Flying through assembly

Manufacturing Engineering, Sep 2001 by Chalmers, Raymond E

New tooling concept drives major advances in aircraft production

Ever read Airframe? The thriller by Michael Crichton involves a fictional aircract manufacturer's struggles over tooling, manufacturing, equality assurance, and at least one chase through a massive aircraft assembly building. Real-life aircraft assembly may not be the stuff of best-sellers, but the results are thrilling in their own right: a major shift in how commercial aircraft are built, saving millions of dollars and significantly reducing assembly time.

The Long Beach, CA, division of Boeing is the home of the 717, Boeing's commercial aircraft designed for short-haul, high-frequency routes, which carries approximately 100 passengers. Described by the company as the only new 100-seat jetliner in service today, the 717 actually has a design history going back 30 years as a variant of the McDonnell Douglas DC-9 aircraft, formerly built in Long Beach. Following the Boeing/MD merger in 1997, Boeing cancelled the MD-80, MD-90, and MD11 programs, leaving the 717 as the only commercial aircraft built on site.

Assembly takes place in Building 80, with more than 580,000 ft^sup 2^ (53,940 m^sup 2^) of assembly area. The best time achieved for assembling MD-80 and MD-90 aircraft there, from the start of joining "barrels" (fuselage sections) until the completed aircraft was ready for paint, was 65 days. In June 1999, Boeing's Lean Manufacturing office met with production, tooling, and industrial engineering departments to establish requirements for cutting the 717's assembly time 50%.

Originally, seven aircraft at a time were assembled in Bldg. 80. Aircraft were parked at an angle at each assembly station, and equipment and workstands, all with individual power stations, were brought to each one. Line moves, or moving aircraft to the next assembly station, took 10 hours over two shifts, and involved downjacking the aircraft from its workstation, bringing in a powered cart and driver, towing the aircraft to its next station, and jacking it in place for the next operation.

Boeing's production department envisioned a moving assembly line, towing seven aircraft in a straight line through Bldg. 80 on their own landing gear, each aircraft pulled by a powered cart. Support equipment and workstands would be pulled along with each aircraft.

On initial investigation, though, Boeing's tooling department quickly discovered there were no AGVs [automatic guided vehicles] existing that were powerful enough to tow the aircraft along with its support equipment. "As you go through the build process, the weight of the aircraft almost doubles, and CG [center of gravity] changes, too," says Kenneth F. Farah, tooling engineer. Also, Boeing was not content with the long lead times involved with engineering custom AGVs for their use, as well as determining safety requirements, vehicle reliability, and the need for spare AGVs. As precision alignment of each aircraft is critical during assembly, it also was discovered (by investigating the concept of towing the aircraft through the factory following a guide wire in the floor) that the 717 would not track straight on its landing gear.

According to David R. Potts, tool engineering manager, many McDonnell Douglas and Boeing assembly ideas from the past involving moving tooling were investigated, including "skate tools" from the DC-9, moving docks from the DC-10, and a "wing-tip dolly" from the 767. To supply power to the moving assembly line, overhead buss bars and air hoses on inertia reels were proposed, eliminating the need to dig any trenches in the floor for powered guide rails. This alternative wasn't popular with production, though, because it prevented use of the overhead crane.

By August 1999, Boeing's tooling engineers began investigating what came to be called the "carwash concept," keeping the aircraft jacked on moving line dollies throughout the assembly process while a conveyor chain in the floor moves the dollies down the assembly line, eliminating carts entirely.

According to Craig Anderson, tooling engineer and project manager for the moving line tool design team, the advantages of the car-wash concept quickly became apparent. "What sold the concept to management was the possibility of doing pulsed line moves to each assembly station (the initial phase before moving to a continuously moving assembly line) in an hour. We stopped pushing everything off on third shift." Additionally, the concept of moving seven aircraft at a time on dollies connected to a chain conveyor made it easier to keep equal distance between aircraft and to keep them at the same height for assembly operations. Putting a chain in the floor also introduced the opportunity of using an underground buss bar to supply constant power and provide fire safety supervision. Electric air compressors could ride on the workstands and power pneumatic tools.

By November 1999, Boeing tooling engineers were touring such plants as Toyota's truck plant in Paramount, CA, Saturn's plant in Spring Hill, TN, and SST Trucking in Garland, TX, all users of powered conveyors for assembly operations. Not only did they gather information on methods for pulling vehicles, but also part-installation processes and staging, and safety and emergency protocols. By January 2000, a formal tool-design study covering process, cost, and scheduling for a moving aircraft assembly line towing seven aircraft on dollies with workstands on rails, all pulled by an underground chain conveyor system, was submitted to Boeing management and approved.