Kawasaki Heavy Industries 47G3B-KH4 , VH-KEB

The Bureau of Air Safety Investigation (BASI) released
occurrence brief 9501019 on 2 February 1996. On 27 April 1999,
following receipt of new information from the helicopter
manufacturer concerning the identity of the part number of the fan
fitted to KEB, a revised occurrence brief was issued by BASI.

The owner of KEB drew attention to a number of potential safety
deficiencies in the maintenance and inspection of Kawasaki
47G3B-KH4 helicopter engine-cooling fans.

In response to the issues raised, and in recognition of the
hazard to flight safety created by engine cooling-fan blade
fracture, the Australian Transport Safety Bureau (formed on 1 July
1999, and which integrated the Bureau of Air Safety Investigation
in a multi-modal agency) then undertook an analysis of the failure
of Kawasaki 47G3B-KH4 engine cooling-fans. This analysis did not
consider operational or piloting aspects, but was confined to the
analysis of the cooling fan. As a result of technical analysis of
this and other engine-cooling fan blade failures, the following
recommendations are made:

R19990195

The Australian Transport Safety Bureau recommends that the Civil
Aviation Safety Authority alert all aircraft maintenance engineers,
especially those conducting non-destructive inspections, to the
detrimental effects that may be created in critical components by
the abrasion of surfaces during general cleaning or cleaning in
preparation for nondestructive inspection.

R19990196

The Australian Transport Safety Bureau recommends that the Civil
Aviation Safety Authority alert helicopter maintenance
organisations and all aircraft maintenance engineers to the
potentially detrimental effects of painting engine cooling
fans.

R19990197

The Australian Transport Safety Bureau recommends that Kawasaki
Heavy Industries clarify the bolt-tightening instructions contained
in Service Bulletin 305 to establish if an allowance should be made
for the frictional torque effects of self-locking nuts.

The loud cracking noise and the restriction in the cyclic
control reported by the pilot may have resulted from the fatigue
failure of the engine-cooling fan. The investigation was unable to
determine if the damage to the control rod would have been
sufficient to adversely affect the flight controls.

Although it is likely that the specified torque was previously
applied to the fan retaining bolts, the required clamping force may
not have been achieved due to frictional torque effects and the
painted surface of the fan.

The difference in the surface finish from that specified by the
manufacturer would be expected to have a detrimental effect on the
resistance of the fan to fatigue failure. Differences in assembly
practices from those specified by the manufacturer may alter the
magnitude of the alternating stresses created in the fan. A
determination of which difference was the most significant, or
whether failure resulted from the combination of the effects of
several differences, would require further testing and analysis by
the manufacturer.

Revised Occurrence Brief, 8 December 1999

The pilot of Kawasaki 47G3B-KH4 helicopter, registered VH-KEB,
reported that following a normal take-off, he heard a loud cracking
sound and felt some restriction through the cyclic control. The
pilot placed the helicopter in an autorotation descent from about
50 ft above a tidal estuary and landed in about 2 ft of water.
Following contact with the water, the helicopter nosed over and
rolled through 360 degrees, coming to rest upright. The pilot
exited the substantially damaged helicopter unaided.

The damage to the helicopter was consistent with impact with the
water and riverbed, except for damage to the engine-cooling fan
assembly, which was consistent with pre-impact failure. Several
blades had fractured and separated from the fan assembly. It was
also evident that the collective control rod located in front of
the cooling fan had been damaged by the impact of a released blade.
The fan was identified by the serial number A34-08516 etched onto
the inner edge of the cooling fan disc.

Analysis of the cooling-fan fractures established that a
two-bladed segment of the fan separated from the assembly as a
result of the initiation and growth of fatigue cracks. A single
blade separated as a result of the development of an excessive
stress in the region of the blade root; this fracture was
considered to be secondary to the separation of the two-bladed
segment by fatigue-crack growth.

The cooling fan, s/n A34-08516, was physically consistent with
the fan, part number 47-661-029-7, specified for this helicopter.
However, significant differences in the surface and assembly of the
fan fitted to KEB were evident when compared to the "as
manufactured" condition of engine-cooling fans and the assembly
requirements of the helicopter manufacturer. The fan disc surface
had been shot-peened. Fans manufactured by Kawasaki Heavy
Industries (Kawasaki) were not shot-peened, and the Bell Helicopter
Company did not specify shot peening as a surface treatment for
engine-cooling fans. The surface of the fan had been abraded in
areas around the boltholes. The resistance of a component to
fatigue is sensitive to surface finish. Abrasion of a shot peened
surface would be expected to reduce the resistance to fatigue.

Significant factors that affect fan assembly stiffness, and
consequently fatigue, are the establishment of the required
(designed) clamping force in each fan assembly bolt and the
retention of the required clamping force during operation. The
tightening instructions included in the Kawasaki service bulletin
KSB-BELL-305 did not indicate whether the final torque included, or
should be adjusted for, any frictional torque effects. The
frictional torque of nut self-locking features will affect clamping
forces established by the applied torque method. Clamping forces
may be reduced during operation by the deformation of materials
included in the bolted joint. In addition, the fan had been
painted, and deformation of the paint film under the heads of the
bolts had occurred. In a letter dated 18 September 1998, Kawasaki
stated that it did not recommend the painting of cooling fans part
numbers 47-661-029-2 and -7 for Kawasaki/Bell model 47G3B-KH4
helicopters. The investigation was unable to find such a reference
in the manufacturer's manuals.