1 FACTUAL INFORMATION
1.1 Sequence of events1
At about 1100 Central Standard Time on 30 May 2004, a Robinson
R22 Mariner II helicopter, registered VH-MIB, crashed and caught
fire while being operated on a fence inspection flight at
Tobermorey Station, NT. The pilot sustained fatal injuries and the
passenger was seriously injured.
The passenger reported that the purpose of the flight was to
inspect a fence line bordering the property and then to conduct
cattle mustering operations. The helicopter was refuelled to full
tanks prior to departure. The weather was fine and sunny, with a
slight breeze from the south-east.
The passenger reported that while overflying a section of the fence
line about 45 km south of the homestead at 30 to 40 ft above ground
level, the pilot initiated a turn back to the left to enable a
closer look at a particular section of fence. Part way through the
turn there was a loud bang from behind the cabin, followed by
"horrendous vibration" and the helicopter immediately began to yaw
left and descend. The ground marks showed that the tail rotor
blades contacted the ground first and then the forward section of
the helicopter's right skid struck the ground, disrupting the front
section of the cabin. The helicopter then came to rest on its right
side and fire rapidly spread to engulf the cabin area.
The pilot was able to free himself from the helicopter through
the broken front section of the cabin, but the passenger
experienced difficulty undoing his safety harness and remained
trapped. He was eventually able to free himself and joined the
pilot at some distance from the wreckage. The passenger reported
that he assessed the pilot to be badly injured and directed him to
a nearby water hole. The passenger then walked to a water bore
approximately 8 km from the accident site, where he met other
station personnel. The pilot was deceased when medical assistance
arrived at the accident site some hours later.
The passenger reported that he had flown in the helicopter
several times. During the last few flights before the accident, and
during the accident flight itself, he had detected a vibration that
he considered abnormal. The passenger advised the investigation
that he had conveyed these concerns to the pilot, who advised that
he conducted a good check of the helicopter and was satisfied that
there were no problems.
1.2 Wreckage examination
Examination of the wreckage by the Australian Transport Safety
Bureau (ATSB) investigation team at the accident site (Figure 1)
confirmed that the helicopter was yawing left and moving forward
when it struck the ground. The tail rotor blades contacted the
ground first. The helicopter then contacted the ground slightly
nose down and heavily onto the right skid, causing it to separate
from the helicopter. The ground impact marks showed that the
helicopter continued to yaw left though about 180 degrees after it
struck the ground, before coming to rest on its right side. The
majority of the cabin and engine bay, including the entire floor
area and cabin structure beneath the pilot and passenger seat
positions were destroyed by fire.
Figure 1: Helicopter wreckage (arrow indicates bulkhead
deformation behind pilot seat position)
The extent of fire damage meant that a complete examination of
the helicopter was not possible. Some aluminium components such as
tubing in the flight control system had been destroyed. However,
all steel components in the control systems for the main and tail
rotor were identified and damage to all of these components was
consistent with impact forces or fire. The main rotor blades
exhibited damage consistent with low rotor energy at impact. Both
tail rotor blades had fractured approximately 1/4 span outboard of
the rotor hub centre drive (Figure 2). The failed section of one
blade was found adjacent to the main wreckage. The failed section
of the other blade was found subsequently about 70 m from the main
wreckage. Both blade sections were taken to the ATSB laboratories
for further examination.
Figure 2: Tail rotor damage
All of the engine drive system components were identified within
the wreckage. The flex plate for the forward flexible coupling of
the main rotor drive system was fractured at one of the two
attachment points to the main rotor gearbox yoke (Figure 3 and 4).
The flex plate, including the clutch shaft, were retrieved from the
accident site, for further examination. The flex plate for the
intermediate flexible coupling was intact and showed evidence of
rotational damage consistent with partial drive system power at
impact. The rear flex plate and coupling components were also found
intact.
