NORWEGIAN PILOTS are to vote on whether to reject formally the findings of the investigation into the fatal loss of a de Havilland Canada DHC-6 Twin Otter.
A second report, this time by Dutch investigators, has failed to persuade UK metallurgical specialists whose earlier evidence caused the Norwegian inquiry to be re-opened. Those experts believe that the aircraft may be fatally flawed.
Bombardier's de Havilland subsidiary insists that it is satisfied with the Norwegian, Dutch and Canadian Aviation Safety Board findings. More than 600 examples of the highly regarded Twin Otter remain in operation.
Norwegian member of parliament Inge Myrvoll is expected to put down a formal parliamentary question about the Wideroe Flyveselskap crash in April 1990 which killed all five occupants (Flight International, 22-28 June, 1994).
UK consultant metallurgist Hugh Tyrer, who has examined the latest - and ostensibly the final - official report, says that it remains fundamentally flawed. "I am afraid a situation exists which could possibly kill somebody," he states.
Head of the Norwegian pilots' union Ulf Larsstuvold, a Wideroe Twin Otter captain, says that the union's committee plans to vote on sending a letter of no confidence in the report to the Norwegian transport ministry.
Evidence about the accident wreckage, presented by Tyrer and by Fred Jones, an aircraft structures specialist with 40 years of accident-investigation experience with the UK's former Royal Aeronautical Establishment, had forced the Norwegian accident investigation board (AIB) to have the information re-examined.
The AIB passed what it had retained of the crash wreckage - much of which had been "shredded" after the original investigation - to the Netherlands National Aerospace Laboratory (NLR).
The NLR report has now been published, and confirms the basic verdict of the original report. Tyrer, however, claims that the verdict remains invalid because the NLR also failed to examine scientifically the evidence, which caused the investigation to be re-opened. The Norwegian AIB and the investigators from the NLR have been unavailable for comment.
The Twin Otter went out of control just after take-off in bad weather and crashed into the sea, according to the AIB report. The AIB maintains that the tailplane failed under aerodynamic loads, causing loss of control. It says that a crucial aft-section elevator control-rod fracture was the result of the combination of tail-plane failure and crash-impact forces.
Tyrer says, that the loss of control was precipitated by a highly specific type of metal fatigue, which caused the break, depriving the pilots of all pitch control.
This push/pull rod, with a bearing at its end connected to a fork on the elevator torque-tube, acts as a bell-crank to rotate the elevator torque- tube and so deflect the elevator.
The fatigue failure might have been caused, Tyrer says, by water in the rod-end bearing-race freezing. Evidence for the water contamination, says Tyrer, is bearing corrosion, which is visible in the official-report photographs, but not mentioned in the reports. The freezing could have happened frequently without detection in northern-Norway conditions, he says.
Larsstuvold agrees, explaining that the pilots could overcome the increased bearing friction with strength.
Meanwhile, Tyrer maintains, the resistance or seizing in the bearing - which he says could also have had several other causes - produced bending loads in the rod-end which, in the Wideroe Twin Otter's case, eventually broke it as the pilot fought the gusty conditions.
Jones, meanwhile, rejects the AIB theory that the tailplane was broken before impact. He has examined the tailplane leading-edges and says that the fact that water-impact damage was uniform along both of them proves that they were intact on impact.
He says that if the final elevator -rod fracture had occurred at impact, as the AIB claims, it would have been a torsion fracture. Tyrer's examination shows that it was a bending fracture.
Source: Flight International
