Boeing says an emergency airworthiness directive issued 12 March for the Boeing next generation 737 family is not connected to a previous directive aimed at the same components - the aircraft's two elevator-mounted trim tabs.
"Past issues affecting the elevator tab have been addressed through maintenance actions and modification programs; they are not directly related to the mounting lug of the elevator tab control mechanism," says a Boeing spokeswoman, referring to a 2003 airworthiness directive (AD).
"The recent in-flight vibration event is the first reported incident related to the mounting lug in the Next-Generation 737's 12-year history," she adds.
The emergency AD, which affects 737 variants from the -600 upward, calls for an inspection to detect a loose bearing in the aft lug of the elevator tab control mechanism. The 737 has tabs on the trailing edge of both elevators, devices used to assist the pilot with elevator control and to provide backup pitch control if the elevator controls fail.
The 2003 AD, which affected 737-600, -700, -700C and -800 series aircraft, called for operators to incorporate elevator tab mechanical modifications and to observe speed brake limitations in order to prevent the tabs from vibrating and possibly separating from the aircraft.
Boeing says the latest AD was based on a continuing investigation of an in-flight vibration event on a 737-800. "The airplane diverted and landed safely with no injuries to passengers or crew," the airframer says in a statement. "During ground inspection, excessive movement of the left elevator tab was found. Subsequent investigation, which is ongoing, determined that the loose tab was caused by fractured mounting lugs on the elevator tab control mechanism."
The AD requires next generation 737 operators to inspect the elevator tab control mechanism, work that Boeing recommended that operators perform in an all-operator message and alert service bulletin sent out 10 March and 12 March, respectively. The inspections must be completed within 12 days on certain aircraft, and within 30 days on others.
"Boeing is committed to the safe operation of its airplanes, and is working with FAA and its airline customers to assure compliance with this rule," says the airframer. "We constantly monitor the in-service fleet and is committed to providing the highest level of continuous safety when issues arise. The Next-Generation 737 is safe to fly on. The interim measures serve to ensure that safety continues at the highest levels while we work diligently to determine a permanent resolution for this issue."
Source: Air Transport Intelligence news
