By Lily Bearings
©2004-2008
The U.S. Federal Aviation Administration has paralleled an Airworthiness Directive filed by Frances' Direction generale de l'aviation civile (DGAC) involving Eurocopter France EC120B helicopters. The FAA AD is FAA-2008-0489.
Eurocopter is a division of EADS; the EC120 is a relatively popular "executive" five-seat single-engine three-rotor-blade helicopter which has been offered around the world since 1998. EC120's are used for everything from medical transport to border patrol to military training.
The FAA's AD, "follows upon the discovery of a batch of spherical thrust bearings which prove to be unfit for flight." They are not airworthy, "because they were not manufactured in accordance with an approved type design."
Eurocopter employs a Spheriflex main rotor hub on its EC120 and several other helicopter series. The Spheriflex rotor uses a titanium hub and elastomeric spherical thrust bearings which reduce vibration, do not need lubrication and are easy to maintain.
The bearings involved are part number 7050A3622036. Affected serial numbers are LK0130, LK0142, LK0155, and LK0158. Neither the FAA, or Eurocopter, or the DGAC named the bearing manufacturer. Similarly, none of the organizations gave the reason that batch of bearings was found not to have been manufactured to type.
These main rotor bearings are, arguably, the most important mechanical components on the aircraft.
Most troubling, this Airworthiness Directive was initially issued February 2006. At that time, all EC120s were to be grounded until inspection and replacement could be carried out. [AD here; PDF file]
In this latest update, the FAA said:
These are critical parts that retain the main rotor to the M/R hub and flexes to allow the M/R blades to pitch. We were previously informed by the manufacturer that all affected spherical thrust bearings had been recovered by Eurocopter France. However, we recently learned that some affected spherical thrust bearings have not been recovered and may still be installed on some helicopters.
Bearing failure is outlined in a rather understated way:
Failure of a spherical thrust bearing during flight could cause the main rotor system to separate from the helicopter, which would be catastrophic.
A similar Canadian notification describes the bearing and failure scenario:
An STB consists of several steel cups laminated between thin rubber sheets, forming an elastomer. The elastomer is bonded between two aluminum frames forming the STB. These bearings carry the main rotor blade centrifugal loads when the rotor is turning. They are flexible in torsion, flapping, and drag, but rigid in compression. STBs are important components of the main rotor hub, as all motions and loads pass through them.
When an STB starts to fail, the rubber progressively squeezes out from between the metal plates, forming blisters on the exterior and an extrusion "tail" on the interior. The squeezed-out rubber causes the bearing to become shorter. Because of the bearing's location in the main rotor head, the rotor blade progressively shifts away from the centre of rotation; the centre of gravity (C of G) of the blade moves farther outboard than the other two blades, and causes a one-per- revolution vibration which increases as more rubber is squeezed out and the bearing grows smaller. When the rubber debonds from the frames, and the centre elastomeric section suddenly pops out, the affected rotor blade suddenly shifts outwards a distance equal to the thickness of the missing bearing section. The one-per-revolution vibration will suddenly become very intense, and control of the helicopter may be lost.