(03-13) 17:48 PST Seattle (AP) --The Federal Aviation Administration has issued an emergency airworthiness directive demanding that airlines check a mechanism that controls tail flaps on about 600 Boeing 737s.

The directive was issued Friday and concerns flaps on the horizontal tails of the jets. On March 2, a Ryanair 737-800 en route from the Netherlands to Madrid, Spain, experienced severe vibrations in flight and had to make an unscheduled landing in Belgium.

Inspection afterward found "extensive damage" to the left elevator, which is a movable flap on the horizontal tail that controls the pitch of the airplane, up or down.