Amateur Built Aircraft Canadian Safari, VH-VDB

Damage to the main rotor system was consistent with both main
rotor blades having failed in upward bending overload, in excess of
design limits, and the main rotor diverging from its normal plane
of rotation and contacting the tail boom and canopy.

Examination of the helicopter and its transmission and rotor
systems found no evidence to indicate any pre-existing defect that
could have contributed to the in-flight break-up. Witnesses
reported hearing the engine running before and after the break-up
occurred.

The pilot was seen waving just moments before the helicopter
broke up. The investigation was unable to determine if flight
control input by the pilot or passenger, or lack of corrective
control, had contributed to the development of the accident.
Although either low rotor RPM or abrupt manoeuvring can result in
air loads on the blades exceeding their design limit, the reason
for the excessive upward bending of the blades could not be
determined.

The NTSB special investigation report NTSB/SIR-96/03 -
Robinson Helicopter Company - loss of main rotor control
accidents, which analysed accident data from 31 fatal
accidents, concluded that in the absence of any evidence of defects
or component failures, other possible factors such as the
sensitivity and responsiveness of the helicopter's flight controls
combined with limited pilot skills, proficiency, or alertness, be
considered. Although that report concerned a different helicopter
type from the Safari, its conclusions were directed to all
lightweight helicopters with sensitive and responsive controls,
characteristics shared by both types.

The installation of a governor and an aural low rotor RPM
warning, as noted in the NTSB special investigation report
NTSB/SIR-96/03 - Robinson Helicopter Company - loss of main
rotor control accidents, had contributed to the greatly
reduced incidence of low rotor RPM related accidents in that
helicopter type.

Sequence of events

The pilot and a passenger of a kit-built Canadian Home Rotors
Safari (Safari) helicopter, registered VH-VDB, operated as an
experimental aircraft¹, were making a private flight from
the pilot's property to a nearby airstrip.

At about 1415 Central Summer Time, witnesses reported seeing the
helicopter flying in a south-westerly directio<p>The
Australian Transport Safety Bureau did not conduct an on-scene
investigation of this occurrence. The report presented below was
prepared principally from information supplied to the Bureau.

One witness, who knew the pilot, reported that he saw the
helicopter commence a gentle left turn and the pilot
waving² to him
moments before it broke up in flight. That witness also reported
seeing the cockpit bubble shatter and a cloud of white dust
appearing from the area of the cabin. Witnesses reported having
heard the engine operating immediately prior to and following the
in-flight break-up. Other witnesses heard a loud metallic sound,
and described the helicopter almost stopping, pitching nose-up and
a main rotor blade folding, before pitching nose-down and
descending steeply. The helicopter collided with a large tree and a
shed before impacting the ground at the rear of a residential house
block. There was no fire. Both occupants were fatally injured.

Wreckage information

The main wreckage consisted of the forward part of the
helicopter, main rotor and transmission assembly and the forward
section of the tailboom. The tail rotor, its gearbox and the rear
part of the tail boom had lodged in the roof of the shed beside the
main wreckage. The rear section of the damaged tail rotor drive
shaft was lodged in a tree.

Debris from the helicopter was distributed over a distance of
approximately 100 m. Components from the tailboom, tail rotor drive
system, fragments of acrylic windshield and items from the cockpit
were found along the wreckage trail. A damaged section of the
windshield exhibited rotational scuff marks consistent with a main
rotor blade strike.

Evidence at the accident site was consistent with the main rotor
blades being stationary at impact. Paint transfer and deformation
of the tailboom tubing and corresponding damage to the leading
edges of the main rotor blades, were consistent with the tailboom
being severed by the main rotor blades at the second rearmost bay.
A ballast weightwas later found approximately 150 m northwest of
the main wreckage site.

Examination of the engine did not reveal any sign of mechanical
failure. The carburettor, magneto and ignition harnesses were
damaged in the impact sequence and could not be tested.
Environmental conditions at the time of the occurrence, when
plotted on the Carburettor Icing Probability Chart, were in the
moderate zone for cruise power settings.

