Southwest Airlines ( LUV), which posted second-quarter earnings earlier that missed Wall Street expectations, announced that its CEO and vice chairman, James Parker, retired.

After 18 years at Southwest, Parker's departure comes in the wake of the carrier's 53rd consecutive quarter with a profit. It was a surprise, given the low-cost carrier's long standing history of positive employee relations and an extremely stable executive suite. Southwest said the Parker resigned for "personal reasons," and said Gary Kelly would replace him as vice chairman and CEO.

"Our entire board salutes Jim for his myriad accomplishments and for being an outstanding individual," said Herb Kelleher, chairman of the board and founder of the airline. "We will all greatly miss him."

Parker's replacement, Kelly, is also an 18-year veteran at the company, joining as controller in 1986 before assuming the role of CFO in 1989, a post he has held since then. Southwest said that Kelly had been tapped to replace Parker as part of its succession plan, but noted that Parker's decision to leave had accelerated the timetable.

In reaction to the executive shake-up -- and the disappointing quarter -- shares of the carrier fell 13 cents, or 0.9%, to $14.93.

Parker's departure is just the latest in a long string of executive departures. Since the World Trade Center attacks in 2001, nearly every major airline has announced a new CEO. In the last few months, Delta Air Lines ( CEO) has lost its CFO and COO, US Airways ( UAIR) has replaced former CEO David Siegel, while Continental Airlines ( CAL) has announced the retirement of current CEO Gordon Bethune.