Four ceramic-matrix composite flap inserts were evaluated following ground testing on a General Electric F110 turbofan engine. Three of the composites accumulated similar to 117 h of engine time. The fourth composite, a Nextel(TM) 720 material with aluminosilicate matrix, accumulated similar to 40 h. Large through-thickness cracks developed along the longitudinal edges of a NicalonTM/Al2O3 insert and the Nextel 720/aluminosilicate insert. The cracks developed because of high tensile stresses caused by the steep in-plane thermal gradients induced across the flap width during afterburner lights. The Nextel 720/aluminosilicate insert also exhibited severe surface wear associated with the acoustic environment and contact with the adjacent divergent seals. Neither a Nicalon/silicon nitrocarbide insert nor a Nicalon/C insert exhibited significant signs of distress.