Pilot information

The pilot held a private pilot licence (helicopter) endorsed
with the Robinson R22 helicopter type. Records held by CASA showed
that the pilot's class 2 medical certificate had expired on 31
January 2002. The pilot had flown an estimated total of 308 hours
of which 241 hours were on the Safari helicopter. Post mortem and
toxicology reports did not find any condition that would have
affected the pilot's performance.

Examination of components

The flight control system was examined for integrity. Although
many of the control tubes were bent and fractured they were all
correctly attached. A friction device was fitted to the pilot's
collective control lever. All fracture surfaces, when examined,
exhibited failure in overload, consistent with impact forces. One
of the high-density plastic 'droop stops', found separate from the
main wreckage, exhibited damage that indicated a significant
in-flight main rotor grip impact.

The main rotor transmission, main rotor mast and main rotor
assembly, including the main rotor blades, were removed for
examination at the ATSB's technical laboratory. None of those
components exhibited any identifiable pre-existing defect. However,
during examination it was noted that neither of the pitch control
horns had the indexing ball bearings installed. A detent, machined
into each blade grip, allowed a ball bearing to align with a slot
machined into the clamping surface of the pitch control horn. That
was to facilitate accurate alignment of the flight controls during
rigging. Although the reason for the missing pitch control horn
indexing balls was not determined, the kit supplier advised that
their use was not essential.

Both main rotor blades exhibited extensive permanent upward
bending. The blades were constructed of an extruded aluminium
leading edge spar to which was bonded an upper and lower carbon
fibre skin. The area between the upper and lower skin behind the
spar was unfilled, except for the trailing edges which were bonded
with an expanded foam adhesive. The deformation of both blades was
similar, although the leading edge extrusion of one blade had
fractured at two locations. Examination indicated that bending was
consistent with both blades having been subjected to excessive air
loads while the rotor system was powered. The examination concluded
that when bending of the blades exceeded normal design limits, the
rigid, less ductile carbon fibre skins, separated. The upper skins
separated almost instantaneously, releasing the cloud of fine
dust-like particles from the disruption of the foam bonding
material.

Subsequent examination of the blades by an independent composite
structure specialist found that the carbon fibre surfaces had
separated from the spar due to shearing forces produced by
excessive upward bending. The blades did not exhibit any
pre-existing manufacturing defect or delamination failure in the
composite material of the blades.

Centre of Gravity

The Centre of Gravity (C of G) of the helicopter was calculated
to be within normal operating limits.

Experimental aircraft

Civil Aviation Regulation CAR (1998) Part 21.191 (g) permitted
construction and operation of the kit built Safari helicopter as an
experimental aircraft solely for the education and recreation of
the constructor. As the constructor of the major portion of the
helicopter, the pilot was considered to be the manufacturer and was
responsible for the acceptance and use of all the helicopter's
components. That permitted the use and modification of components
without the need for lengthy and expensive development normally
associated with certification.

The helicopter kit supplier did not approve any modification or
alteration to the Safari helicopter or its components, other than
those made by the factory. The kit supplier reported that following
any factory modification to the helicopter, an extensive program of
hover and flight testing was undertaken before acceptance. They
also recommended that when constructors made modifications or
substituted components, a similar testing program to that used by
the manufacturer should be adopted.

The pilot was reported to have modified and replaced some
components that had been supplied as part of the kit. Although the
kit supplier reported that many of the changes had been made in
consultation with them, the absence of a complete set of component
modification records for the helicopter did not allow the
investigation to authenticate information about those modifications
or the testing of them.

Experimental aircraft were not required to have a Flight Manual,
but it was expected that sufficient information for the safe
operation of the aircraft was available to the pilot. A copy of a
Safari flight manual was with the pilot's documentation provided to
the investigation team. The manual was not identified as being
specific to VH-VDB and the accuracy of information contained in it
could not be validated. The provision and accuracy of information
relating to the operation of the helicopter was the responsibility
of the constructor.