Figure 3: Forward flexible coupling as found in
wreckage
Figure 4: Components of failed forward flexible
coupling
The fuel system and engine ancillaries were destroyed by the
fire. There were vertical cuts puncturing the inside wall of the
right fuel tank and the horizontal stainless steel firewall above
the engine. The cuts in the right tank and the firewall aligned
with the forward flex plate plane of rotation (Figure 5). The left
fuel tank was destroyed by fire.
Figure 5: Flex plate puncture of stainless steel
firewall above engine.
1.3 Personnel information
The pilot held a commercial pilot (helicopter) licence and was
appropriately endorsed on the R22. He was issued with a private
pilot (aeroplane) licence in 1974 and a commercial pilot
(helicopter) licence in 1990. The pilot was issued with a
commercial pilot (aeroplane) licence in 1995. He held a stock
mustering rating and a valid class 1 medical certificate. He
completed a flight review in the occurrence helicopter on 19 April
2004. At the time of the accident, the pilot had approximately
10,400 hours aeronautical experience. He flew 31 hours in the
occurrence helicopter between 17 and 30 May 2004.
1.4 Medical and pathological information
Post mortem and pathology reports did not indicate that the
pilot was suffering from any condition that might have affected his
performance during the flight. The most significant injuries
sustained by the pilot were the result of impact forces rather than
fire.
1.5 Fire
There was a fire affected area (sooting) on the ground that
extended up-slope from the wreckage (Figure 6). The sooting formed
a swirl pattern of decreasing radius in the direction the
helicopter was yawing when it contacted the ground.
Figure 6: Sooting pattern adjacent to wreckage
(arrow indicates approximate direction of flight at
impact)
1.6 Survival aspects
Three-point lap/sash type safety harnesses were fitted to both
seating positions in the helicopter. The passenger reported that
both he and the pilot had their harnesses fastened during the
flight. Fire damage precluded a detailed assessment of the seats
and performance of the crush zones beneath them as well as the seat
belt harnesses.
Severe crush damage to the lower cabin bulkhead was evident
immediately behind the pilot's seat. (Figure 1).
Following the accident, no Emergency Locator Transmitter signal
was received (refer section 1.7.6). There was no mobile telephone
coverage in the area and the passenger reported that they did not
carry any other communications aids, such as a portable satellite
telephone.
1.7 Helicopter information
1.7.1 Helicopter data
The helicopter was manufactured in August 2002 as Serial No
3357M. The most recent maintenance release for the helicopter could
not be located. It was reported to have been kept in the
helicopter. If so, the maintenance release would have been
destroyed in the post-impact fire. Based on other maintenance
records and information contained in the pilot's personal diary,
the total time in service of the helicopter on 30 May 2004 was
estimated to have been 506 hours.
1.7.1 Main and tail rotor drive system
In the R22 helicopter, power to drive the main and tail rotors
was transmitted from the engine to the rotor drive train via a
multiple Vee belt drive and clutch system. A shaft transmitted
power forward from the clutch to the main rotor gearbox and aft to
the tail rotor gearbox (Figure 7).
Figure 7: Main components of main and tail rotor
drive systems2
A forward flexible coupling, which includes a flex plate,
connected the drive shaft to the main rotor gearbox. The tail
rotor drive system also included an intermediate and a rear
flexible coupling. Yoke assemblies at the end of each drive shaft
section connected the shaft to the flex plate via bolted joints
(Figure 8). The purpose of these flex plates was to accommodate
small differences in shaft axial alignment during drive shaft
rotation. The flex plates and the bolted joints were critical
elements in drive system integrity.
Figure 8: Components of the forward flexible
coupling
The helicopter manufacturer published procedures for assembling
and aligning the drive system components, including the allowed
tolerances. The design loads of components could be exceeded if
those tolerances were not met.
1.7.3 Helicopter manufacture
The helicopter arrived in Australia partly disassembled for ease
of shipment. The main drive system components within the engine
compartment were assembled during manufacture and not subsequently
disturbed for this method of international shipment.
