de Havilland Canada DHC-8-102, VH-TNG

FINDINGS

The investigation identified the following contributing safety
factors.

• The part number A44700-009 roll spoiler actuator, as fitted to
  TNG, was susceptible to fatigue cracking as a result of
  deficiencies within the design of the actuator cylinder base
• The actuator ruptured as a result of fatigue cracking initiated
  due to the design deficiency and the accumulation of a sufficient
  number of flight cycles
• Rupture of the actuator allowed the loss of fluid contents from
  the number-2 hydraulic system, and the subsequent failure of that
  system.

SAFETY ACTION

Aircraft manufacturer

On 18 August 2004, the manufacturer issued In-Service Activity
Report (ISAR) 2004-07-2760 advising that it considered the overall
risks associated with the failure of the actuator as low, and to
raise operator awareness of the actuator failure mechanism.

Transport Canada

Transport Canada, being the certification authority for the Dash
8 series of aircraft, conducted its own investigation of the issues
associated with actuator failure. In communication with the ATSB,
Transport Canada concurred with the aircraft manufacturer's risk
management position and agreed that on the basis of current failure
rates, no additional action was warranted.

Australian Transport Safety Bureau

The ATSB reviewed the responses and positions of the aircraft
manufacturer, Transport Canada and the aircraft operator in
relation to the potential for roll spoiler actuator failure to
affect operational safety of the Dash 8. While it supports the risk
management approach adopted by the manufacturer and Transport
Canada, the ATSB believes that the issue of landing performance
limitations imposed by a hydraulic system failure is not adequately
addressed by this approach. Therefore, the ATSB issues the
following safety advisory notice.

Safety Advisory Notice SAN20050012

The Australian Transport Safety Bureau notifies operators of
Dash 8 aircraft fitted with part number A44700-007 and/or
A44700-009 roll spoiler actuator components, of the increased risk
of the cracking and/or rupture failure of the actuator cylinder
section as the actuators accumulate service cycles. Failures have
been sustained by actuators with service lives ranging from 27,909
to 41,716 cycles. Cracking or rupture of an actuator cylinder
results in the loss of contents and subsequent failure of the
associated aircraft hydraulic system. Given that the loss of an
aircraft hydraulic system will result in extended landing distance
requirements, operators are advised to consider the safety
implications of operating affected aircraft to regions where the
increased landing distance requirements may not be easily
accommodated by available primary or alternate aerodromes.

ANALYSIS

Actuator failure

The investigation determined that the roll spoiler actuator from
TNG had failed as a result of the growth of fatigue cracking from
within the actuator cylinder. Originating from the internal threads
at the end of the cylinder, the cracking propagated under service
pressure cycles to a point where it compromised the cylinder wall
and subsequently caused the rupture of the housing.

ATSB research found records of 16 actuator failures within the
world fleet, of the type and nature sustained by TNG. All such
failures had occurred in part number A44700-007 or -009 actuators
and all had occurred in units that had accumulated over 27,000
flight cycles. There was no record of failures of the newer part
number 1556000-1 actuators.

On the basis of the investigation findings, it was apparent that
the design of the A44700-007 and -009 actuators renders the units
susceptible to fatigue cracking and failure under service
conditions. The nature of design influenced fatigue failures is
such that they typically present a range of cycles-to-failure that
is characterised by a lower threshold value and an increasing
number of failures as cumulative cycles increase. The distribution
of known actuator failures reflects these features and as such, it
is a reasonable expectation that the number of future failures will
increase with the age of the Dash 8 fleet equipped with the A44700
series actuators.

Operational implications

Following the TNG actuator failure and increasing awareness of
the actuator deficiencies, the aircraft manufacturer reviewed the
issues from a risk management perspective and elected not to pursue
any direct course of safety action, on the basis that the
probability and risks presented by the events were low. From the
information available, it was not apparent whether the aircraft
manufacturer, in its decision not to implement any direct safety
action, had considered the likelihood of an increasing frequency of
actuator failure as the number of component flight cycles build
across the fleet. Nor is it known if the manufacturer had
considered the performance implications of actuator failure and
hydraulic system loss where the aircraft was being operated into
areas with limited runway distance availability.

Shortly after reaching a cruising altitude of 12,000 ft (FL120),
the flight crew of a de Havilland Canada DHC-8-102 (Dash 8),
registered VH-TNG, that was operating a scheduled passenger service
from Brisbane to Roma (Queensland), were alerted to the failure and
rapid loss of contents from the number-2 hydraulic system. Electing
to return the aircraft to Brisbane, the flight crew carried out the
relevant quick-reference handbook (QRH) checks, and after some
initial difficulty, manually extended the landing gear.