The US Federal Aviation Authority has reviewed a service bulletin sent to NG series 737 operators by Boeing on Friday and upgraded it to an emergency airworthiness directive or AD.

This makes the Boeing bulletin compulsory, and follows a recent serious inflight incident involving part of the horizontal stabiliser of a near new Ryanair 737-800.

Qantas operates 38 Boeing 737-800s and the Virgin Blue fleet includes around 30 of the type, as well as a large number of the smaller 737-700 model.

The inspections are unlikely to affect schedules but there has been no statement by either carrier as yet.

This is the first part of the FAA directive:

AGENCY: Federal Aviation Administration, DOT

ISSUE DATE: March 12, 2010

AD#: 2010-06-51

Emergency airworthiness directive (AD) 2010-06-51 is sent to all owners and operators of The Boeing Company Model 737-600, -700, -700C, -800, -900, and -900ER series airplanes.

The FAA received a report of failure of the aft attach lugs on the left elevator tab control mechanism, which resulted in severe elevator vibration. The flightcrew diverted from the intended route and made an uneventful landing. Subsequent investigation revealed extensive damage to the elevator tab control system. Severe vibration in this attach point is suspected of allowing rapid wear of the joint, and resulted in failure of the attach lugs. This condition, if not corrected, could result in a loss of aircraft control and structural integrity.

Explanation of Relevant Service Information
We have reviewed Boeing Alert Service Bulletin 737-27A1296, dated March 12, 2010. The service bulletin describes procedures for a detailed inspection to detect discrepancies of the inboard and outboard aft attach lugs of the elevator tab control mechanism. Discrepancies include movement or rotation of the spacer, and gaps between the swage ring and the aft attach lug or between the spacer and the aft attach lug. The service bulletin describes procedures for replacing any discrepant elevator tab control mechanism, including performing the detailed inspection on the replacement part before and after installation. For certain airplanes, the compliance time for the inspection is 12 or 30 days, depending on airplane line number, total accumulated flight cycles, and approval for operation under extended twin operations (ETOPS).

FAA’s Determination and Requirements of this AD
We have evaluated all pertinent information and identified an unsafe condition that is likely to exist or develop on other airplanes of this same type design. Therefore, we are issuing this AD to detect and correct a loose bearing in the aft lug of the elevator tab control mechanism, which could result in unwanted elevator and tab vibration. The consequent structural failure of the elevator or horizontal stabilizer could result in loss of aircraft control and structural integrity. This AD requires accomplishing the actions specified in the service information described previously, except as discussed under “Differences Between this AD and the Service Bulletin.” This AD also requires reporting the inspection results to Boeing.

The second part of the FAA bulletin discusses differences between its emergency AD and the Boeing service bulletin, which was issued the same day.

ADs like this are a part of a global air safety oversight system that in general, works very effectively at picking up and correcting issues.

The unusual aspects of this directive are that the manufacturer didn’t see a need to make it compulsory, and that it relates to an issue with comparatively new jets.

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