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.