During a charter flight from Broome, WA to Cone Bay, WA, the
pilot of an amphibious (float and landing gear equipped) Viking Air
Ltd DHC-3-T 'Turbo-Otter' aircraft (VH-OTV) reported an unusual
movement within the control system, followed by a sudden downward
pitching motion, leading to a rapid and uncontrolled descent.
With the assistance of the front seat passenger, the pilot was able
to arrest the descent and regain control of the aircraft, before
making a precautionary landing at Lombadina Station, WA.
An engineering assessment of the aircraft found that the right
elevator servo tab had broken away from the control rod and horn at
the outboard end. Damage to the elevator trailing edge and
tearing of the tab through the mid-span was consistent with gross
oscillatory movement (flutter) of the tab after it had become
disconnected from the rod and horn.
Aerodynamic flutter within the elevator trim and servo tabs of
the DHC-3 aircraft type had been known since the 1960s, however the
development of turboprop engine conversions for the aircraft had
resulted in an increased potential for tab failure as a result of
the increased airspeeds and control surface loads associated with
the re-engined aircraft. A series of engineering solutions to
the flutter problem had been subsequently developed, and in April
2004, a US Federal Aviation Administration airworthiness directive
(AD) mandated the modification of the DHC-3 elevator tab assembly
for US registered aircraft.
At the time of the occurrence, VH-OTV had not undergone the
elevator tab modifications. The maintenance organisation
stated that it was unaware of the FAA actions and had not received
any information as to the availability of flutter prevention
modifications from the aircraft type certificate holder or the
certificate of registration holder.
Safety action taken by the maintenance provider after the
occurrence included the implementation of systems to more
adequately alert the organisation to the existence of important
safety bulletins and airworthiness directives affecting the
aircraft. An airworthiness directive for the elevator tab
modifications issued by Transport Canada the month before the
occurrence, became effective on 31 March 2006 and, on 1 March 2006,
the Civil Aviation Safety Authority (CASA) introduced an AD to
mandate the prospective Transport Canada requirements from 31 May
2006.