By Graham Warwick in Fort Worth & Guy Norris in Los Angeles

Advanced composite/metallic manufacturing and automotive moving-line assembly techniques are needed to build a next-generation fighter affordably

A buggy ride through the cavernous assembly building at Fort Worth shows a changing of the guard getting under way. The F-16 line, which filled this hall in its heyday, is winding down from its recent peak of nine aircraft a month to three a month – a rate Lockheed hopes to maintain until F-35 production ramps up.

Spaces are beginning to be filled as Lockheed installs tooling to support the 12-a-month rate due to be reached by the end of system development and demonstration (SDD) in 2013. The first aircraft has already left the building, and there is now a gap of several months before the next one begins to move down the assembly line, but the sense of a quickening pace is there.

BAE tails AA-1 
© BAE Systems

Taking shape

The acceleration is already evident at partners Northrop Grumman and BAE Systems, where subassemblies are taking shape for the next several aircraft in line. Responsible for the centre fuselage, which has the longest lead time, Northrop has six aircraft “in flow” at its Palmdale, California plant, and will have eight by the end of the year – and 18 by the end of next year. Leading the line is the centre fuselage for BF-1, the first STOVL aircraft and first optimised airframe, which is due to be delivered to Fort Worth in November.

Centre fuselage design work is based at Northrop Grumman Integrated Systems in El Segundo, California, which is also where the composite, bifurcated inlet ducts are fabricated, beginning the process of building an F-35. But the bulk of the company’s assembly work is performed at a new production site in Palmdale at the US Air Force Plant 42 complex.

Northrop has reached the 90% design release point on the STOVL centre fuselage and is “down just to the last brackets and tubes”, says vice-president and F-35 programme manager Janis Pamiljans. Following the five STOVL F-35Bs down the assembly line is the first CTOL F-35A with optimised airframe, AF-1. “We have the new lightweight CTOL going down the assembly line at Palmdale now,” he says.

Meanwhile, “build-to” packages for the composite inlet ducts on the first CV aircraft, CF-1, have just been issued, but the design release level for this third, F-35C, variant of the JSF is less than 10%. “In the summer we will start winding the first ducts, and we plan to begin jig loading on 3 November, when we will have 24 frames around the duct for the outer mould-line skins,” says Pamiljans.

Over in the UK, “the train is leaving the station”, says Tom Fillingham, BAE Systems vice-president and JSF programme manager. Responsible for the aft fuselage and horizontal and vertical tails, the company began assembly activity on BF-1 at the end of May, at its Samlesbury plant in north-west England. “Every six to eight weeks we will start another aircraft in assembly. We are into the heartbeat process,” he says.

Design work is split over three BAE sites: Brough for the horizontal tail, Woodford for the vertical tail, and Samlesbury for the aft fuselage and the outer wing panels for the CV aircraft, which the UK company will also produce. The assembly layout at Samlesbury is copied from the Eurofighter Typhoon line, “and the benefits were evident straight away”, says Fillingham. “The parts went together hand in glove.”

BAE is in the final detail design of aft fuselage and tails for the CTOL aircraft and will release build-to packages towards the end of the year and start assembly of AF-1 early next year. Meanwhile, the CV aircraft is in early design and moves into detail design late this year. Assembly activity on the first aircraft, CF-1, will start towards the end of 2007, says Fillingham.

Assembly activity

Back in Fort Worth, Lockheed is beginning assembly of its own sections of BF-1 – the forward fuselage, redesigned centre wing and outer wingbox. Forward fuselage subassembly is under way, the outer wingbox was loaded on 19 June and assembly of the centre wing will begin on 1 August. In the hiatus between completing AA-1 and beginning final assembly of BF-1 in January 2007, the company is finishing up the installation of the tooling and facilities needed to support rate production.

weapon bay 
© Lockheed Martin  

Self-rigging weapon bay doors simplify assembly

“We are ramping up to 12 a month in 2012, so we need rate tools in place,” says Edward Linhart, Lockheed vice-president F-35 production operations. Although there are fewer subassemblies in an F-35 compared with an F-16, Lockheed is putting in two of every key tool, to ensure a problem will not halt the line. To make room, the F-16 line will be moved elsewhere and, at full rate, the F-35 line will occupy the building, he says.

