The crew of British Aerospace Jetstream 31, VH-TQJ, had flight-planned from Williamtown to Sydney via Mt McQuoid at flight level (FL) 120. The crew of Beechcraft Super King Air B200, VH-KCH, had flight-planned from East Sale, to Williamtown via Mt McQuoid at FL 250. Both aircraft were equipped with dual very high frequency (VHF) radios and Mode C transponders, which were operating at the time. A transponder is a radio device which, when triggered by a secondary surveillance radar signal, transmits a response that provides, when selected to mode C, altitude and positional data on a radar display for air traffic controller reference.

The routes flown by the two aircraft were within the Class G demonstration airspace detailed in the Aeronautical Information Publication (AIP) Supplement H66/98 of 5 November 1998. The route segment from Williamtown to Mt McQuoid included airspace inside the mandatory broadcast zone for the Williamtown control zone up to and including 5,000 ft. It also included the mandatory transponder area from 5,000 ft to 8,500 ft for a radius of 30 NM centred on Williamtown, and Class C airspace above 8,500 ft to the south-west. The crew of the King Air were descending on air traffic services route Whisky 170 (W170) on track from Mt McQuoid to Williamtown. W170 was a low-level two-way route. The crew of the Jetstream tracked to intercept W170 to Mt McQuoid after departing from Williamtown and were on climb.

Williamtown was listed in paragraph 4.3 of AIP Supplement H66/98, which required aircraft not receiving a radar information service to "monitor and, when required, use the frequency specified", which was 118.3 Megahertz (MHz), the Williamtown mandatory broadcast zone frequency when inside the Williamtown mandatory transponder area. The crews of both aircraft attempted to monitor and use the mandatory broadcast zone frequency while they were inside the mandatory transponder area.

The meteorological area forecast for Williamtown issued at 1800 Eastern Summer Time (ESuT) was for 3 octas of stratus at 1,000 ft, 6 octas of cumulus at 1,800 ft and 6 octas of stratocumulus at 2,500 ft. The trend type forecast included moderate turbulence below 5,000 ft from 1800 to 1900 ESuT, and from 1900 to 2100 ESuT the visibility was expected to reduce to 4,000 m in drizzle with broken cloud at 1,000 ft. The reported weather was a varied cloud base from 1,200 ft to 1,600 ft, with the tops 4,000 ft to 5,000 ft. A higher level cumulo-nimbus cloud cell was reported to the west of Williamtown.

While taxiing at Williamtown, the crew of the Jetstream made "all stations" broadcasts on the national advisory frequency and the mandatory broadcast zone frequency. The crew of the King Air did not hear these transmissions because they had not selected or transferred to those frequencies at that time, nor were they required to. The King Air was established in Class C airspace and the crew was communicating with the air traffic controller located in the Brisbane Area Control Centre.

The crew of the King Air selected and monitored the mandatory broadcast zone frequency at approximately 31 NM south-west of Williamtown, when the aircraft was passing approximately FL 150. AIP Supplement H66/98 required crews to monitor the mandatory broadcast zone frequency when below 8,500 ft, the upper limit of the mandatory transponder area, except when receiving a radar information service.

At 18:45.41, the crew of the King Air was cleared by Brisbane Centre to leave control area on descent, was given the area QNH, and informed that control services would terminate passing 8,500 ft. They were also advised that Williamtown was operating on mandatory broadcast zone procedures. The crew acknowledged this transmission. During this period, the crew of the Jetstream broadcast their departure on the mandatory broadcast zone frequency on their VHF 1 radio. On their VHF 2 radio, they had the Brisbane Control frequency of 126.9 MHz, selected, which they had been monitoring, together with the MBZ frequency, for four minutes prior to departure. The crew of the King Air reported that they did not hear the Jetstream's departure broadcast.

At 18:46.41, the controller asked if the crew of the King Air was still on the control frequency, and then provided the crew with traffic information regarding a Beech 1900 inbound to Williamtown from the north that was descending through 6,000 ft. The crew of the King Air acknowledged the traffic information. The King Air was passing 8,700 ft on descent when the traffic information was passed.

At 18:47.01, the controller attempted to provide the crew of the King Air with additional information about unidentified traffic (the Jetstream), 3 NM south-west of Williamtown. The transmission was over-transmitted by another aircraft and was unreadable. The crew of the King Air heard only the last few words of the transmission, and did not consider it relevant to their flight. The King Air was passing 8,500ft, which meant that it was leaving Class C controlled airspace and entering the Williamtown mandatory transponder area in the Class G demonstration airspace. During the high workload at that period of the flight, the crew was preparing for an instrument arrival into Williamtown due to the weather conditions. The controller did not follow up this "unacknowledged" transmission, because the crew of the Jetstream reported on the control frequency, its reported position identifying it as the previously unidentified traffic.

The mandatory broadcast zone at Williamtown was established in Class G (uncontrolled airspace). While within the mandatory broadcast zone, pilots were required to maintain a continuous listening watch on the mandatory broadcast zone frequency. They could arrange mutual separation within the mandatory broadcast zone. The frequency was not normally monitored by air traffic services. Pilots of IFR aircraft were required to report "Changing to Williamtown mandatory broadcast zone 118.3 MHz" when they were inbound to the Williamtown mandatory broadcast zone area.

