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.
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.
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.
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.
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.
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.
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.
The kit supplier advised that several constructors had adapted commercially available governor units and fitted them to Safari helicopters. The kit supplier reported that a governor system was in development for future use in their Safari helicopters.
The kit supplier also advised that an improved rotor speed tachometer, with an expanded operating band for better interpretation, was being developed. A warning system that included both audio and visual alarms was to be incorporated into the tachometer. The RPM limits marked on the tachometer, and the alarm settings, were adjusted to indicate a low rotor RPM between 400 and 460 RPM and an overspeed condition above 502 RPM. Normal operation was between 461 and 501 RPM. When testing has been completed, the tachometer and warning system will be made available to Safari owners and to kit constructors.
As a result of this investigation the Australian Transport Safety Bureau issues the following Safety Advisory Notice.
Safety Advisory Notice SAN20040076
The Australian Transport Safety Bureau advises constructors of Canadian Safari and other lightweight helicopters to review the safety benefits of installing an engine speed governor.
Safety Advisory Notice SAN20040077
The Australian Transport Safety Bureau advises constructors of Canadian Safari and other lightweight helicopters to review the safety benefits of providing discernibly different aural warning tones to differentiate between main rotor RPM overspeed and underspeed conditions.
|Date:||29 March 2003||Investigation status:||Completed|
|Time:||1415 hours CSuT|
|Location:||4 km SW McLaren Vale|
|Release date:||03 November 2004|
|Report status:||Final||Occurrence category:||Accident|
|Highest injury level:||Fatal|
|Aircraft manufacturer||Amateur Built Aircraft|
|Aircraft model||Canadian Safari Helicopter|
|Type of operation||Private|
|Damage to aircraft||Destroyed|
|Departure point||6 km N McLaren Vale, SA|
|Departure time||1410 hours CSuT|
|Destination||Aldinga, (ALA) SA|
|Role||Class of licence||Hours on type||Hours total|