NTSB Factual Report

On July 10, 1995, at 0800 eastern daylight time, a Bell 47G- 3B-1, helicopter, N48316, was destroyed during a forced landing and collision with trees near Hammonton, New Jersey. The commercial pilot was fatally injured. Visual meteorological conditions prevailed, for the positioning flight that departed Hammonton, New Jersey, about 0750. No flight plan had been filed for the flight conducted under 14 CFR Part 137.

According to a company employee, after the pilot completed an application mission, he landed at the field site, rinsed out the chemical tank, and refueled the helicopter. The pilot then departed in N48316, for Mays Landing, New Jersey.

According to a Federal Aviation Administration (FAA) Inspector, a witness reported hearing the helicopter engine, and then heard the helicopter impact the trees.

Examination of the wreckage by the FAA Inspector confirmed that the engine could be rotated, and valve train continuity and compression were present on all cylinders. The carburetor bowl was full of fuel, and the fuel screen was absent of debris. No pre-impact failure of the engine was observed. Drive train continuity was confirmed from the engine, through the transmission, to the main and tail rotor systems. According to the FAA Inspector's statement, "Excessive travel was noted in the collective control system."

A certified airframe and power plant (A&P) mechanic, employed by the State of New Jersey, examined the collective control system at the accident site. His report stated:

Collective pitch levers could not be moved more than 1 inch travel, up and down. Movement was restricted and did not feel normal. Collective pitch change link was removed. Excessive vertical play was found in collective levers. Collective levers were removed. Collective pitch control yoke was then checked and it was found to have excessive play in all directions... Collective support was then moved upward on mast to expose pitch control bearing. Bearing lock nut appeared to be tight and intact. Movement of support dislodged three roller bearings from the bearing, and numerous thin slivers of metal. The grease was very black.

The main rotor mast and swashplate assembly were removed for further examination, and shipped to the Bell Helicopter Textron facility, Forth Worth, Texas. The Bell Helicopter Engineering Laboratories Report stated that the collective yoke ball bearings had rolled onto the bearing shield, and the collective pitch sleeve yoke spacer. The report further stated:

Spherical impressions and deformation were evident around the inner diameter of the yoke spacer which indicated bearing rotation at the time the bearing came apart...The bearing outer ring had an uneven raceway pattern as well as deformation over the edge of the raceway...The heaviest wear and deformation was in line with the loading arms on the collective pitch control yoke and may have been the exit point of the ball bearings. The raceway and other inside diameter surfaces on the outer ring were corrosion pitted and worn...The bearing inner ring was also corrosion pitted and worn...Eight of the balls that were recovered were examined...all the balls were corrosion pitted...Grease samples...were analyzed and found to be similar to Mobil 28 (Mil-G-81322)...

The helicopter was manufactured for the U.S. Army in 1966, as a TH-13T. On October 21, 1982, the helicopter was converted from a TH-13T, to a Bell 47G-3B-1. On January 24, 1986, the helicopter was completely rebuilt by Tulsa Helicopters Inc. (THI). During the rebuild, a 1,200 hour inspection was also completed, which included the installation of a new collective sleeve.

In July, 1990, a dampener arm failed and resulted in a hard landing. The helicopter was then returned to THI for repair, and a sudden stoppage inspection. The inspection did not require the removal of the collective sleeve assembly, or inspection of the collective yoke bearing. The helicopter was returned to service on April 24, 1991, with a completed annual inspection.

Since the 1986, annual and 1,200 hour inspection, the helicopter had 9 annuals conducted by 6 different mechanics. The 1,200 hour inspection was the only inspection that required the disassembly and examination of the collective sleeve and yoke bearing.

The annual completed on March 23, 1993, by Rotorcraft Inc., Black Rock, Arkansas, included a 600 hour inspection. Disassembly and inspection of the collective yoke bearing was not accomplished, or required, during the inspection.

The Bell Helicopter Maintenance and Overhaul Manual, required that either Mil-G-25537, or Mil-G-81322, grease be used inside the collective sleeve and the swashplate support. It also stated that intermixing of greases was prohibited. The grease removed from the inside of the collective sleeve was black; however, grease samples were analyzed and found to be similar to Mobil 28 (Mil-G-81322).