At Fort Worth, work begins with the arrival of carbonfibre skins for trimming and machining – from Alliant Techsystems for the upper wing, Vought Aircraft Industries for the lower wing and in-house for the forward fuselage. Whereas AA-1 had a one-piece upper wing skin, the new design has seven pieces. A flexible overhead gantry picks up each skin panel and a vacuum tool holds it in place while, after a temperature soak at 23°C (73°) to stabilise the tool, a five-axis machine trims the edges, cuts recesses and drills co-ordinating holes.

Fuselage skins are then flipped over and their inner surfaces machined where they will touch the metallic substructure. “Low observability requires we not have gaps or mismatches,” says Linhart. “We have half the tolerance of the F-22 – 10thou [0.25mm] versus 20thou. The machine is certified to hold 8thou, but is performing better – the 35ft-span [10.7m] wing skin [on AA-1] was trimmed one end to the other within 2thou.”

Afterwards, the tools are washed and dried “like a carwash”, and the composite particles captured, compacted and recycled. The skins then go to the laser ultrasonic test (LaserUT) machine to be scanned for defects. Developed by Lockheed, LaserUT scans composite skins at 10 times the speed of conventional water ultrasonic testing, and no tool is needed to hold the part in precise alignment with the sensor head.

LaserUT uses two lasers: one to heat the part to generate ultrasound through thermoelastic expansion; and the other to detect the ultrasonic vibrations. The system can work up to 45° off-axis, allowing complex shaped parts to be scanned. Lockheed is building LaserUT machines for partners BAE and Northrop, as well as Italy’s Alenia, which is expected to build half of all F-35 wings at rate production.

Northrop’s Pamiljans says assembly of AA-1 showed that metallic and composite parts could “fit brilliantly” together. The normal process is to attach skin to structure and drill through both, then separate them to deburr the holes, but BAE’s Fillingham says the accuracy of the electronic design database is such that the company has begun to drill the parts separately then assemble them. “They line up perfectly. It’s the first time we’ve been able to do this.” Such continuing improvements in manufacturing during development are necessary, he says, to reduce the manhours required to assemble the aircraft and keep it affordable.

AA-1 centre fuselage
© Northrop Grumman 

Northrop's centre fuselage for AA-1 shows the waterline mate (rear) and hard splice (front) since abandoned

Assembly autodrill

Lockheed’s forward fuselage is built in three subassemblies – aft equipment bay, cockpit and forward equipment bay. After mating, liquid shims where the structure touches the skin are machined to maintain the inner mould line and control the gaps. The forward fuselage is then mounted on an autodrill pedestal, the skins temporarily attached, and 3,000 holes drilled and countersunk. The skins are removed and the forward fuselage stuffed with tubing, wiring and systems. The skins are then reattached and the completed forward fuselage tested – including mounting and cycling the nose gear, Linhart says.

Simulation played a key role in developing the assembly concept for the F-35. “We did as much as we could before building anything,” says Linhart. ERGOman digital manikins were used to make sure access holes were provided to allow assembly workers to get to equipment. “We did work-ups in tight places such as the IPP [integrated power package] bay, where we found there was a problem with how the precoolers were installed. You had to drop the whole thing to get them in. We caught that in simulation.”

Simulations used Delmia, the manufacturing software developed by Dassault Systemes and built on its Catia V5 three-dimensional design system. Moving parts like gear and flaps were simulated using the design data to check for interferences. “It still takes proficiency to take Catia into a simulation of components that move against each other,” says Linhart. But the effort paid off. “The only problem when we swung the gear on AA-1 was an actuator that needed an eighth of an inch more stroke. It had not been simulated.”

The redesign to reduce weight significantly changed the wing tooling concept. Beginning with BF-1, the new inner wing module is assembled in a vertical fixture, the work platform moving up as the wing is built up from the rear spar. Below it, a secondary platform provides access to any area needing additional work. The platform system was built by a Canadian company that normally works for Circle du Soleil. “It was their first aerospace job,” says Linhart.

