To attain its remarkably rapid production rate of P-51 Mustangs, North American employs a component assembly technique calling for a minimum of "final assembly." Equipment and accessories are virtually completely installed before the components are joined.
There are five major assembly lines: Engine accessories, fuselage, fuselage tail cone, empennage, and wings. A number of minor assembly lines turn out landing gear, pilot housings, fuselage fire walls, and the like.
The Mustang laminar flow wing consists of two outer panels joined together without a wing center section. As a production component, it consists of four quarters: Right and left leading edge sections, and right and left trailing edge sections.
Assembly of leading edge sections is started in stationary jigs, with two or more parts being joined. These, in turn, are joined, step-by-step, until a substantial section is completed.
Spars, for example, start as short, preformed channel sections to which stiffeners and brackets are added. These are joined an outboard to an inboard section to form a leading edge or main spar. Then begins the addition of ribs, stringers, and skin, all done in stationary jigs. To the spar is added already assembled outer and inner portions and the main landing gear casting. Combined, these represent roughly 30 percent of a completed wing.
When securely joined, the leading edge section of a right or left wing panel is attached to a floor carrier, the casters running on the floor itself, where it will move continuously through 24 stations of two legs of a line, to emerge with a major portion of its installations already in place, for joining to a trailing edge section. The latter, likewise, begins in jigs and is transferred to a floor carrier line, but this one has only one leg, ending at the joining jig for union with the leading edge section.
The two "quarters" are removed from their carriers and literally "stuck" together in another carrier, in a vertical position with the leading edge held in the cradle of the carrier and the trailing edge resting on top of it. In this carrier, the stuck-together sections they have not been riveted or in any way actually fastened together as yet move into a master joining jig, where they become a right or left wing panel, as the case may be.
Upon emerging from the master jig, the now securely joined panel moves into the first of two legs of a continuously moving line containing 17 stations, where tie-together work is finished and installations are made. After emerging from a paint booth at the midway point (the right and left panels alternating on the line) they are joined before progressing back toward the starting point of this two-leg line.
On this second leg, major installations are made, including the landing gear and the control stick mechanism. When the wing emerges, still on its carriers, from the last station of this line it is trundled by a crew of handlers to the head of the final assembly line where it is transferred to another carrier, this time in a horizontal position, and the fuselage is lowered onto it and joined, in the first station.
A feature of the leading edge line that has contributed considerably to its efficiency is the extreme to which the work is broken down. So specialized are the jobs that only one, two, or three persons work in each station. Because they do only one or two small phases of the work in each of these stations, and there is no congestion of workers, they become highly proficient and can do their assigned jobs while the line moves slowly through the station.
Another feature of this line is overhead compressed air and electric lines, with tools suspended from them on trolleys that travel with the work through the station. This eliminates floor congestion and prevents excessive breakage of tools and snarls of hose and electric cords that would interfere with the movement of the workers. The carrier casters run on the floor astraddle a single angle iron rail which guides them by means of a slotted extrusion of the frame which slides along the rail. Power is applied through a dragline with connecting links between carriers.
One of the unusual features of the master jig, where trailing and leading edge sections of the wing panels are riveted together, is the accessibility provided. Critical points are established and the work is firmly held at the ends, overhead, and in the center at the bottom. There are no side members and very little framing in this jig. Workers can get close to each area and perform their tasks in normal standing or sitting positions.
The final wing line, too, is of rather unique design. Here the carriers roll along the floor through a well between working platforms. Although only 30 in across, this well provides room for some of the workers to stand on the floor while others are on the platform. Work progresses on both top and bottom of wing surfaces simultaneously as the sections pass steadily through the stations of the line.
Throughout the plant, which of course includes the wing lines, stock is stored adjacent to its point of use. There is no routine or paper work involved in obtaining it. The installer merely reaches over to the bin and takes out the part as he needs it. Production control responsibility ends with the delivery of parts to these point-of-use racks and bins. Responsibility for damage and stock-on-hand is placed on the shoulders of leadmen and other supervisors.
An example of the team work between designing and production departments, mentioned earlier, occurred recently when installation of certain hydraulic equipment was shifted from the fuselage to the wing departments. Production found that it could do its work more efficiently if the location of certain hydraulic valves and tubing could be shifted from the fuselage to the wing. Design promptly ordered the change and found a place in the wing for the bothersome equipment. Similarly, the control stick installation, normally made when the fuselage is in place on the wing, was shifted to the wing department because this permits rigging of the aileron by the wing department, a function normally accomplished by final assembly.
