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.
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 guage 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.
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.
The flight had progressed normally until the pilot reported the rough-running engine and associated high-indicated cylinder head temperature. The pilot could not determine the problem with the engine and decided to conduct a forced landing. As the pilot was seriously injured, the carriage of a portable ELT was ineffective in that the able-bodied passengers could not locate it in the aircraft.
The engine, while still within the overhaul period, had required several previous cylinder changes due to cylinder head cracking. The engine failure leading to the accident had also resulted from cylinder head fatigue cracking due to overheating.
Overheating of the cylinder head could be attributed to several factors.
- The operation of the engine at a higher than normal operating temperature. This is possible due to varying pilot engine operating techniques over a number of flights. Only limited, and possibly inaccurate, engine temperature information was available to any pilot because of recorded defects with the cylinder head temperature indicating system.
- The ring-type cylinder head temperature sensor was fitted to the number one cylinder head with the normal spark plug washer, effectively reducing the spark plug length. This may have contributed to a change in the normal combustion process, producing localised overheating near the plug and altering the thermal transfer within the cylinder. However, as this sensor was not fitted to other cylinders, this would not have occurred on the previous number four and number six cylinder failures.
- The engine cooling baffles, being in poor condition, would have been less efficient in providing cooling airflow to the cylinders. The number one cylinder, being at the rear of the engine, would have been more susceptible to overheating. The engine manufacturer advises that poor baffling will have a significant effect on cylinder lives.
- The prior cracking and replacement of the number four and number six cylinders indicates that the engine may have been subjected to higher than normal operating temperatures or that the cylinders had a reduced thermal cycle tolerance due to their age and time in service. At the time of the accident, the failed number one cylinder was seventeen years old.
The occurrence and subsequent occupant injuries were a result of the failure of the number one cylinder through fatigue cracking and the subsequent forced landing. Inaccurate cylinder head temperature indications may have contributed to the failure of the cylinder.
There are ongoing reported structural failures of piston engine cylinders as documented in the CASA MDR and ATSB occurrence data. Some of these failures have led to forced landings, which increases the risk of injury to aircraft occupants.
Aircraft piston engines must be maintained in accordance with CASA requirements and have their time in service history recorded in the engine logbook. However, this is not the case for piston engine cylinders, which are not required to have serial numbers for maintenance history documentation.
This report also highlights the limitations of carrying a portable ELT should the pilot become incapacitated and the need for comprehensive passenger briefings on the location and use of emergency equipment.
At present, there are no recommended or mandatory life requirements for piston engine cylinders in Australia for aircraft operated in any category of operation by either the piston engine manufacturers or the Civil Aviation Safety Authority. There is also no requirement for piston engine cylinders to be identified with serial numbers to document their history and time in service.
Safety Action Statement
As a result of this occurrence, the Australian Transport Safety Bureau issues the following safety advisory notice to the Civil Aviation Safety Authority.
The Civil Aviation Safety Authority should note the safety deficiency identified during this investigation and consider introducing methods to identify and record time in service of piston engine cylinder heads particularly for those cylinder heads utilised in passenger carrying operations.
|Date:||30 October 1998||Investigation status:||Completed|
|Time:||1407 hours CSuT|
|Location:||4 km S Gumadeer|
|Release date:||05 April 2001||Occurrence category:||Accident|
|Report status:||Final||Highest injury level:||Serious|
|Aircraft manufacturer||Cessna Aircraft Company|
|Type of operation||Charter|
|Damage to aircraft||Substantial|
|Departure point||Maningrida, NT|
|Departure time||1300 hours CSuT|
|Role||Class of licence||Hours on type||Hours total|