A bright yellow automatic guided vehicle, navigating via lasers and angles on the building walls, picks up the wing and moves it to final drill where, Linhart says, “the mother of all autodrills” – a six-axis gantry with dual independent heads – drills and countersinks 6,700 holes in the lower wing and 4,400 in the upper, through skin and structure. “We’ve had very few defects in autodrill – orders of magnitude less than in manually drilled legacy aircraft,” says Bobby Williams, air vehicle team lead.

One last step before mating is to apply a low-observable (LO) coating to those areas that will be inaccessible after assembly. This is performed by an eight-axis robot in a climate-controlled room. “By hand, you could not hold the accuracy,” says Linhart. “This is a heavy material – 1thou too thick over the whole aircraft would add 400lb - and it needs to go on at 400°F.” An LO facility to coat the completed aircraft is being built at Fort Worth, where the radar cross-section measurement building is already complete.

Electronic alignment

The F-35 comes together in the electronic mate and alignment system, where Northrop’s centre fuselage is spliced first to the forward fuselage and then the centre wing, to which BAE’s aft fuselage is then mated and, finally, the horizontal and vertical tails are joined. The airframe sections are supported by servo-driven jacks that are part of a laser-guided alignment system that automates mating. “When we did the AA-1 aft-to-centre mate, the lasers said we were 12thou too far right. The operator keyed it in and it mated,” says Linhart. “The system provides micrometer-level readings. They’ve never done that before; older laser alignment accuracy was not that great.”

Where to join the aircraft was a lesson learned from the F-22, says Linhart. “There was big problem on the F-22 where the inlet runs across a manufacturing joint and requires close tolerances. On the F-35 we made sure we did not have an inlet mated across a manufacturing joint. The forward-to-centre fuselage joint is ahead of the inlet.” The F-35’s diverterless inlets are also easier to manufacture. “We learned how to take care of hammer shock without drilling thousands of holes in the inlet,” he says. “The bump on the forward fuselage eliminates moveable parts and holes.”

Self-rigging doors

Another first-time technology that has simplified assembly is the self-rigging weapon bay doors. Although extremely rigid and rapid-acting, these doors carry missiles and there can be some deflection under load, but there can be no mismatch of the edges to the fuselage. A system of sensors, drive motors and cams in the hinges pull up the doors so they need no adjustment by mechanics on the line. “It is quite a contraption, but it has worked every time,” says Linhart.

Once mated, the airframe joins what will become a moving final assembly line. At each station, the aircraft is connected up to a swivelling boom that carries all air, cooling, electrical and hydraulic supplies. At the end of the boom’s arc, the aircraft is unhooked and connected to the next station in line. By the final station, all systems have been installed and tested and the aircraft is ready for painting. “We designed it with the same Toyota experts that did Boeing’s 737 moving line,” says Linhart.

A moving line adds pace to assembly and urgency to solving problems that halt the line. Lockheed will experiment with pulsing the line during SDD, and will it start moving in low-rate initial production – just 25mm (1in) per hour at first. By full-rate production the line will be moving at 1.2m (4ft) per hour. “We have not figured out how to install the engine when the aircraft is moving,” says Linhart.

Partners and suppliers are gearing up to feed the moving line. “The rate has to be designed so that the line only moves as fast as the slowest builder. The supply line also has to be ‘just in time’,” says Northrop’s Pamiljans. Using Toyota production system techniques and an integrated assembly line designed by a Detroit-based company specialising in automobile production lines, Northrop is gearing up to deliver around 200 centre fuselages a year to Fort Worth.

Reducing intervals

The line, which will eventually move on rails, will have 82 positions, or stations, and is designed to produce one centre fuselage a day. “On current plans, that’s what we should be producing by 2012-13,” Pamiljans says.

Northrop has already reduced the production interval for the centre fuselage from 30 to 25 working days and will go from “25 to 20 to 15 days” during SDD, Pamiljans says. The interval will further reduce during LRIP until it reaches the planned full-rate production level of around one per day. “That seems aggressive, but it’s not really as it allows you to cycle people, improve efficiency and cut costs even more,” he says. ■

Source: Flight International