Main rotor speed

Main rotor speed was one of the critical factors that determined
the amount of lift generated by the main rotor of a helicopter.
Engine power provided the force to overcome the drag of the rotor
blades and attain a specific main rotor RPM. The pilot controlled
engine power via a twist grip throttle control on the collective
control lever. Although a mechanical linkage, described as a
correlator, automatically adjusted engine power to approximate the
power requirement for the collective control position, the pilot
was required to make minor adjustments with the twist grip
throttle. Despite the installation of a collective lever friction
lock, the helicopter was not fitted with a governor that
automatically maintained engine RPM and therefore main rotor
RPM.

The rotor speed limits in the pilot's copy of the Flight Manual
for the Safari were a maximum of 520 RPM and minimums of 475 RPM
(power off) and 450 RPM (power on). The Flight Manual cautioned
pilots that catastrophic rotor stall occurred if rotor RPM dropped
below 390 RPM. Low rotor speed could result in excessive rotor
'coning' and if uncorrected, would ultimately lead to blade failure
from excessive upward bending. In the event of a low rotor RPM, the
kit-supplier reported that 'A strong engine will allow recovery.
Lowering collective will lessen the time required. Using less right
pedal will also help.'

The pilot was reported to have developed a main rotor RPM
warning system, after having experienced difficulty monitoring the
kit-supplied dual-pointer, combined rotor and engine RPM gauge. The
system was described as providing both aural and visual warnings.
Visual warnings were provided by an indicator, installed on the
centrally mounted instrument panel. A red light at the top of the
display indicated rotor overspeed and an amber light positioned at
the bottom of the display indicated rotor underspeed. Four green
lights arrayed across the centre of the instrument illuminated
sequentially to provide a rotor RPM deviation trend. A single tone
was generated to indicate both overspeed and underspeed rotor
conditions. The warning indicator was too badly damaged to
determine its operating status. The damaged sender unit from the
warning indicator was tested and found to provide a low rotor speed
signal at 459 RPM and an overspeed rotor signal at 494 RPM.

A Flight Manual limitation stated 'Use maximum power-on RPM [520
RPM] during take-off, climb or level flight below 500 ft AGL or
above 5,000 ft density altitude'. That had the potential of a main
rotor overspeed warning remaining continuously activated while a
pilot adhered to the recommended rotor RPM during the
above-mentioned phases of normal flight.

Maintenance

As the constructor of the helicopter, the pilot was approved to
carry out his own maintenance. The kit supplier reported that the
pilot had received maintenance training on the Safari helicopter
from the kit supplier's test pilot when he attended the
constructor's property to conduct the initial flight tests. The
pilot was reported to have performed his own maintenance, but no
maintenance documentation, including the current maintenance
release, was found. The kit supplier reported that the pilot seldom
asked about the Safari maintenance but would contact them for
information when needed. A Rigging and Balance Manual for the
Safari was available to owners. That manual contained information
about the flight control rigging and balancing and tracking
procedures for the main and tail rotors.

The main rotor blades were manufactured in the US by a separate
manufacturer and sold to the pilot by the Canadian kit supplier.
The pilot's family reported that the main rotor blades were
obtained as part of the helicopter kit. When unpacked, one of the
blades was found to have a 2 mm depression on the upper and lower
surfaces at the mid-chord position along most of the blade length.
The pilot had contacted the kit supplier about the problem and the
blade manufacturer subsequently advised that the increase in air
pressure, between the pilot's near sea-level property and the
significantly higher elevation of their US facility, had resulted
in the slight depression. The blade manufacturer advised the kit
supplier that the pilot should drill a small hole in the outer end
of the blade to allow the pressure to equalize, and then fill the
hole. The pilot was reported to have performed the repair according
to the instructions. Although the kit supplier had offered to
replace the blades with a matched pair, the pilot subsequently used
the repaired blade.

The pilot had recently replaced the pitch control bearings of
each blade due to excessive wear. That required the removal and
refitting of the main rotor blades. As noted in the Examination of
Components, the indexing ball bearings had not been installed. The
investigation was unable to determine if any misalignment between
the pitch control horns had existed before the in-flight break-up
occurred.

