Debris originating from the starter failure was not contained by the starter casing and severed the number one engine B-sump oil scavenge pipe.
The manufacturers of the engine and air turbine starter have committed to implementing a number of changes that will address this issue going forward and have demonstrated that the ongoing risk to the fleet is acceptably low.
Honeywell performed a risk analysis to assess the possibility of an inflight shutdown (IFSD) and the risk was within the acceptable limit. Starter containment is a certification requirement for the manufacturer. Failed starters have generally been benign at the aircraft level and the starter does have design features (like turbine blade clashing and a containment ring) to mitigate that failure mode.
The single pawl and ratchet clutch design has also been superseded, with the replacement design being much less susceptible to this type of failure.
GE agreed that starter failures were not desirable (contained or uncontained), and has continued work to understand the failure and make improvements as noted. Relative to certification, of particular note is that the core compartment is defined as a fire zone (due to the presence of flammable fluid and a potential ignition source), so its boundaries are to be fireproof. As a result of work on a relatively recent engine certification program (different engine model), data was collected from a number of representative legacy in-service fires, which demonstrated they are of limited extent (size and/or duration). Furthermore, the associated oil volumes or quantities were relatively small, so that typical oil-related fires do not last very long, sometimes not even annunciated or alerted to the flight crew.
The action taken with regards to the change in design will eliminate those air turbine starter failures associated with single pawl and ratchet crash engagements, however it will not completely eliminate failures (contained or otherwise) from all causes. Nevertheless, as a result of these safety actions, the ATSB is satisfied that the likelihood and consequence associated with these starter failures will remain low.
Additional safety action
Whether or not the ATSB identifies safety issues in the course of an investigation, relevant organisations may proactively initiate safety action in order to reduce their safety risk. The ATSB has been advised of the following proactive safety action in response to this occurrence.
The air turbine starter manufacturer has initiated an engineering change to remove maintenance information from the data plate, so that maintenance information will only be found in the associated documentation.
The engine manufacturer advised that they were in the process of including changes to Engine Component Maintenance Manual (CMM) to reflect the new oil quantity limits. The CMM update will also include the addition of new installation tooling and clarification of the installation and pinning of the turbine pinion bearing locking nut. It is anticipated that the manual update will occur in early 2016.
The aircraft manufacturer advised that changes had been made to the oil servicing instructions in the Aircraft Maintenance Manual due for release in January 2016.
The aircraft operator has updated the task cards to be more proactive with their management of the air turbine starter oil system. The new cards include requirements above and beyond those specified in the AMM.
