Jumbo Jet (747-class)

Overview

Boeing's standard line about the 747 is that it contains six million parts, half of them fasteners, and the second half of the sentence explains the first. A jumbo jet is not complicated because any one piece is exotic; it is complicated because a 70 m pressurised aluminium structure is held together by rivets installed one at a time, wired with 274 km of cable terminated one crimp at a time, and finished with an interior whose clips and brackets are counted in the millions. This BOM distributes those six million parts the way the aircraft actually carries them.

Structure

The fuselage is built as five great barrels, sections 41 through 48 in Boeing numbering, joined at circumferential splices. Each Fuselage Section is a lattice of Fuselage Frames and Stringers under a chem-milled Fuselage Skin Panel shell, and each carries a rivet population in the hundreds of thousands: every skin-to-stringer, frame-to-clip and splice joint is a separate Rivet / Lockbolt hole, drilled, filled and inspected. The five sections together account for over two million fasteners. The structure holds an 8.9 psi pressure differential, which at cruise altitude means the cabin breathes at an effective 2,400 m while the air outside would be unconscious in seconds. Ten plug-type Passenger Doors and 400 fail-safe dual-pane Cabin Windows penetrate the pressure vessel.

Each Wing is a fuel tank that flies. Three Wing Spars and 48 sealed Wing Ribs box a volume that holds most of the 216,840 L fuel load, skinned with machined Wing Skin Panel planks up to 32 m long. Wing fastening, skin planks to stringers, spar chords, rib feet, adds another 430,000 fasteners per side. Thirteen High-Lift Surfaces per wing, triple-slotted Fowler flaps behind and Krueger flaps ahead, nearly double the lift coefficient so a 285 t landing weight can arrive at about 290 km/h. The Empennage repeats the construction in miniature, with a Horizontal Stabiliser that doubles as a 12,490 L trim tank on the 747-400, letting the flight management system shift the centre of gravity aft in cruise to cut trim drag.

Engines

Each of the four High-Bypass Turbofans is a ~25,000-part machine in its own right. The 2.4 m Fan Module supplies about 80% of the thrust; behind it the Compressor Module packs 4,300 Compressor Aerofoils and vanes into fifteen stages for a ~30:1 pressure ratio. The Combustor burns roughly 3 kg/s at takeoff, and the Turbine Module extracts ~60 MW with Turbine Aerofoils of single-crystal nickel running above their alloy's melting point on internal film cooling. Most of the engine's count, though, is the 18,700-strong population of Engine Fastener / Small Parts, the seals, tube runs, clips and bolts that plumb and dress the core inside its Nacelle & Thrust Reverser.

Systems

The Landing Gear Set puts 18 wheels on the pavement: a two-wheel Nose Gear and four four-wheel Main Gear Bogie bogies, two hung from the wing and two from the body. The body gears steer to keep the tyres from scrubbing in tight turns, each Shock Strut is forged from 300M steel, and each wheel carries a Carbon Brake Assembly sized so the set can absorb on the order of a gigajoule in a rejected takeoff.

The Electrical System is where the wiring legend lives: 274 km of wire, run as 750 Harness Looms containing 170,000 individual Harness Wires, every end finished by the Wiring Terminations: machine-crimped Crimp Contacts, in-line Wire Splices and Terminal Lugs that outnumber the wires three to one. Four 90 kVA Engine Generator (IDG)s, one per engine, feed the 115 V / 400 Hz buses. The Hydraulic, Fuel & Air Systems group runs four independent 3,000 psi hydraulic circuits, any one can land the aircraft, plus the fuel and air systems, and its 16,000 Tube Segments are joined and restrained by Fluid-Line Fittings, the couplings, B-Nut Union unions and clamps that run to a six-figure population.

The Cabin Interior seats 416 to 524 across two decks. Its visible furniture, 467 Seat Assembly units, six Galleys, fourteen Lavatory Module modules, 120 Overhead Stowage Bins and 1,450 Cabin Lining Panels, is dwarfed by what holds it on: nearly a million Trim Screw / Clips, clips and seat-track fittings across ~520 m² of cabin. On the 747-400, the Avionics Suite suite of six display units and dual flight management computers retired the flight engineer's panel and brought the crew to two.

The arithmetic of six million

Roll the tree up and the number falls out of ordinary hardware: about three million rivets and lockbolts in the airframe, six hundred thousand electrical terminations, six hundred thousand fluid fittings, close to a million interior fasteners, and the engines' small-parts populations. At peak 747 production Boeing's Everett plant, itself the largest building in the world by volume, built for this aircraft, assembled that count into a flyable machine in roughly four months per airframe.