In order to assist the investigation, the manufacturer supplied
the investigation with the itemised build records, which included
digital photographs of the rotor drive system, for the occurrence
helicopter. Those records were reviewed as part of the laboratory
analysis of the flexible coupling failure. The review of the build
records and photographs for the forward flexible coupling in the
occurrence helicopter showed that NAS6605-6 bolts had been used,
and a spacer washer had been included in each bolted joint and one
thin washer had been installed under the nut, with a palnut
(locking nut) fitted to each. The build records showed that the
build up of the bolted joint at the time of manufacture of the
helicopter was correct and in accordance with the assembly
procedures.
The 'Daily or Preflight Checks' section 4-2 and 4-3 of the
manufacturers Pilot's Operating Handbook identified the requirement
for a visual check of the flex coupling to ensure there are 'No
cracks and 'Nuts tight'. Also required is a check of the yoke
flanges for cracking. It further advised, in part, that 'During the
following inspection, check the general condition of the aircraft
and also look for any evidence of leakage, discolouration due to
heat, dents, chafing, galling, nicks, corrosion and especially for
cracks. Also check for fretting at seams where parts are joined
together. Fretting of aluminium parts produce a fine black powder,
while steel produces a reddish brown or black residue'.
1.7.4 Maintenance history
Maintenance records indicated that an Australian certificate of
airworthiness for the helicopter was issued on 11 October 2002,
after assembly in Australia, following manufacture and acceptance
flights in the US. At that time, the total time in service
was 5.1 hours. A summary of subsequent maintenance conducted on the
helicopter is as follows. All references to drive system
adjustments and/or maintenance have been included.
-
11 March 2003. Total time in service 55.1
hours. 50 hourly engine inspection -
27 March 2003. Total time in service 98.5
hours. 100 hourly inspection. Maintenance carried out
included adjustment of the engine sheave alignment.3 -
30 June 2003. Total time in service 198.2
hours. 100 hourly inspection. Maintenance carried out
included checking and adjustment of the engine sheave alignment and
intermediate flex plate shimming to within limits. -
15 September 2003. Total time in service
296 hours. 100 hourly inspection. -
17 February 2004. Total time in service 384.1
hours. Civil Aviation Safety Authority Airworthiness
Directive (AD) R22/51 'Main Rotor Clutch Shaft', dated 12 November
2003, was incorporated. AD/R22/51 was applicable to all R22
helicopters. It required disassembly of the main rotor yoke (A907)
to the clutch shaft joint (A166) (see Figure 6) and inspection of
the shaft and yoke for damage including fretting4 of bolt holes, cracking in the area of the
bolt holes, and the presence of an unapproved jointing compound in
the mating surfaces. The helicopter maintenance worksheet indicated
that no fretting was evident but that the incorrect jointing
compound had been used. The worksheet stated that the AD had been
complied with and that the clutch shaft and yoke were reassembled
in accordance with the maintenance manual. The worksheet also
recorded that a duplicate inspection of the clutch shaft
installation and the yoke (A907) assembly had been performed. The
licensed aircraft maintenance engineer who carried out the AD
reported that he disconnected the yoke (A907) from the forward flex
plate, but did not disconnect the flex plate from the main rotor
gear box yoke (A908). He stated that he did not perform any
maintenance on the bolted joints at the connection between the main
rotor gear box yoke and the flexible coupling. -
27 March 2004. Total time in service 396
hours. 100 hourly inspections. Maintenance carried out
included engine sheave alignment. -
12 May 2004. Total time in service 476.1
hours. 100 hourly inspections.
The documentation showed that maintenance had been performed on
27 March 2003, 30 June 2003, and 17 February 2004 in the vicinity
of the forward flexible coupling that, while it did not
specifically necessitate bolt removal, provide opportunities for
the forward flexible coupling bolts to be disturbed.
