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Interim Factual

Summary

History of the flight

At about 1425 Western Standard Time1 on 22 October 2005, the pilot of an Air Tractor Inc AT-602 (AT-602) aircraft, registered VH-NIT, was fatally injured when his aircraft impacted terrain, approximately 150 m south of the Ballidu airstrip, WA. Figure 1 depicts the township of Ballidu, the airstrip, the approximate flight path of the aircraft and the location of the accident site.

Figure 1: Ballidu townsite, airstrip and accident site2

figure 1

The pilot had not operated the AT-602 prior to the day of the accident. The series of flights that day were for familiarising the pilot with the aircraft's handling characteristics, and were carried out under the supervision of the company owner/chief pilot.

The chief pilot reported that the aircraft was fully fuelled before the first in the series of flights. That flight had included ground taxying operations, takeoffs, landings and general aircraft handling, including simulated spray runs along the strip and practice operating the hopper door. The accident occurred during the second flight of the day.

During those flights, the pilot returned to the operating pad several times, during which the chief pilot asked the pilot how he was progressing and, when necessary, replenished the contents of the hopper. The chief pilot reported that the aircraft was being operated with about 200 L of water in the hopper, which was the minimum quantity to prevent the hopper's seals from drying out. He recalled that he had replenished the hopper several times that day.

A number of witnesses sighted the aircraft flying over the airstrip at low altitude and saw water drop from the underside of the aircraft. The witnesses observed the aircraft gain a small amount of height before the nose of the aircraft suddenly pitched downwards and the aircraft descended steeply towards the ground.

Witnesses

The witnesses that reported either seeing or hearing events associated with different portions of the flight immediately prior to the accident included:

  • Three witnesses that were located at two separate positions, who observed water dropping from the underside of the aircraft and recalled that the nose of the aircraft pitched steeply upwards after completion of that drop, with the aircraft gaining a small amount of height.
  • The chief pilot, who sighted the aircraft climbing out as it flew past his hangar, but recalled that the nose attitude did not appear abnormally high. He did not recall hearing any unusual noises from the aircraft engine or propeller.
  • A witness who sighted the aircraft as it flew abeam her location and recalled that the nose of the aircraft was at a level attitude, before the nose 'dropped'. The sound from the aircraft appeared normal and at a constant level until the witness heard the sound of an impact.
  • A witness who was familiar with dropping operations and was watching the aircraft as it approached the airstrip. From the aircraft's position relative to the airstrip and the location of the runway thresholds, he surmised that the pilot was making a practice water drop. The aircraft disappeared from his view as it descended to what he perceived to be the drop zone before reappearing again, having made what he presumed to be either a water drop or a dummy water drop. The witness recalled that the aircraft was climbing steeply, but that the nose attitude of the aircraft was close to horizontal, before pitching steeply down towards the ground. The witness recalled hearing an unusual noise from the aircraft's engine or propeller as the aircraft started to descend, which he described as being similar to the noise when a pilot reversed the propeller's pitch on landing, or made a significant change to the power setting of the engine.
  • A witness who was adjacent the airstrip and thought that the aircraft's engine sounded as if it was not producing much power as the aircraft started to climb away, when compared to water drops that he witnessed earlier that day.
  • Two residents close to the site of the accident who heard the noise of an aircraft approach and then an increase in engine noise, similar to when a pilot applies power to enter a climb or the noise that the aircraft makes after it lands. A few seconds of silence followed, and the residents recalled thinking that the aircraft must have landed. They then heard the sound of an impact and immediately realised that the aircraft had crashed.

All of the eyewitnesses were consistent in their recollection of the aircraft's final descent towards the ground, recalling that the nose of the aircraft suddenly and violently pitched downwards, and the aircraft descended steeply towards the ground.

