The ATSB has issued three safety recommendations arising from the ongoing investigation into the circumstances in which a Piper PA31-350 Chieftain ditched in Spencer Gulf SA with the loss of eight lives during a regular public transport (RPT) service from Adelaide to Whyalla on 31 May 2000 (Occurrence 200002157). The recommendations relate to mixture leaning procedures and the carriage and use of life saving equipment.
Immediately prior to the accident the pilot gave a MAYDAY report to Flight Service indicating that the aircraft had experienced two engine failures. The investigation found mechanical damage to both engines. The left engine had failed following a fatigue fracture of the crankshaft at the No. 6 connecting rod journal. Cracks of this type are created by the generation of thermal stresses in the journal surface.
The No. 6 connecting rod "big end" bearing had failed and it was evident that engine operation had continued after the bearing shells had been broken down. The surface of the journal, and the journal radii, had been damaged extensively by the rotation of the journal against the connecting rod. Extensive thermal cracking was evident over the entire journal surface.
In the right engine a hole had developed near the top of the No. 6 piston, allowing combustion gases to bypass the piston rings. The hole had been created by an exposure to temperatures within the melting range of the piston material. There were no failures of any other structural components of the right engine.
The ATSB is examining a number of recent occurrences involving Textron Lycoming TIO-540 series turbo-charged engines, similar to those fitted to the PA31-350. Engineering analysis indicates that the engines had typically been operated at or near peak exhaust gas temperature (EGT).
The fuel mixture leaning practice adopted by the operators during cruise flight was based on EGT settings ranging between 50 degrees (F) rich of peak and 50 degrees lean of peak EGT. While this practice is in accordance with the PA31-350 pilot operating handbook, early results suggest that operations in that EGT range, in combination with other possible factors, may have contributed to induced engine damage due to detonation.
This is in contrast to other operators who have not experienced similar problems, and use a more conservative leaning procedure by setting EGT at around 100 degrees rich of peak.
Induced damage may manifest itself through low compression, loss of power, erratic operation, metal contamination in filters or even complete engine stoppage. The underlying reasons for these symptoms can include burnt pistons, stretched or 'tuliped' valves, cracked spark plug ceramics or distressed bearings.
Operators can minimise the likelihood of such damage by eliminating the possibility of detonation. It can be insidious, and a pilot may not be aware that detonation is occurring.
Pending the outcome of its investigation of these issues, the ATSB suggests that, in addition to following the guidance provided in the pilot operating handbook, the operators of all turbo-charged engines avoid high cylinder temperatures through the adoption of a conservative approach to fuel mixture leaning practices.
Civil Aviation Orders 20.11 paragraph 5.1.2 details requirements for the carriage of life jackets for over-water flight. Multi-engine land aircraft authorised to carry nine passengers or less on RPT or passenger charter operations are not required to be equipped with life jackets or equivalent flotation devices unless the aircraft is operated over water and at a distance from land of greater than 50 NM. The Adelaide to Whyalla route was less than 50 NM from land.
Preliminary evidence indicates that the occupants of the Chieftain would have had sufficient time to don life jackets had they been provided. At least two of the occupants may have escaped from the aircraft after it ditched, but subsequently drowned. Had life jackets or equivalent flotation devices been available it is possible that their chances of survival would have been greatly increased.
The Australian Transport Safety Bureau has recommended that:
the Civil Aviation Safety Authority alert operators of aircraft equipped with turbo-charged engines to the potential risks of engine damage associated with detonation, and encourage the adoption of conservative fuel mixture leaning practices. [R20000250].
the Civil Aviation Safety Authority amend Civil Aviation Order section 20.11 paragraph 5.1.2 to remove the restriction that it only applies to aircraft authorised to carry more than nine passengers. [R20000248].
the Civil Aviation Safety Authority ensures that Civil Aviation Orders provide for adequate emergency and life saving equipment for the protection of fare-paying passengers during over-water flights where an aircraft is operating beyond the distance from which it could reach the shore with all engines inoperative. [R20000249].
|Type:||Educational Fact Sheet|
|Publication date:||3 March 2000|