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Regular as a heartbeat
Pulse line improves C-17 engine pylon work flow
BY DARYL STEPHENSON
Assembly of C-17 engine pylons is now taking place on a pulse line at Boeing St. Louis. It's the latest in a series of continuous improvements that have employed Lean manufacturing and affordability initiatives to reshape virtually every aspect of the pylon assembly process.
The pulse line is expected to reduce engine pylon assembly costs by 10 percent, improve the assembly cycle by 20 percent and save $6.7 million through production of the 180th aircraft, said Jeff Webb, the C-17 program's Lean and affordability focal in St. Louis.
A pulse line is a type of moving assembly line, but it's one step short of a continuous moving line. In a pulse line, products are positioned sequentially in a straight or U-shaped line on the shop floor. The products stay at those positions until all the planned work is complete.
Then they move to their next stations—in a pulse-like fashion. The work flows faster because of standard processes, visual control systems, and point-of-use delivery of parts and tools.
That's the way it is in C-17 engine pylon assembly, where there are 14 assembly positions in a U-shaped line. All of the engine pylons move along the same line in a process that takes approximately 49 days.
The pulse line began in the fourth quarter of 2000, when members of the C-17 pylon High Performance Work Organization assembly teams, their support organization and representatives from the St. Louis Lean Office participated in a value stream mapping session at the Boeing Leadership Center. They put together a future map that defined a pulse moving line as their goal and a risk mitigation plan to address technical issues.
With management approval and funding, the team conducted a study to identify a technically sound tooling approach that would eliminate unweildy platform jigs that had been used to locate and drill interchangeable features. After the study was completed in March 2001, the assembly team spent two months to mature the tooling concepts and prepare detailed cost and savings estimates.
Now that the pulse line has been implemented, the assembly team and support staffs are continuing to meet each week to review progress and resolve technical issues.
Previously, the C-17 team employed two separate lines to assemble the engine pylons. One line was for the inboard pylons (which fit under the wings near the aircraft fuselage) and the other line was for the outboard pylons (which go under the outer portion of the wings). Each C-17 has four engine pylons.
Under the old assembly configuration, the team had to use complex platform assembly jigs in the middle of each pylon line to perform key locating and drilling operations for various interchangeable features. There were smaller, less complex assembly jigs to locate, drill and assemble components or install electrical and hydraulic systems.
The team also had to use an overhead crane to move the pylons from one assembly jig to another. The use of the crane created a bottleneck when pylons were moved between jigs and also risked damage to components and tools.
The engine pylon line, under the old system, "was inherently imbalanced and required adherence to a rigid schedule sequence," said John Chandler, superintendent of pylon production.
Today, there are not many moving lines like this one in the aerospace industry. It has a number of unique aspects, one of which is that it involves the assembly of complex structures, Webb said.
"Moving lines are gradually being implemented within aerospace. But most are on commercial programs where rates are fairly high or in final assembly where the work revolves around installations," he said.
Because of the relatively low rate of C-17 production, about 15 aircraft per year, it's not likely that the pulse line will be converted to a continuously moving line. Even so, it's having the desired effect. "The very nature of a moving line, either pulsed or continuous, creates a sense of urgency," Webb said. "It ensures that the desired velocity is maintained throughout the assembly cycle."
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