Zero stress for Boeing B777 – Maintenance procedure of airframe cracks
A cracking problem on Boeing 777s prompted a warning. Reports of cracks on the underwing longerons came up. This could result in fuel leakage into the forward cargo area and consequent increased risk of a fire or, in a more severe case, could adversely affect the structural integrity of the airplane.
Longerons usually carry larger aircraft loads and help to transfer skin loads to the internal structure. Longerons nearly always attach to frames, ribs, or the skin of the aircraft. Fatigue and stress cracks eventually appear around the fasteners used to secure longerons to structures. Such cracks risk going undetected because they are small, they are near and under fastener heads and often hidden beneath surface coatings. These situations present some interesting technical problems. First, potential cracks existing near fasteners are short, they spread in all directions and they are often subsurface, making them difficult to detect. Second, hidden corrosion between the layers of multilayered aluminum structures is hard to locate.
The cracks have been reported in the Service Bulletin 777-53A0081 by EASA and the Federal Aviation Administration (FAA). The corresponding Airworthiness Directive states that extra checks are needed to detect and correct cracking and corrosion in the fuselage skin, which could lead to rapid decompression and the loss of structural integrity of the airplane.
Risk of cracks – The consequences for maintenance
Boeing proposed to require repetitive inspections of the left and right side underwing longerons for any crack followed by related investigative and corrective actions if necessary. The implication of this is that the airframe of the aircraft must not be exposed to any kind of loading during maintenance. The otherwise normal lifting procedure using tripod-jacks, which bends each aircraft slightly, must not be used.
Figure: Each of the main landing gears is lifted with two axle-jacks of the same model
As a substitute for the customary three tripod-jacks used for a B777, the nose landing gear is lifted slightly with a type RS5032-001 axle-jack and each of the main landing gears is lifted with two axle-jacks of the same model. The axle-jack on the nose landing gear is also equipped with a load measuring cell to monitor the loads. The axle-jacks for this application have been even further refined and they all include a lock nut that prevents unintentional lowering. To stabilize the fuselage allowing the aircraft to be jacked up without loading for a longer time, the fuselage must then be secured with several shoring cradles. Three of them are necessary in front of the wings and one for the tail area.
Figure: The fuselage has to be secured with several shoring cradles
Maintenance process approved by Boeing
The full process has been approved by Boeing. The Airworthiness Directive (AD) from Boeing applies to all model 777 airplanes. The FAA estimates that this AD affects 201 airplanes of U.S. registry alone.
Important to know for all MRO and Airlines
The issued Airworthiness Directive from the Federal Aviation Administration requires repetitive inspections of the left and right side underwing longerons for any crack followed by related investigative and corrective actions if necessary. This AD has been issued to address the unsafe condition on the aircraft. Additionally a service bulletin has been sent out to all B777 customers.
It is important that all MROs and Airlines follow this process for jacking a B777. Tripod jacks can no longer be used on the B777 for aircraft safety reasons.
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