1.7.5 Forward flex plate bolted joint component
specification
A review of the diagrams contained in the manufacturer's
Maintenance Manual and the Illustrated Parts Catalogue (IPC)
revealed a difference in the specifications of the parts in the
bolted joints. Notes contained within the IPC explained that bolts
of different grip lengths and washers of different thickness were
to be used in the flex plate bolted joints to expose between two
and four threads beyond the end of the nut. A table comparing the
different specifications between the Maintenance Manual table and
the IPC is provided in section 4.1.3 of the ATSB technical analysis
report attached as Appendix 1.
The manufacturer advised that the bolt length identified in the
Maintenance Manual was for use in an earlier version of the manual
and was out of date. Corrective action to update this information
was scheduled by the company for November 2005, but at the time of
writing of this report had not been accomplished.
The manufacturer advised that the company did not publish any
warning to maintenance organizations about the discrepancy in bolt
length between the Maintenance Manual and the Illustrated Parts
Catalogue. The discrepancy was not considered by the
manufacturer to be critical in that the use of either a NAS6605-5
or a NAS6605-6 bolt with the appropriate combination of spacer and
washers would give the correct clamp up for proper joint integrity.
In the few cases where the -5 bolt did not allow proper
installation of the B330-16 palnut, the problem would be self
evident. The manufacturer believed that any engineer performing the
installation where the bolt was too short to install a palnut would
install a longer bolt or make inquiries to resolve the problem.
1.7.6 Emergency locator transmitter
The maintenance records indicate that the helicopter was
imported from the US and subsequently operated by various owners
without a fixed Emergency Locator Transmitter (ELT) unit being
fitted. This fact was noted on the maintenance releases issued at
5.1 airframe hours total time in service (TTIS) on 11 October 2002
and 98.5 hours TTIS on 27 May 2003, which required the pilot to
observe the requirements of CAR 252 and carry a personal ELT. No
further entries of this nature were found on maintenance releases
issued after this date, nor could evidence be found in the aircraft
logbooks that an ELT had been fitted. A search for both a fixed and
personal ELT within the wreckage and surrounding accident site was
conducted but nothing was found.
The passenger stated that the pilot normally carried a personal ELT
on him. No personal ELT was identified among the pilots clothing or
personal effects and no emergency signal was received by AusSAR
from that location on the day.
1.8 Specialist examination of the failed components
The forward flex plate and the broken sections of the tail rotor
blades were subject to detailed examination by the ATSB. The report
on those examinations and analysis of the failures is attached as
Appendix 1.
The metallurgical evidence confirmed that the failure mode of
both tail rotor blades was very similar and was the result of
contact with the ground during the impact sequence. The rocky
material embedded in the blade tips provided clear evidence that
the blades had struck the ground while rotating. The blade that was
found about 70 m from the wreckage was thrown that distance as the
result of tail rotor rotational energy.
The specialist examination found that the flex plate in the
forward flexible coupling fractured as a result of the propagation
of a fatigue crack at one of the bolted connections between the
plate and main rotor gearbox yoke. Final fracture of the flex plate
occurred during operation and not as a result of the collision with
the ground. No crack growth or wear damage was observed at the
three remaining boltholes. Examination of the bolt installed at the
failure location revealed that extensive fretting wear had occurred
around the entire circumference of the bolt, in the region adjacent
to the flex plate and the regions adjacent to the reinforcing
plates. Fretting wear was also evident on the washer surface
adjacent to the bolt head.
The bolted joint at the flex plate failure location was found to
have a single thin washer under the bolt head and nut, and no
spacer washer between the yoke and flex plate. This spacer and
washer combination was different from that specified by the
manufacturer for use with a NAS6605-6 bolt.
- Only those investigation areas identified by
the headings and subheadings were considered to be relevant to the
circumstances of the accident. - Diagram with permission of Robinson Helicopter
Company. - Drive Vee belts sometimes stretch when new and
adjustments are then necessary to maintain the correct drive system
alignment. Engine sheave alignment is part of that adjustment
process. - The AD defined major fretting as 'any evidence
of the machining marks in any of the bolt holes being partly or
fully obliterated'.