A point it might be well to stress here is that the layout of the several departments adheres to a rule that the travel of finished components is never far, generally not over 20 ft, before the components reach their next point of use. The wing department is laid out right and left of a horizontal center line, with leading edge work being done on one side and trailing edge on the other. Both flow toward the center. Final wing assembly line starts at this point and returns to it when the wing is completed. It is out through this center line, actually an aisle, that the completed wing is trundled to the final/assembly line for joining to the fuselage.
In the fuselage department, the breakdown of components is carried to a greater extreme than is common in aircraft manufacture. The main fuselage section begins in stationary jigs where the air scoop, bottom, top deck, and side panels are assembled as small units, each growing by the addition of other assemblies until a completed component is obtained, in most instances with a major portion of its installation in place. Here, too, the jigs are notable because of their wide open design that permits the fullest possible accessibility. Installations begin, as a rule, in the third or fourth step through which such assemblies pass. The wide open design of the jigs makes possible this early start on installation work.
The tail section or "cone", having few installations, starts in a stationary jig and then passes through a short, four-station line, the last one of which is for installations. Four of these lines, side-by-side, provide the necessary volume.
Side panels, likewise, start in stationary jigs but are transferred at an early stage to carriers on which they progress through three stations before entering a paint booth where the interior side is sprayed. Upon emerging from the paint booth, the side panels, in two lines (rights in one, lefts in the other) progress through four installation stations before entering a master jig. Here the two sides, the top and two bottom panels, one for the radio equipment and the other the structural members of the pilot’s cockpit, are joined into a main fuselage section.
Upon emerging from the master jig the now joined fuselage is mounted on a castered carrier for the trip through two legs of a rather long, continuously moving line where, in the stations of the second leg, the tail cone, empennage, and engine are added.
The carrier used on this double-back line rolls on four grooved caster wheels which run on an inverted angle iron track. A triangular extension on the front end, to which two additional, smaller, casters are attached, provides additional support and stability when the heavy engine assembly is added. Work stands, two on each side of the upright supports to which the assembly is attached by means of bolts through brackets that will later line up with other brackets on the wing, slide along the lower side platforms, permitting their being shifted to desired locations.
Engines mounted on castered stands enter the assembly area at right angles to, and across the head end of, the initial side panel lines. After progressing past those lines, they make a change of direction from sidewise to lengthwise, the front end forward, where equipment, accessories, cowl framework, and finally cowling are added in twelve stations. From the last engine line position the power plant moves through an aisle over to the second leg of the final fuselage line where it is bolted in place.
With the engine joined, the now nearly completed fuselage progresses through five additional stations where connections are completed and electrical continuity and mechanical installations are checked before the fuselage is moved sidewise into the first station of the final assembly line. There it is raised from its carrier and lowered onto the wing (already mounted on a final line carrier about normal landing gear height) and the two are joined.
Traveling backward, tail foremost, the ship passes through ten stations where final hook-up of the wing fillets is completed, its propeller added, and final clean-up is performed. Station No 8 is company inspection, Station No 9 cleans up "squawks", and No 10 is government inspection. After that comes camouflaging, installation of armament, tune-up, shake down, and test flights.
After acceptance, planes to be shipped to distant fronts proceed to the shipping departments where wings, propeller, empennage, and other large components are removed and boxed for shipment. Those going to airfields in this country are, of course, turned over to the AAF for flight delivery.
Not only is the Mustang "designed for production as well as tactical purposes," but Royal Air Force authorities say it is the easiest plane to reassemble that they import. Because of the design that makes possible such rapid assembly and reassembly, the Mustang is also an easy plane to maintain and repair. In wartime, when, because of difficult operating conditions, damage to planes is frequent, that is a factor of more than usual importance.
This article was originally published in the July, 1943, issue of Aviation magazine, vol 42, no 7, pp 162-167, 325.
The original article includes 15 photos of the P-51 assembly line, and a full-page flow diagram of the Mustang assembly line.
Photos are not credited but are certainly from North American.
The model shown appears to be the P-51 or P-51A.