The pilot had reported to other constructors that he had
experienced vibrations associated with the main rotor with either a
near full fuel load, or with low fuel quantities. During the
previous three months, four of the flight entries in the pilot's
notebook were annotated as 'track and balance'. The pilot was
reported to have also weighed and balanced the blades in an attempt
to eliminate a possible source of vibration. A family member
reported that the pilot was attempting to eliminate all the
vibrations, not because of any risk to flight safety, but in order
to satisfy a personal ideal. That family member reported that
during a flight in the helicopter earlier that week it had flown
normally. A damaged hand-held vibration analyser unit was found in
the wreckage. When examined, the unit contained no stored data. No
relevant record of track and balance data was found. It could not
be determined if there had been any tracking and balance testing
during the accident flight.

NTSB Accident study of light weight
helicopters

In 1996 the United States of America (US) National
Transportation Safety Board (NTSB) released special investigation
report NTSB/SIR-96/03 - Robinson Helicopter Company - loss of
main rotor control accidents. The report reviewed 31
occurrence reports where loss of main rotor control was identified
but where no evidence was found of the specific event that caused
or allowed the main rotor blades to diverge from their normal plane
of rotation and strike the airframe. The report stated that the
sensitivity and responsiveness of the helicopter's flight controls
when combined with limited pilot skills, proficiency, or alertness
could have been a factor in some of the 31 accidents the NTSB
reviewed. Although the NTSB report was solely concerned with the
R22 helicopter type, it noted the need for continued research to
study flight control systems and main rotor blade dynamics in light
weight helicopters with highly responsive controls.

The report also noted that in March 1995, the US Federal
Aviation Administration (FAA) technical panel recommended that the
R22 helicopter type be reconfigured with an electronic engine RPM
governor. The report further recommended that in normal flight
operations, switching the governor off should be prohibited. The
manufacturer adopted those recommendations and fitted governors to
their fleet. The NTSB report cited that those measures were some of
the many that had subsequently resulted in a reduction to the R22
accident rate.

¹
Civil Aviation Safety Authority (CASA) Advisory Circular
21.1(1) Aircraft Airworthiness Certification - Categories and
Designations Explained stated that:

Experimental aircraft by their very nature are not
type-certificated. Experimental is not a category per se, rather it
is a designation. It is also important to note that an experimental
certificate does not attest to an aircraft being fully
airworthy, despite being grouped under the special Certificate
of Airworthiness (CoA). The Experimental certificate system
replaces some elements of airworthiness control previously covered
by Civil Aviation Regulation 134 Permit to Fly, and Civil Aviation
Order 101.31 developmental category CoA.

² The
helicopter was flown from the left seat. When seated on the left
side of the helicopter, the pilot's left hand operated the
collective control and twist-grip throttle. The cyclic control was
operated with the other hand and unlike the collective control,
generally required constant input.

³ The
Safari helicopter had a relocatable 6.3 Kg ballast weight
consisting of a lead filled stainless steel tube. The ballast
weight was positioned according to the helicopter loading, to
maintain the Centre of Gravity within permissible limits. Mounts
were located on the forward section of the right landing skid and
on the tail boom tubing in the second bay from the rear. When not
required for helicopter trim, the ballast weight could be carried
in the cabin.

⁴ Bureau
of Air Safety Investigation, Asia Pacific Air Safety Journal
No. 22, June 1999.

⁵ Coning
is the upward bending of the blades caused by the resultant forces
of lift and centrifugal force. At rest, the main rotor blades have
a preset Coning Angle. As rotor RPM increases, the coning angle
reduces due to the centrifugal force exerted on the blades. When
weight is transferred to the blades, the lift produced by the
blades increases, increasing the coning angle. Within the range of
normal operating RPM, centrifugal force acting on the blades
prevents excessive coning.

⁶ Civil
Aviation Regulation 42ZC(6) of CAR 1988 permitted authorised
persons to conduct maintenance on the aircraft and engine. CASA
Instrument 545/00, effective from 1 January 2001, gave that
permission to constructors of kit built aircraft who fabricated and
assembled the major portion of that aircraft and if the aircraft
was used solely for that person's own education or
recreation.