9 July 2003
Damaged piston seals
Transport Canada has recently received a Service Difficulty Report (SDR) describing an incident where smoke in the cabin resulted in the pilot's decision to shutdown the LH engine in flight.
Following shutdown, the engineer immediately noticed oil on the LH cowling. The source of the oil leak was later traced to a failed propeller piston O-ring. This allowed high pressure oil into the spinner area and eventually into the engine intake and entered the aircraft air conditioning system.
Transport Canada (TC) carried out a service history review which revealed numerous SDR reports on propeller piston and dome O-ring failures. On many occasions, an inflight engine shutdown had to be carried out due to resultant low engine oil pressure.
In an effort to reduce these occurrences, TC has contacted the manufacturer who provided the following information. The propeller system incorporates a single governed high pressure engine oil supply feed (beta) tube acting on the rear side of the piston. The pitch change mechanism is hydraulically operated towards fine and reverse pitch and mechanically assisted to coarse and feathering pitch by coil springs and blade counterweights. Experience has demonstrated that a damaged and/or failed propeller piston seal will allow the hydraulic fluid into the normally dry front half to the cylinder (dome) and then out through the non-hydraulically sealed start latch mechanism.
The propeller piston oil seal can be damaged by engine oil-sourced, fluid-borne contaminants. The seal can also be damaged due to cylinder bore scoring. When oil debris enters the cylinder, this contaminated oil will be separated outwards by centrifugal forces and then collect on the cylinder wall. This debris may then contact the seal causing seal damage in the form of seal rolling, flat spots and cuts/scoring.
Another source of seal damage can be caused by internal metallic debris which is produced by the start latches not fully engaging in the actuation piston during engine shutdown. Partial engagement of the start latches with the edge of the piston can produce sharp slivers of metal. This type of damage is associated with inappropriate engine shutdown procedures, contrary to the aircraft operating instructions. It is necessary to select reverse pitch during shutdown so that hydraulic effort can keep the piston forward against the spring loads while the start latch springs can push them inwards against centrifugal loads (around 25% propeller RPM).
The propeller manufacturer does have an improved composite seal kit available (P/N P340701) which has provided better protection against seal damage, whether from latches or fluid borne contaminants. Please refer to Dowty Rotol Service Bulletin SB 61-1089.
Note: The R333 propeller hub/piston configuration installed on the Jetstream 3100/3200 is the same configuration as installed on the Swearingen SA227 and CASA 212 series aircraft. Therefore these operators may experience similar seal damage.
Transport Canada reminds maintainers/operators that damage to piston seals can be reduced by paying close attention to aircraft engine start and shutdown procedures. It is essential that, for this installation, reverse pitch is selected on engine shutdown to ensure that starting latches engage on the piston for subsequent restarting. Good visual checks around the dome area for oil leakage and any early signs of oily smell or vapours in the cabin may be a sign of impending piston seal failure.
Any defects or further occurrences should be reported by sending a Service Difficulty Report to Transport Canada, Continuing Airworthiness, Ottawa.
For further information, contact a Transport Canada Centre, or Mr. B. Caldwell, Continuing Airworthiness, Ottawa, telephone (613) 952-4358, facsimile (613) 996-9178 or e-mail firstname.lastname@example.org.
For Director, Aircraft Certification
Acting Chief, Continuing Airworthiness