At 18:47.24, the crew of the Jetstream reported their position to Brisbane Centre and that they were passing 4,000 ft on climb. The controller instructed the crew to squawk (transponder) ident, to squawk code 1201, and to maintain 5,000 ft due to inbound traffic. The squawk code and altitude restriction were read back correctly by the Jetstream crew, although the crew then inadvertently selected the wrong code of 1207. During the period of the Jetstream's transmission, the crew of the King Air was making an "all stations" broadcast on the Williamtown mandatory broadcast zone frequency. The Jetstream crew reported that they did not hear the broadcast.

At 18:47.56, the controller again attempted to provide traffic information about the Jetstream to the crew of the King Air, together with an instruction to maintain 6,000 ft. That transmission was over-transmitted by another aircraft and was not heard by the King Air crew. The over-transmission was confirmed by the crew of the Jetstream, who transmitted "two in together". The controller reported that he was "95% sure" that the King Air crew had read back "maintain 6,000 ft", which was not supported by analysis of recorded audio data. The transmission "two in together" was recorded. At the time of the controller's transmission, the Beech 1900 crew initiated communication with the King Air crew on the mandatory broadcast zone frequency and between them they subsequently arranged mutual separation.

The aerodrome circuit was active with a number of aircraft conducting circuit training, or inbound for landing. Both runways 35 and 30 were in use. Runway 30 intersected runway 35 approximately 750 m from its threshold. The tower was staffed with three controllers operating the aerodrome control (ADC), coordination and surface movement control positions.

The ADC had instructed the crew of a De Havilland Dash 8 (Dash 8) to line up for a departure from runway 35. The pilot of an American Aircraft Corporation Cheetah (Cheetah) had been instructed to continue approach for runway 30. The ADC cleared the Dash 8 crew for take-off and then looked towards the final approach path for runway 30 to monitor the approach of the Cheetah. As the ADC completed the transmission to the Dash 8 crew, he saw that the Cheetah was on short final and immediately transmitted an instruction to the Dash 8 crew to hold their position. This instruction was over-transmitted by the acknowledgment of the take-off clearance by the pilot in command of the Dash 8. The ADC commenced to issue a land-and-hold-short instruction to the pilot of the Cheetah; however, seeing that the Dash 8 was starting its take-off roll, he ceased the transmission and issued a further instruction to the Dash 8 crew to hold position. This instruction was over-transmitted by the Cheetah pilot reporting that he was initiating a go-around. The Dash 8 became airborne prior to the intersection of the runways, however, the crew held their aircraft at approximately 50 ft above ground level while the Cheetah passed approximately 100 ft above and slightly behind the Dash 8.

The automatic voice recording of the radio transmissions from the ADC during this period indicated that there were few or no intervals between any of his transmissions. The pilot of another aircraft that was operating in the circuit heard the hold-short instruction. The crew of the Dash 8 reported that they did not hear either of the hold-short instructions.

The ADC had formulated a traffic plan but this changed when a turbojet aircraft he thought was going to conduct a full-stop landing on runway 35, conducted a touch-and-go. The ADC had instructed the crew of the Dash 8 to line up on runway 35 after the approach of the turbojet aircraft, and had amended the approach of the Cheetah to runway 30 to facilitate the departure of the Dash 8. The ADC did not instruct the pilot of the Cheetah to report at a position that would have enabled him to adjust the traffic sequence if required. There was also a delay between the time the ADC approved the Dash 8 crew to line up, and the eventual issue of the departure instructions and a clearance. This delay was due to radio transmissions from the pilot of the turbojet aircraft, as well as the coordination of inbound traffic with the approach controller. During this period, the Cheetah pilot had continued his approach to runway 30. The ADC did not check the position of the Cheetah in relation to the intersection of the runways prior to issuing the take-off clearance to the Dash 8 crew. That was done as he was completing the transmission of the take-off clearance to the crew. As a final action to resolve the conflict between the aircraft, the ADC proposed to issue a land-and-hold-short instruction to the pilot of the Cheetah; however, the landing distance available (590 m) to the pilot did not meet the distance required (900 m) for land-and-hold-short operations.

The Manual of Air Traffic Services details the radiotelephony phraseology (RTF) to be used by controllers to cancel take-off clearances or to stop a take-off in emergency conditions. The ADC used only part of the required RTF for the cancellation of the take-off clearance. He did not use the RTF for an emergency situation.

A Boeing 747 (B747) was en route between Sydney and Perth, tracking via air route Q158. The crew contacted Perth air traffic control at waypoint BIDDY, and were issued with a standard terminal arrival route (STAR) to Perth. The controller and the crew entered into a discussion regarding Perth STARs. The crew were asked if the aircraft was Area Navigation (RNAV) equipped and replied that the aircraft was not so equipped. Aircraft that are not RNAV equipped are not permitted to track on air route Q158 from Sydney to Perth.

The investigation subsequently revealed that the aircraft was RNAV equipped and therefore, permitted to use Q158. There was no infringement of separation standards.

