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.
|Date:||15 February 2006||Investigation status:||Completed|
|State:||Western Australia||Occurrence type:||Loss of control|
|Release date:||25 January 2008||Occurrence class:||Operational|
|Report status:||Final||Occurrence category:||Serious Incident|
|Highest injury level:||None|
|Aircraft manufacturer||de Havilland Canada|
|Type of operation||Charter|
|Damage to aircraft||Minor|
|Departure point||Broome, WA|
|Destination||Cone Bay, WA|