Fixing a previously undisclosed 787 design flaw has now emerged as a contributor to Boeing's decision to freeze shipments from suppliers until early June.

Shear ties, which affix the fuselage frames to the skin of the aircraft, now require replacement or rework, after the initial design failed to take into account thermal fatigue loading of the aluminium parts.

The parts in question are located in the aft most part of the fuselage, inside sections 48 and 48 aft, which is the tail cone of the 787. Boeing discovered the problem in December of last year after it was found that repeated cooling and warming of the unpressurized Section 48 and 48 Aft would result in the shear ties pulling away from the skin of the fuselage, potentially compromising the structural integrity of the aircraft.

Boeing says the solution, which will be incorporated on all deliverable aircraft, is a combination of new thicker shear ties, while an additional programme source adds that radius fillers will be employed along side some existing shear ties.

The airframer says that the changes to the shear ties will not impact the current flight test or delivery schedules, and does not present an immediate safety of flight issue to the four flying 787s.

"It is a structural issue in that we are modifying the airplanes but once complete, the structure has full integrity and will meet all FAA requirements," says Boeing.

Rework on Airplane 23, say programme sources, has already begun in Charleston, South Carolina and will be completed in Everett, Washington, as will Airplane 24. The same changes are required for Airplanes four through 22 as well, including three of the six flight test aircraft, which will eventually be delivered. A permanent design solution is planned for introduction beginning with Airplane 55.

Section 47 and 48 are both fabricated by Boeing Charleston, formerly Vought Aircraft Industries, while Section 48 Aft is made by Korean Aerospace in South Korea.

Starting with Airplane 25, shear tie modification of the Section 48 and 48 Aft will be done in Charleston and Korea, respectively.

The 24-manufacturing day hold does not represent a new production schedule as the 787 ramp up continues, rather, the pace of deliveries will quicken as the time between rate breaks is reduced.

One programme source says the shear tie issue was an important driver for the 24-manufacturing day hold, while Boeing downplayed the issue as being on a longer list of contributing factors.

Continued part shortages and design changes were identified by Boeing as the cause of the 24-manufacturing day hold announced on 27 April that will keep structures for Airplane 23 at suppliers until early June.

While accurate, programme sources say that the stoppage can be ultimately traced to the aft fuselage shear tie replacement and rework. Holding structural sections at suppliers, particularly the aft fuselage, allows for the completion of required engineering along with significantly easier access by crews to address the rework.

Once in Everett at final assembly and delivery, the horizontal stabilizer is installed and access to the frames and shear ties in the aft fuselage becomes "extremely limited" they add.

Compared to a year ago, says Boeing VP of airplane programmes Pat Shanahan, design changes to the 787 have declined by a factor of ten or greater, though he acknowledges changes, even small ones, continue to be disruptive to the manufacturing process.

During manufacturing in Charleston, Sections 47, which is the pressurised aft passenger cabin, and Section 48 which houses the aft pressure bulkhead and horizontal stabilizer, are joined, stuffed with systems, wiring, insulation and ducting before being shipped to Everett for final assembly.

Programme sources say Boeing and Vought, which still supplies engineering support to Boeing, are blaming each other for the design oversight.

Boeing has encountered previous disruptions in 787 production from shear ties in 2007 and 2008 when a shortage of shear ties built by Boeing Winnipeg prevented structural frames from being installed, prompting a limited structural completion of assembly at the time of delivery.

Source: Air Transport Intelligence news