This study systematically evaluates the burn-through resistance of 2618-T61 aluminum alloy plates (3, 4, 5, 6, and 8 mm thicknesses) under standard flame impingement (1100 ± 80 °C) for aero-engine applications. Results reveal a 409.4% increase in burn-through time and 96.5% reduction in burn-through area when thickness grows from 3 to 8 mm. Plates ≥ 4 mm form protective Al2O3 films after initial burn-through, significantly impeding flame penetration. A critical thickness threshold is observed at 5 mm, with ≥ 6 mm specimens showing 54.2% smaller burn-through areas versus 5 mm plates, and 8 mm plates demonstrating near-complete burn-through resistance within 15 min. Burn-through time and melt delay exhibit linear thickness dependence, while burn-through area follows an S-curve pattern. These findings provide essential airworthiness certification data for firewall design, quantitatively establishing thickness as the dominant factor in fire resistance.