The investigation revealed that THI used Mobil 28 grease when the collective sleeve was installed, in 1986. The pilot also used Mobil 28 during his greasing of the helicopter, between the 100 hour and annual inspections. During the 600 hour inspection, the pilot supplied Mobile 28 grease to Rotorcraft Inc., to complete the inspection.

A review of the helicopter's log books revealed that during the past 9 years, the pilot maintained the helicopter's daily logs. He performed oil changes and helicopter greasing, between the annuals and 100 hour inspections. He also performed the cleaning and washing of the helicopter.

The log books also revealed that the helicopter experienced periods of low usage during the winter months. Between October 1993, and April 1994, 2 hours were logged on the helicopter. Also, between September 1994, and April 1995, only 9/10 of an hour was logged on the helicopter. The last entry in the helicopter's log book was dated April 3, 1995. According to a separate log, and the helicopter's Hobbs meter, 57 hours were flown on the helicopter after the April entry.

The Bell procedures for inspecting the pitch control sleeve and yoke assemblies were examined. According to the Bell 47D-1 Maintenance and Overhaul Manual, the removal and magnaflux inspection of these components occurred at the 600 hour inspection. The 600 hour inspection criteria, for the Bell 47G-3, stated that complete disassembly and magnaflux is accomplished at the 1,200 hour inspection.

The 600 hour inspection of the Bell 47G-3 addressed disconnecting links and bolts to examine components. The components included; the main rotor blade grips and gimbal ring bearings; the stabilizer bar; the swashplate for freedom of operation, and for indications of roughness or wear in the carrier bearings, gimbal pins and bearings; the scissor levers bushing and bearings for wear; and raising the transmission cap to inspect the main rotor mast bearing. It also included the removal and partial disassembly of the main transmission. There was no description of a procedure to inspect the collective yoke bearing.

The 47G-3 manual listed components that had a maximum hour life. The collective sleeve assembly was listed with a 5,000 hour retirement life. A retirement life was not listed for the collective yoke bearing. The yoke bearing was a condition item that would be changed when it was determined to be unserviceable, by disassembly and visual inspection. The criteria used to inspect the collective yoke bearing would be interpreted from the Bell 47G-3B-1 Maintenance and Overhaul Manual.

The 1,200 hour inspection criteria, stated that the inspection of all bearings for smooth and free operation should be accomplished before and after assembly. It also stated to check parts dimensionally as necessary to determine the extent of wear, or for proper fit of mating parts. Under Mast and Controls, it stated to remove, disassemble, and visually inspect parts for wear and damage, which included all parts of the swashplate.

In Section III of the manual, Mast and Controls, the collective yoke bearing was listed as 1 of 12 different types of bearing assemblies that made up the swashplate assembly. The inspection procedure stated to visually inspect the swashplate parts for wear or damage, to inspect bearings for chips, cracks, brinelling, and splines for chipped, cracked, broken or worn teeth. It also stated, "Inspect parts dimensionally." A chart then listed components with inner and outer diameter dimensions. The only collective yoke bearing components listed were the inner and outer race dimensions.

The inspection criteria did not specifically address the collective yoke bearing. It did not identify a method to check the condition of the bearing while installed on the helicopter. It also did not specify a ball bearing wear tolerance. The A&P mechanic was left to decide what constituted excessive wear.

The collective yoke bearing had accumulated about 839 hours since the 1,200 hour inspection, and 208 hours since the 600 hour inspection. There were no records available to determine if the collective yoke bearing was replaced at the 1,200 hour inspection.

The Maintenance and Overhaul Manual for the Bell 206 specified special lubrication requirements. The manual stated:

After each day of operation in rain showers or snow or after washing helicopter, all exposed control bearings should be purge lubricated to remove trapped moisture and ensure that a lube film is applied to susceptible surfaces.

Parking helicopters outside in a heavy dew environment requires that all exposed control bearings be purge lubricate every seven days to ensure that no voids exist that could trap moisture.

If helicopter is stored for a period in excess of 45 days without operation, or service, purge lubricate all bearings.

The Bell 47G-3B-1 manual did not list any special lubrication requirements.

Interviews with Bell 47, agriculture operators and maintenance facilities, revealed that as a practice, they disassembled and inspected the collective yoke bearing during the 600 hour inspection.

NTSB Probable Cause

failure of the collective yoke bearing, which resulted in an uncontrolled descent and collision with trees. A factor relating to the accident was: the manufacturer's inadequate inspection interval of the yoke bearing.