Yesterday, during a round of removing wire segments between the UIM and Top Mass on the main chain, a screw was dropped resulting in one of the fibers breaking on the ITMy.  Once the first fiber broke, the other 3 immediately broke, likely due to the offloaded weight of the mass or possibly flying fiber debris from one side of the suspension to the other.  The fiber guards were in place and set appropriately, as were the shields between the PUM and UIM.  However, in the act of working on the mechanics of the suspension above the monolithic (a game we must play in order to tune it to proper alignment) an 8-32 screw was dropped which then bounced off the tray, falling to the top of the test mass where it then bounced again and into the inside of the fiber guard.  The area between the Test Mass and PUM, and between the fibers is not blocked in any way.  Pictures of the suspension can be found on ResourceSpace at:

https://ligoimages.mit.edu/?c=804

As well, attached is my sketch of the likely trajectory of the screw.

Note, the activity requiring the screw to be free above the monolithic is a typical adjustment to the suspension.  We have highlighted this and a few other risky mechanical maneuvers to SUS and SYS who are working to further mitigate them.  I expect we'll see additional shields, some custom tools, and capturing screws in the near future for these problematic locations of work in the QUAD.

We are mobilizing to remove the lower structure from the QUAD/ISI and push it back into the fiber weld cleanroom for restoration of the monolithic with 4 new fibers.