) specifies a general inspection of the tail boom for cracks, distortion, corrosion, loose rivets, and security. The U..S. Army flies Bell 205A-1 helicopters under the military designation of UH-1. According to the U.S. Army Safety center, the U.S. Army has additional inspection requirements on the vertical fin spars of the UH-1. Completion of the inspection includes a dye penetrant and ultrasound inspection performed every 600 flight hours. A helicopter repair station manager in Canada reported that over many years he has seen and repaired many vertical fin forward spars on Bell 205As. He said the spars always had cracks starting on the left side. He also stated that they all had loose rivets on the left side. The most recent case of cracking he had seen was a few weeks before the date of the accident being investigated. A request was made to the FAA for Service Difficulty Reports (SDR), between January 1, 1990 to the date of the accident, that pertained to vertical fin cracks. None were found in the Safety Data Analysis Section database. Airworthiness Directive (AD) 71-21-02 was issued by the FAA effective October 30, 1971. Amendment 39-2048 was effective December 23, 1974. The AD applied to Bell Models 205A and 205A-1 certificated in all categories and equipped with tail boom assembly, part number 205-032-802-1, -2, -5, -9, or -11. The AD was to detect a possible fatigue crack in the vertical fin forward spar in the area of the first rivet hole above the tail fin and tail boom intersection. Removal and non-replacement of the first rivet was required to facilitate a daily inspection for cracks. If cracks were found, the tail boom was to be replaced. The inspection requirement of the AD could be dispensed with upon compliance with Bell Helicopter Service Bulletin No. 205A-6 date May 11, 1971or a later Revision A dated June 14, 1971. The AD did not apply to N68HJ by tail boom part number and the service bulletin did not apply by aircraft serial number, since the modification was applied during manufacture.
fatigue failure of the vertical fin forward spar in an area that was not accessible to inspection. A factor relating to the accident was: guidance for inspection of the area (where the fatigue occurred) was insufficiently defined by the manufacturer.
Source: NTSB Aviation Accident Database
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