During initial climb, the right propeller of the DHC-8-315 (Dash
8) aircraft auto-feathered. The flight crew retarded the right
engine power lever, declared a PAN (radio code indicating
uncertainty or alert) condition, and completed an uneventful single
engine return to Sydney airport.
The aircraft was fitted with two Pratt and Whitney Canada PW123E
engines. The flight data recorder (FDR) indicated that the right
engine over-torqued to 120 percent for 7 seconds after the
propeller feathered. The FDR also indicated that the left engine
over-torqued to 117 percent for 20 seconds. The engine
manufacturer's transient over-torque limits were not exceeded.
Maintenance personnel found that a loose connection of the right
engine torque signal conditioning unit (TSCU) connector pins
resulted in an intermittent electrical connection. The TSCU was
replaced as a precaution, and the connector was cleaned and
reseated. Following a flight test, the aircraft was returned to
service.
Propeller auto-feathering
The propeller auto-feather system, when selected, was designed
to automatically feather the propeller during take-off if the
engine torque decreased below about 22 percent rated torque.
Interlock features in the auto-feather logic and control circuits
provided arming control and prevented auto-feather of the operating
propeller, once the auto-feather sequence for one of the propellers
was initiated. The system provided for relaying a 'power uptrim'
(engine power increase) signal to the operating engine.
Previous occurrences
The ATSB investigation into two previous occurrences (199905044
and 200002853) determined that in the earlier occurrence the
electronic engine controller electrical connector was contaminated
with water, while a faulty TSCU was found in the other.
The two previous Australian occurrences were also documented on
the Australian Civil Aviation Safety Authority's database. A search
of the engine manufacturer's database and the Service Difficulty
Databases for Transport Canada and the United States of America
Federal Aviation Administration, revealed twenty-three similar
occurrences in the period from 19 June 1993 to 27 October 2001.
Of the twenty-six worldwide events reported, four were confirmed
in-flight engine shutdowns (IFSD). Nineteen were attributed to
electrical problems (harness and/or connector or torque signal
conditioning unit). Fourteen events occurred during initial climb
out and ten during the take-off roll.
Aircraft and engine manufacturer background
information
The aircraft manufacturer advised that their data indicated that
propeller auto-feathering as described in this incident was a
result of loss of torque signal to the TSCU, most likely due to
"connector intermittencies". Improvements to the system included
design changes to strengthen the torque signal, and flight crew
procedural changes. The aircraft manufacturer considered that the
present decrease in reported occurrences reflected the success of
these changes.
The engine manufacturer reported that the occurrences were
associated with an intermittent loss of torque signal. They
recommended, when an operator experienced one or more occurrences,
that the operator conduct a fleet-wide electrical harness
inspection, clean the connectors and enhance connector tightening
procedures.
Service bulletins and operator letters
On 25 May 1993, the engine manufacturer issued Service Bulletin
(SB) 21269 addressing the application of shrink tubing to the TSCU
wiring harness to provide protection from moisture ingress and
loosening of the connectors.
On 19 December 1995, the engine manufacturer issued SB 21456
addressing spurious `uptrims' and activation of the auto-feather
control system when the system was in the armed condition. Those
problems were attributed to the torque sensor air gap not being
optimised. The procedures were described for decreasing the torque
sensor air gap by replacing a spacer in the unit. The modification
improved signal strength and reduced sensitivity to electrical
'noise'.
On 31 January 1996, the engine manufacturer issued SB 21463
addressing fretting of the TSCU electrical connector socket pins.
The modification involved replacing the existing wiring harness
with one that included a more secure connector assembly with
sockets less susceptible to fretting.
On 11 December 1997, the engine manufacturer issued Operator
Message Number (OMN) 464 informing operators of two recent IFSD
auto-feather events and advised them that those events may have
been the result of incorrect tightening torque on the TSCU
connectors. They recommended that the connectors be inspected for
security, and if found loose, should be disconnected and inspected
for contamination and moisture.
On 28 February 2000, the engine manufacturer issued OMN 602
informing operators of recent IFSD auto-feather events and
reiterated procedures currently published in the Aircraft Flight
Manual. The letter strongly recommended that the operator's review,
with their flight crews, the correct procedures to follow with
respect to any engine or propeller malfunction on take-off. The
letter also noted that previous events had indicated that flight
crews often retard the power levers of both engines, thereby
cancelling the `power uptrim' signal to the operating engine.
Compliance with these bulletins and messages was not mandatory,
however the maintenance organisation implemented the requirements
of SBs 21269, 21456 and 21463.