Written by Dominic Gates

Republished from Seattle Times, Aerospace

The Federal Aviation Administration (FAA) issued an emergency airworthiness directive Friday requiring that airlines inspect about 600 Boeing 737s to check a mechanism that controls the flap on the horizontal tails of the jets.

Some of the jets must be inspected within 12 days, and the rest within 30 days. FAA spokesman Allen Kenitzer said about half the affected airplanes are operating in the U.S.

He said the directive stems from an in-flight incident on March 2, when a Ryanair 737-800 en route from Eindhoven in the Netherlands to Madrid, Spain, with 146 passengers aboard experienced “severe vibration” in flight.

The flight crew diverted the airplane, which landed safely and uneventfully in Brussels, Belgium.

An 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.

The FAA airworthiness directive described the damage as “failure of the aft attach lugs on the left elevator tab control mechanism.”

“Severe vibration in this attach point is suspected of allowing rapid wear of the joint and resulted in failure of the attach lugs,” the FAA report said. “This condition, if not corrected, could result in a loss of aircraft control and structural integrity.”

During the required inspection of affected planes, mechanics are instructed to look for damage to the attachment points of the elevator-control mechanism. If they find these lugs damaged, the plane must be grounded until the mechanism is replaced.

The Ryanair jet that experienced the vibration in March was a relatively new airplane.

It was delivered from Boeing’s Renton plant in April 2008 and had completed 4,233 flight cycles. Kenitzer, the FAA spokesman, said the elevator tab on the jet’s horizontal tail was the latest design, one mandated by a previous FAA directive in 2003 that was intended “to prevent severe vibration of the elevator and elevator tab assembly.”

That earlier directive required a retrofit redesign involving 88 hours of work at a cost of more than $5,000 per jet. The FAA said then it was necessary to prevent “severe damage to the horizontal stabilizer followed by possible loss of the elevator tab and consequent loss of controllability of the airplane.”

However, Boeing spokeswoman Sandy Angers said the problem that has now come to light with the elevator tab attachment lugs “is a separate issue.”

Boeing issued a service bulletin earlier Friday recommending that airlines inspect the mounting lugs on all its newer-generation 737s, more than 3,000 of which are flying.

However, the emergency FAA directive mandating the inspections applies only to about 600 of those jets considered more at risk and that must be inspected within a month. Whether a jet must be checked within 12 days or within 30 days depends upon its age, its total accumulated flight cycles and if it is approved to fly extended flights.