Accident site and wreckage examination

The sandy scrub-type terrain in the vicinity of the accident site was flat, with gently upwards sloping terrain on approach to the site. Bushes and small trees, approximately 3 to 5 m high, covered the immediate area and a power line, supported by power poles about 10 m high, ran east to west along a road adjacent the airfield boundary.

Examination of the wreckage and analysis of impact loads through the structure of the aircraft indicated a slightly right wing low, almost nose-level (zero) pitch attitude on impact with terrain. The characteristics of the impact were consistent with a high rate of vertical descent and low forward speed. Damage to the vegetation in the vicinity of the accident site indicated a steep final flight path towards the ground, with the main wreckage located about 23 m along the wreckage trail from the initial point of impact. Figure 2 shows the initial impact point relative to the main aircraft wreckage.

Figure 2: Terrain impact point and aircraft wreckage

figure 2

All structural components and flight controls were accounted for at the accident site. The aircraft was intact prior to impact with terrain. The main landing gear collapsed on contact with the ground and the fuel tanks ruptured during the impact sequence. There was evidence that a significant quantity of fuel had spilled from each of the tanks.

There was no evidence of bird strike or collision with other obstacles prior to the initial contact with terrain.

Weather

Witnesses at Ballidu recalled that the weather conditions around the time of the accident were generally fine with light south-westerly winds.

The Bureau of Meteorology reviewed the available weather data for the afternoon of the accident. That review indicated the presence of south-westerly surface winds and little or no cloud in the Ballidu area at the time of the accident.

The closest official meteorological recording station was at Dalwallinu, about 40 NM to the north-west. At the time of the accident, Dalwallinu recorded a south-westerly wind at 8 to 10 kts and a temperature of about 23 degrees C.The Bureau of Meteorology assessed that, under the prevailing conditions, similar weather would have been experienced at Ballidu around the time of the accident.

Aircraft

The AT-602 was manufactured as a single-seat, specialist type aircraft designed for agricultural and fire fighting (water dropping) operations. That aircraft model had been issued an airworthiness certificate by the US Federal Aviation Administration (FAA) as a Restricted Category3 aircraft, for use in special purpose operations. As the airworthiness certificate was issued by a recognised National Airworthiness Authority, that certification was accepted by the Civil Aviation Safety Authority (CASA) for operating that aircraft type on the Australian aircraft register.

The aircraft was manufactured in the US during 2000 and exported to Australia. It was placed on the Australian aircraft register in December of that year. The aircraft was exported from Australia in April 2002 and placed on the aircraft register of New Zealand. In December 2004, the aircraft was re-imported to Australia and placed on the Australian aircraft register as VH-NIT.

An aircraft Maintenance Release was issued on 22 July 2005 following the last period of scheduled maintenance. The maintenance release recorded 58.7 hours operation since that time. During that period, no defects were recorded on the maintenance release. The aircraft had accumulated approximately 1,650 hours since manufacture.

The AT-602 was equipped with an electrically operated Fowler flap system that also incorporated an aileron interconnection, which symmetrically drooped the ailerons as the flaps extended. The motion of the aileron interconnection was non-linear, in that most of the aileron droop occurred during the first 15 degrees of flap extension. Full flap extension for the AT-602 was 30 degrees, with an associated aileron droop of 10 degrees. Figure 3 illustrates the flap extension and aileron droop in an AT-602.

Figure 3: AT-602, showing extended wing flap and aileron droop

figure 3

The flaps were actuated by a 'rocker'-type switch4 on the control stick (Figure 4). That switch enabled the pilot to extend the flaps between 0 and 30 degrees. The flaps could also be activated by a control switch mounted adjacent to the engine and propeller control levers on the left side of the cockpit (Figure 5).

Damage to the wing flaps was consistent with their being in an extended position at the time of impact.

Figure 4: Control stick for a similarly-equipped AT-602, showing flap 'rocker' switch

figure 4

Figure 5: Flap control switch adjacent engine controls

figure 5

The aircraft was configured for water dropping operations and was equipped with a 630 US Gallon capacity fibreglass hopper (2,385 L). A manually-controlled fire-bomber dump door was fitted to the base of the hopper. Activation of the cockpit drop handle opened the dump door and allowed the hopper contents to drop from the aircraft.

