Boeing is working to determine the cause and scope of two different manufacturing quality problems in its Boeing 787 production process that, when combined, make affected aircraft susceptible to structure failure at loads they should be able to withstand.

The combination of defects has been flagged on eight 787s, Boeing said, prompting the manufacturer to recommend immediate inspections and, likely, repairs that would take about two weeks per airframe. Boeing did not publicly identity the operators or specific airframes affected by the issue, which was first reported by The Air Current.

A source with knowledge of the situation told Aviation Week the affected airframes are from a batch produced sequentially in 2019, and the issues were detected initially when an inspection revealed a problem with shims—material added during assembly that fills gaps between structures or adjusts how pieces fit together to ensure tolerances are met. 

The composite material that makes up the 787 fuselage is extremely stiff when cured. Achieving the correct corner angle between the cured part and final shape is hard to control and predict, so shimming is usually required to make parts of the 787 fuselage sections mate together. In some 787s, Boeing found the shims are not the correct size. 

The second issue is an out-of-tolerance problem with the surface of the inner mold line. The defect areas in the problem aircraft exceeded the 0.005 in. tolerance limit for flatness, the source said.

The eight aircraft flagged by Boeing have both defects.

“Boeing has identified two distinct manufacturing issues in the join of certain 787 aftbody fuselage sections, which, in combination, result in a condition that does not meet our design standards,” the company said in a statement. It informed the FAA and is working to determine “the root cause” of each issue. “In addition, as part of our assessment, we determined that eight airplanes in the delivered fleet are affected by both issues and therefore must be inspected and repaired prior to continued operation,” Boeing added.

Neither problem on its own creates an immediate safety-of-flight issue, Boeing said. But combined in the same physical location, the issues create a situation where the affected aircraft no longer meets limit load conditions, or the maximum load the design is expected to experience in service.

Boeing said it is examining the rest of the fleet to determine the extent of each defect, but it has determined that all other aircraft meet limit-load requirements. It is not clear how many aircraft are affected by only one of the two issues, or what corrective actions might be necessary for them.

“The FAA is aware of the matter and continues to engage with Boeing,” the agency said in a statement

Singapore Airlines (SIA) confirmed that one of its Boeing 787-10s is among the eight aircraft flagged as needing immediate attention. The aircraft concerned “is not in service and we will work closely with Boeing on a solution,” SIA said. SIA has 15 787-10s with 29 more on order, according to Aviation Week Intelligence Network Fleet Data.

The Air Current reported the issue affects the join between the 787’s two composite aft fuselage sections, known as sections 47, which is pressurized, and 48, which is unpressurized and supports the empennage, or tail, section. Both sections are made at Boeing’s North Charleston manufacturing facility, which was purchased from Vought Aircraft in 2009. They are then joined and moved to one of the two 787 final assembly lines—either North Charleston, which assembles all 787 variants, or Everett, Washington, which assembles 787-8s and 787-9s.

Shimming problems at the back of the fuselage are not new. A 2014 review of the 787 program by the FAA noted that Boeing “identified a significantly higher number of nonconformances related to shimming as compared to other fuselage sections” with similar design features. “Aft fuselage shimming issues were identified in production and in the full-scale fatigue test,” the report added.

While most of the problems were found and corrected before delivery, five aircraft entered service with “potentially discrepant shims,” leading Boeing to issue an alert service bulletin to ensure they were fixed.

The 787 fleet is the closest airlines have to a widebody workhorse amid the novel coronavirus pandemic that has gutted demand for air travel, particularly international long-haul services. Aviation Week Intelligence Network Fleet Data showed that 680 of the 987 787s in airline fleets, or about 70%, were active as of Aug. 19, including 86 in a “parked/reserve” status denoting flights one or two times in the previous seven-day period.