Readers will recall that back on Feb 20th we flew N280MM down to Pendleton, OR so that the folks at All Terrain Aircraft could complete the TT strap upgrade on the main rotor head per the FAA’s new spindle AD.  Here I am typing on March 10th and N280MM is STILL in Pendleton.  You read that right… the helicopter has been in maintenance for nearly three weeks!  The mechanics found a couple of worn parts when they were putting everything back together.

We’re replacing several parts at Enstrom Helicopter’s recommendation.  That’s in addition to the $20,000 cost of the TT strap upgrade!  I’ll review the reasons for replacing all these parts and the troubleshooting once I have the helicopter back and get the final tally.  I can say that Enstrom Helicopters and David Styer at All Terrain Aircraft have both been fantastic.  I’m very impressed with the customer support.  Enstrom definitely stands behind their product.

For now, let’s take a look at the TT strap upgrade.

Enstrom Helicopter Spindles

What the heck is a Spindle?

The picture to the left is taken from the Enstrom Helicopter Illustrated Parts Catalog (Figure 8-10).  The spindle assembly is shown as item #1 in the diagram.  It connects to the rotor hub on one end and the blade grip (item #18) at the other end with a Lamiflex Bearing sandwiched somewhere in the middle.  You’ll see that the inboard end has a flapping hinge (vertical pivot) and the entire assembly allows the blade to feather (roll around the horizontal axis).  Since it’s holding the blade on to the rotor head I think we can all agree that the spindle is important!

A Lami-what?

A Lamiflex Bearing is made up of many layers of brass and rubber (sometimes hundreds) sandwiched together.  You’ll see one in the picture to the right.  For scale, the bearing to the right is about half the diameter of a hockey puck and the same thickness.  I’ve tried to research how they actually work, but anything I can find either requires a Ph.D in Engineering or just shows a picture.  What I CAN tell you is that the rubber compound must be made primarily of unicorn tears because the three (3) Lamiflex Bearings in the Enstrom rotor head are $1700 EACH!

Lamiflex and TT Strap

Enter the TT Strap

The spindles are sent off to Airwolf Aerospace where they are machined (shortened).  This removes the threaded end of the shaft where the Lamiflex Bearing is attached.  Notice the difference in length between the two spindles in the photo to the left.  Replacing the Lamiflex and the giant nut on the grip end of the spindle is the TT or Torsion Tension strap labeled #25 in the diagram.  You can see it sticking out the end of the spindle.  Now, instead of the centrifugal force pulling on the Lamiflex Bearing and a giant threaded nut, you have that force pulling on a pin (#32 in the diagram) that passes through the end of the TT strap.

My understanding is that the few spindles that failed due to cracking did so at the threaded end where the giant nut attaches them to the blade grip.  By machining that section off and using the TT straps you eliminate the piece of the spindle that failed.

All new Enstrom Helicopters leave the factory with TT straps.  Most older Enstrom Helicopters will be retrofit with them in order to comply with the spindle AD.  Touted benefits are less maintenance and rotor blades that stay in track much more consistently than with the old Lamiflex.  Time will tell and I’ll be sure to let everyone know how they work for me going forward.

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