The aircraft manufacturer's FAA-approved Airplane Flight Manual (AFM) recommended using 10 degrees of flap and an airspeed of 109 kts on approach and load release during water dropping operations.

The AFM indicated a wings-level stall speed for the AT-602 at an operating weight of 4,173 kg of 76 kts CAS5 (flaps up) and 61 kts CAS (flaps down). The AFM indicated that the maximum altitude loss from a wings-level stall was 300 ft.

At the time of the accident the aircraft's operating weight was estimated to be approximately 3,500 kg.

Engine

The aircraft was equipped with a Pratt and Whitney Canada PT6A-60AG (PT6A) turboprop engine.

The logbooks and maintenance records indicated that the engine had accumulated approximately 1,650 hours since new.

The engine was shipped to the engine manufacturer for disassembly and examination under the direct supervision of the Canadian Transportation Safety Board. The final report from that examination was not available at the time this interim report was written.

Pilot details

The pilot held a Commercial Pilot (Aeroplane) Licence and a Grade 1 Agricultural Rating, and had previously performed water dropping operations in the operator's reciprocating-engine aircraft. A review of the pilot's logbook indicated approximately 6,736 hours total aeronautical experience, including about 400 hours water dropping operations in the reciprocating-engine PZL "Dromader" D-18 aircraft.

Civil Aviation Regulation 5.22 enables CASA to prescribe the aircraft endorsements that must be held by the holder of a flight crew licence. Civil Aviation Order 40.1.0 requires the holder of a flight crew licence to hold an endorsement to operate certain types of aeroplane and provides for aircraft with similar design features to be grouped into aircraft classes.

The pilot recently completed training for, and was issued with a class endorsement on Ayres Turbo (PT6)-type aircraft. The Ayres Turbo (PT6) aeroplane class endorsement includes the Ayres S2R "Thrush" and Air Tractor AT (400, 401, 402, 502, 602 and 802) aircraft types equipped with a PT6A turbine engine.

Protective equipment/survivability

The pilot was not wearing a protective helmet.

The aircraft was fitted with a metal frame seat with a mesh-fabric seat cover and a four-point restraint harness. Vertical impact forces distorted the base of the seat frame.

Testing and examination of recovered components

The jackscrew for the flap actuator was found along the wreckage trail and had separated from the aircraft during the impact sequence. The jackscrew remained attached to the actuator gearbox, but had fractured in the vicinity of the ACME-threaded nut assembly connecting the jackscrew to the flap actuator arm. The jackscrew and ACME-threaded nut were submitted for technical examination.

Laboratory examination of those components revealed a bending overload failure as a consequence of impact forces. The extension of the jackscrew was consistent with the wing flaps being fully extended at the time of the impact with terrain.

The aircraft was equipped with a cockpit instrument that monitored the quantity of fuel consumed by the aircraft's engine. That instrument contained a non-volatile memory and so was recovered from the aircraft for subsequent examination. That examination revealed a total fuel consumption of 227 L and a quantity of 772 L remaining.

An elevator pushrod had fractured at the eye-end bearing fitting and was recovered from the accident site for analysis. Examination of that component did not reveal any evidence of a pre-existing material anomaly. The fracture surface exhibited characteristics consistent with gross structural overload and component failure during the accident sequence.

A number of cockpit instruments were recovered for laboratory analysis. Examination of the instrument face from the airspeed indicator revealed witness marks from the instrument's indicator needle. Those marks indicated that, at the time the indicator needle contacted the face of the instrument, the needle was indicating between 45 and 55 knots.

Other instruments that were recovered for subsequent laboratory examination included the: engine Ng (gas-generator speed), oil temperature and pressure gauges; and propeller RPM and engine torque gauges. No witness marks were evident on those instruments.

