Rigorous studies have characterized the aa-tRNA selection mechanism in bacteria, which is essential for maintaining translational fidelity. Recent investigations have identified critical distinctions in humans, such as the requirement of subunit rolling and a tenfold slower proofreading step. Although these studies captured key intermediates involved in tRNA selection, they did not elucidate the transitions of aa-tRNA between intermediates. Here, we simulated 1,856 aa-tRNA accommodation events into the human ribosomal A site, revealing the requirement of a distinct ~30° pivoting of aa-tRNA about the anticodon stem within the accommodation corridor. This pivoting is crucial for navigating the crowded accommodation corridor, which becomes more constrained due to subunit rolling. Subunit rolling-dependent crowding increases the steric contributions of the accommodation corridor during aa-tRNA accommodation, consistent with the 10-fold reduction in the rate of proofreading. The pivoting of the aa-tRNA enables precise alignment within the accommodation corridor, allowing it to traverse the narrower passage. Furthermore, we found that domain III of eEF1A interacts with the accommodating aa-tRNA through conserved basic residues, providing a steric block to prevent dissociation from the A site. Together, these findings provide a structural framework for understanding the distinctions between bacterial and human aa-tRNA selection and demonstrate that the alignment of the aa-tRNA relative to the ribosomal catalytic sites is a critical determinant of translational fidelity.