American Airlines management clearly doesn't think much of the idea of US Airways' staging a hostile takeover while the former carrier is in Chapter 11 protection. If you were in their position, hoping to still be in charge after a reorganization, you would probably feel the same way. US Airways is fairly open these days that it's interested in pursuing AMR. To help counter that, American released a memo to its CEO from its chief pilot that indicated American pilots ("See, it's not just me, boss, it's the rank-and-file!") question the ability of US Airways to pull off a merger. Read more in an informative article that includes some historical context, on the mess labor relations at both carriers traditionally have been.
UPDATE on Monday, 4-16: An American spokesman suggests this item needs clarification. He says American did not release the memo to its CEO from the chief pilot; Bloomberg, which published the article in the link, acquired the information from another source. Nothing else changes.