The HellHawk incident was an incident in 2009 between Acepedia (AceWiki as it was named back then) and AceCombatSkies, two Ace Combat fansites, over the validity of AceWiki's content especially including the ADF-37 HellHawk.
On January 9, 2009, an anonymous contributor created the "ADF-37 HellHawk" article on AceWiki. Aside from the trivia and categories, the page looked very similar to this by July 2009.
Up to this point, AceWiki (having been created in 2006) was full of fan material and had a reputation as an unreliable wikia. The HellHawk article made that reputation worse when a user believed the HellHawk was real and asked about it on AceCombatSkies. The topic has since been deleted or otherwise made unavailable, but it is still available for public view through the Web Archive. Alpha Juliet, who was Sgt.johnson on AceWiki, had the harshest reaction as a former administrator of the wikia; a few days later, he became an admin again to start cleaning up the content.
In 2010, AceWiki became Acepedia and came under new administration by Protostealth, Koveras Alvane, and Raymorn; the new administrators and users began to clean out the fan material from the wiki and the Hellhawk incident faded into history.