Abstract: At 7:00 a.m. on January 26, 2022, two employees were standing on an economizer, approximately 4 feet off of the ground, attempting to plumb 6 tubing elements. The employees were working on the second "B" of four identical economizers (A,B,C,D). For the "A" economizer, the tubes were installed individually from the bottom up. At the direction of the department foreman, the assembly sequence was optimized for the "B" economizer by building tubular elements approximately 20 feet wide by 5 feet tall by 2 feet, weighing 1,050 pounds each and installing them into a fixture to prevent toppling. Six elements were installed into the fixture and all were held in place with F-Clamps at the top while the bottoms were welded in place to the header pipe. The employees were standing on top of the fixture and the elements were leaning away from them. The employees instructed a coworker, on the ground, to remove the clamps that were holding the tops of the elements in an attempt to plumb and reblock the elements manually. Employee #1 leaned two elements into the vertical position and upon leaning the third element upright, all six elements toppled toward them. The employees were not tied off nor were they required to be tied off. Notably, it had been previously assessed that any fall protection would have increased the risk of injury. Employee #2 was knocked off the fixture and fell to the ground. Employee #1 was pinned under the elements. Both employees were hospitalized with broken bones, broken ribs, fractures, and a punctured lung.