The flight, using a Cessna 210 aircraft, had been arranged as a charter after the scheduled regular public transport (RPT) service aircraft had become unserviceable. Waiting passengers were advised of the cancellation of the scheduled service and that limited seating would be available on a charter flight. Five passengers elected to take the charter flight using their previously purchased tickets and the company agent briefed them on the changed status of the flight.

Subsequently, while cruising at an altitude of 4,500 ft, the pilot reported to Adelaide flight service that the engine was running rough, that the engine cylinder head temperature (CHT) indication had become very high and that she was attempting to locate a clear area for a forced landing. The pilot landed the aircraft in a cleared gravel pit. During the landing roll however, the aircraft struck large mounds of gravel and was substantially damaged. The pilot and several passengers sustained serious injuries.

The aircraft came to rest upright, resting on the lower fuselage and right wingtip, with the tricycle landing gear folded back under the aircraft. The left wing was damaged with fuel escaping from the wing fuel tank. The engine had separated from the airframe and came to rest inverted in front of the wreckage.

The passengers evacuated the aircraft and assisted the unconscious pilot from the cabin. Adelaide flight service arranged for overflying aircraft to locate the accident aircraft and coordinated a rescue team to be dispatched from Darwin. A search-and-rescue (SAR) helicopter attended the scene and took the occupants to Darwin for medical attention.

The aircraft carried a manually operated GME Electrophone MT310 portable emergency locator transmitter (ELT), however the passengers were not aware of its location. Although the injured pilot attempted to direct the passengers to the ELT in the aircraft, the passengers were unable to locate it and as a result the ELT was not activated.

Aircraft engine

The aircraft was fitted with a Teledyne-Continental IO-520-L engine, serial number 567962, that had completed 734.5 hours since being overhauled. The engine was removed to a suitable engine maintenance facility for examination which revealed that the number one cylinder head had cracked, resulting in the observed high cylinder head temperature and rough running reported by the pilot. The cylinder head, part number TM642320F, had 10/81 stamped on the rocker boss, indicating that the cylinder head had been manufactured in October 1981. The history of the cylinder head, including the total time in service, could not be determined.

A detailed metallurgical examination of the cracked cylinder head determined that the cylinder head had failed as a result of fatigue cracking due to overheating (See Technical Analysis Report).

The engine was fitted with a six-point cylinder head temperature sensing system with a temperature sensor probe in the head of each cylinder. However, this system was inoperative at the time of the occurrence. A single, ring-type temperature sensor was fitted to the number one cylinder lower spark plug, in addition to a standard spark plug washer. Normally, a ring-type temperature sensor replaces the spark plug washer. This sensor provided cylinder head temperature information to the aircraft cylinder head temperature gauge. A maintenance release entry, dated 27 September 1998, stated "CHT gauge drops to zero periodically".

The engine baffles, used to direct cooling airflow over the engine cylinders, were in a generally poor condition with several cracks, broken mounting rings and worn areas. In one place, a baffle had been worn through by contact with an engine hose.

The engine had been overhauled on 18 August 1997 and fitted with six overhauled cylinders. On 11 August 1998, the number four cylinder head had failed because of cracking and was replaced. On 28 August 1998, the number six cylinder head had similarly failed because of cracking and was replaced. On 14 October 1998, 16.4 flying hours before the accident, the engine had passed a cylinder pressure leak check during routine maintenance.

Piston engine maintenance requirements

The Australian Civil Aviation Safety Authority (CASA) airworthiness directive, AD/ENG/4 (Amdt 7), details continuing airworthiness requirements for piston engines. That directive requires piston engines, operated in charter operations, to be overhauled in accordance with the engine manufacturer's requirements or in accordance with an approved system of maintenance. The directive also requires that the engine service history be recorded in the engine logbook. The engine manufacturer, Teledyne Continental Motors (TCM), does not specify retirement lives for engine cylinders but advises that "at some point in operating hours, generally very high, the aluminium cylinder head will fatigue due to the cylinder pressure loads and temperatures." TCM also advise that for cylinders in excess of two overhaul cycles "past that, duty cycle and the nature of field repairs make it difficult to project future life."

TCM also report that cylinder head life can be affected by any detonation/preignition event during the cylinder life, and that hot cylinder operation due to poor baffles or fuel mismanagement has a significant effect. CASA's information brochure titled "Piston Engine Cylinders" also recognises that those components have a limited life and suggests that cylinders be replaced every second or third engine overhaul. However, piston engine cylinders are not required to have serial numbers and recorded life history in the same manner as their parent engine.

Data review

A review of ATSB data covering the 10-year period January 1990 to December 1999 revealed that there had been over 60 reported occurrences, including a number of forced landings, involving piston engine cylinder head structural failures. Only one other reported cylinder head failure during this period resulted in occupant injuries. A review of the CASA major defect reporting (MDR) system for the same period revealed that there had been about 250 reported piston engine cylinder head structural failures, an average of approximately one failure every 2 weeks. The data only records the proportion of cylinder failures that have been reported through formal reporting procedures. Other cylinder head failures, such as those detected during routine maintenance, are possibly not represented.