American Airlines (AMR) said an internal investigation has revealed improperly installed clamps caused some seats to loosen in six of its aircraft and would inspect the seats in other planes.
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On Monday the company said it had temporarily grounded eight aircraft after some seats came loose on two flights.
"Our maintenance and engineering teams have discovered that the root cause is a saddle clamp improperly installed on the foot of the row leg," the company said in a statement.
The airline, whose parent AMR Corp filed for bankruptcy protection in November, is inspecting all 47 of its 102 Boeing 757 aircraft which have the same type of seat.
The Federal Aviation Administration is aware of the internal review and its findings, the company said.
The airline has already suffered disruptions to its flights due to a long-running labor dispute with its 8,000 pilots, who are expected to authorize a strike this week.