Australian investigators uncovered a potentially serious design flaw in nearly 40 early Bombardier Dash 8-100 turboprops while probing a mysterious landing event involving the type.
Inspection of the aircraft revealed that a knob used to adjust throttle-lever friction could interfere with the flight-idle gate mechanism.
This gate mechanism normally prevents inadvertent selection, during cruise, of power settings below flight-idle - known as the beta range - which could lead the propellers to overspeed and result in engine failure.
But the design flaw meant that the gate could be bypassed without the pilots' having to use the triggers normally needed to access beta mode. This could happen if the friction knob was wound completely out.
"In effect the flight-idle gate could be rendered ineffective at protecting the propeller system against inadvertent activation of propeller ground beta range in flight," says the Australian Transport Safety Bureau.
It had been looking into a landing at Cairns on 30 December 2011 during which the crew of a Skytrans-operated Dash 8-100 perceived that the aircraft had slowed more rapidly on touchdown than expected.
Even though the crew had not selected reverse thrust, the captain believed the aircraft's propellers might have entered the reverse range.
Engineering inspections did not identify any technical problem with the aircraft (VH-QQA). Flight-recorder information confirmed reverse-thrust had not been used and showed no abnormal operation of the engines or propellers.
While no safety reason could be found to explain the crew's perception, the inquiry found that the friction-knob problem affected the first 39 Dash 8-100s built. Although a service bulletin was issued in 1986 to address it, the ATSB says it was "ineffective" because it omitted a requirement to modify a specific component.
It points out that the problem took 28 years to emerge and carried a "very low" risk, because the friction knob would probably not be fully wound out during normal operation. It adds that other subsequent safety modifications, including a beta-range warning horn, would alert pilots to the problem.
But it states that the discover "highlights the importance of crews reporting occurrences and other perceived problems". Bombardier corrected the problem through a service bulletin issued last year.