Operators of around 1,200 Bell 47 helicopters still flying have been hit by an emergency airworthiness directive (AD) requiring expensive inspections of the main rotor blade grips.
Bell disputes the need for the AD, issued by the US Federal Aviation Administration following investigation of an August 1998 accident in which a blade grip failed due to a fatigue crack.
The manufacturer issued a service bulletin in 1985 halving the blade grip service life to 1,200h, but "disagrees with" the FAA's decision to require non-destructive inspection of the grips every 200h.
Dye penetrant or eddy current inspection of the grip threads requires removal of the rotor and will cost around $2,000, says Bell, concerned this could prove too expensive for some operators of the elderly helicopter.
In the 1998 accident, still the subject of litigation involving Bell, a grip failed after around 200h in service. Following the accident, the AD says, "an analysis of the field service data revealed fatigue cracks in 70% of the grips inspected".
The AD requires inspection within 100h of initial installation, or within 10h for grips with more than 100h in service. Bell says it does not have many replacement grips in stock, "but we're trying to ramp up [production]".
The FAA has also issued ADs on the Agusta A109E and Eurocopter BO105 and EC120B. The A109E emergency AD follows two incidents of fatigue failure of the main rotor rotating scissors assembly caused by incorrect installation. The AD requires immediate visual inspection of the scissors assembly, and removal and inspection of the attachment bolt within 50h.
The BO105 AD establishes a life limit for the main rotor tension-torsion strap, after an accident in which a rotor blade separated from a Eurocopter/Kawasaki BK117 due to fatigue of the same strap.
Straps in service for 120 months, or 40,000 flights, must be replaced by year-end. The EC120B AD requires adjustment of the cabin sliding door and follows the loss of a door in flight.
Source: Flight International