Indicator globes from the aircraft's annunciator panel were recovered and analysed for evidence of any filament stretch that could indicate the illumination of the lights at the time of ground impact. The following indicator globes were examined:

  • Propeller beta - illuminates when propeller blade angle is in the "beta" range6.
  • Fuel filter - illuminates when the fuel filter is partially blocked.
  • Low fuel quantity - illuminates in a low fuel condition.
  • Air filter - illuminates to indicate restricted airflow to the engine's air inlet.
  • Chip detector - indicates metal particles in contact with detector terminals and the possibility of other metal particles in the engine lubricating oil. A brittle fracture was observed in that filament.

There was no evidence in any of the examined indicator globes of filament stretch, or illumination of any of the associated indicator lights at the time of the aircraft's impact with the ground.

The aircraft was equipped with an emergency locator transmitter (ELT), which separated from its mounting bracket during the impact sequence. Although the ELT was armed, it did not automatically activate.
The ELT activated normally when tested after the accident.

Fuel sample testing

A sample of fuel was recovered from one of the ruptured fuel tanks. Testing of that sample indicated a blend of diesel and aviation turbine fuel. Particulate matter within the fuel sample was consistent with contamination of the fuel with bacterial organisms.

Propeller examination

The engine was equipped with a five-bladed Hartzell Propeller Inc., HC-B5MP-3C constant speed propeller that was fully feathering and reversible in pitch.

The propeller was recovered from the accident site and examined under the direct supervision of investigators from the Australian Transport Safety Bureau (ATSB). When reconstructed as a set, all of the propeller blades showed a progressive and marked increase in axial twist and rearward, out-of-plane bending around the sequence of rotation. That damage indicated a steep angle of impact with terrain (ie low horizontal speed, high vertical speed) and a rapid cessation of rotation.

The pitch change mechanism for the propeller blades was damaged during the accident sequence and each of the blades showed evidence of moving independently during the terrain impact sequence. Witness marks on the hub of the propeller blades indicated blade rotation beyond the assembly limit of -11 degrees.

There was no evidence to indicate that the propeller was operating in the beta or reverse blade angle range at the time of the collision with terrain.

Ongoing investigation

The investigation is continuing, including in the following areas:

  • assessment of engine operation at the time of the accident, pending the results of the examination/testing performed under the supervision of investigators from Canadian Transportation Safety Board
  • examination of survivability issues associated with the accident
  • the assessment of aircraft handling characteristics and operational factors associated with the accident flight, including the aircraft's configuration immediately prior to the loss of control.

  1. The 24-hour clock is used in this report to describe the local time of day, Western Standard Time (WST), as particular events occurred. Western Standard Time was Coordinated Universal Time (UTC) + 8 hours.
  2. Aerial photograph reproduced by permission of the Department of Land Information, Perth, Western Australia, Copyright Licence 33/2006 www.dli.wa.gov.au
  3. The Restricted Category certification was on the basis of airworthiness complying with US Federal Aviation Regulation 23, excluding those sections deemed inappropriate for the special purpose use of agricultural spraying, dusting and seeding and for the special purpose use of forest and wildlife conservation (fire fighting).
  4. That switch was installed subsequent to the manufacture of the aircraft and following the aircraft's initial importation to Australia, in accordance with Engineering Order ADG-AT602-EO2125.
  5. CAS is calibrated airspeed and is the indicated airspeed corrected for instrument and position errors. At those speeds, the AFM indicated that the calibrated airspeed is within about 1 knot of the indicated airspeed.
  6. Beta refers to operation of the propeller blade at fine blade angles, during which the propeller blade angle (and consequently thrust) is directly controlled by movement of the power lever. In this operating range, the propeller does not operate at a constant speed and propeller blade angle is coordinated with fuel flow, according to the power lever position. The beta operating range extends from just below flight idle on the power lever, through ground idle and reverse.
 
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