The US Federal Aviation Administration will require operators of 384 US-registered Bell Helicopter models ranging from the Bell 205 to the Bell 412 to perform frequent visual checks of tail rotor blade assemblies for cracks, and potentially replace the blades at a cost of $12,000 per helicopter.
In a separate airworthiness directive, also issued on 6 May, the agency is requiring 96 operators of Eurocopter EC120B helicopters to inspect their main rotor spherical thrust bearings for defects. Replacement of the bearings, expected to be required on about four helicopters, will cost $4,820 per helicopter.
The final Bell AD requires operators to clean and visually inspect using a 3X magnifying glass the tail rotor blade skins, leading-edge spar, doublers, grip plates and trailing edge for cracks, corrosion and other damage, including nicks, scratches or dents, at least every 25 flight hours or 30 days, whichever occurs first.
The FAA says the short compliance time is needed because a cracked or damaged tail rotor blade can "adversely affect the structural integrity and controllability of the helicopter".
The FAA had proposed but not finalised a similar rule for fewer models in 2006 after reports of fatigue cracking on five Bell 412s and three Bell 212s, three of which failed in flight.
In one case, the FAA says a section of the blade separated from the helicopter during cruise flight at 5,500ft (1,675m), after which the aircraft was "reported to have violently turned down and to the left", levelling out at 1,000ft and setting down in water.
Before the proposed rule could be finalised however, the FAA received another report of a cracked blade on a Bell 412EP, prompting the agency to issue the new final rule without completing the normal public input process.
The FAA says replacing damaged blades with a "redesigned, more damage-tolerant tail rotor blade" will eventually negate the need for frequent inspections. Bell has said it is working on such a design.
The Eurocopter AD was prompted by "the discovery of a batch of spherical thrust bearings which proved to be unfit for flight", says the FAA. The bearings retain the main rotor to the main rotor hub and flex to allow the main rotor blades to pitch. "Failure of a spherical thrust bearing during flight could cause the main rotor system to separate from the helicopter," says the FAA.
"We were previously informed by the manufacturer that all affected spherical thrust bearings had been recovered by Eurocopter France," the agency says. "However, we recently learned that some affected spherical thrust bearings have not been recovered and may still be installed on some helicopters."
Source: Flight International
