Types of Aircraft Fuselages: Truss, Monocoque and Semimonocoque

Four Fuselage Structures

Truss: A Frame Wearing a Skin

The FAA's pilot handbook says older aircraft used an open truss of wood, steel or aluminum tubing. Lengths of tubing called longerons are welded in place to form a braced framework. Vertical and horizontal struts are welded to the longerons and give the structure a square or rectangular shape when you look at it from the end. Extra struts resist stress that can come from any direction, and stringers and bulkheads, also called formers, shape the fuselage and support the covering.

The maintenance handbook adds that the frame is usually welded steel tubing, put together so that every member can carry both tension and compression. Its drawing of a Warren truss shows mostly diagonal bracing. Some light, single-engine aircraft use aluminum alloy instead, riveted or bolted into one piece, with solid rods or tubes for cross-bracing.

The main drawback, according to the pilot handbook, is that a truss has no streamlined shape of its own. Designers fixed that by enclosing the truss, first with cloth fabric and later with lightweight metals such as aluminum. In some cases the outside skin ended up carrying all or a major part of the flight loads, and that's where the next two structures come from.

Monocoque: The Beverage Can

Monocoque is French for "single shell." The FAA compares it to an aluminum beverage can. The skin supports almost all the loads, so there's little internal bracing, which saves weight and leaves the inside open. Formers, frame assemblies and bulkheads give the shell its shape, with the heaviest ones placed where loads are concentrated and where the wings, powerplants and stabilizers attach.

The can comparison also shows the weakness. A can holds up considerable force at its ends, but if the side is dented slightly while it's carrying a load, it collapses easily. The handbook says a monocoque isn't tolerant of surface deformation, and that keeping enough strength inside an allowable weight is the biggest problem with the design.

One early solution came from Jack Northrop in 1918, for the Lockheed S-1 Racer. Two molded plywood half-shells were glued together around wooden hoops or stringers. To make each half, three large sets of spruce strips were soaked in glue and laid in a semicircular concrete mold shaped like a bathtub. A rubber balloon was inflated under a tightly clamped lid to press the plywood against the mold. A day later the smooth half-shell was ready to join its partner, and each half was less than a quarter inch thick.

The FAA says pure monocoque construction then went quiet for several decades because of the complexity involved. Its everyday survivor is the unibody car.

Semimonocoque: Why Most Aircraft Look This Way

Semimonocoque, "partial" or "one-half" monocoque, keeps the stressed skin and adds a substructure to share the work. The bulkheads or formers and the stringers take some of the bending stress off the skin. The main section of the fuselage also includes the wing attachment points and, on single-engine airplanes, a firewall behind the engine. That's a fireproof partition, usually heat-resistant stainless steel, that protects the pilot and passengers from an engine fire.

The maintenance handbook spells out the parts:

• Longerons are the heavy longitudinal members. They usually run across several frames and help the skin carry the primary bending loads. They're typically aluminum alloy, either one piece or built up.
• Stringers are more numerous and lighter, in a variety of shapes, usually aluminum extrusions or formed aluminum. They have some rigidity but mainly give the skin its shape and something to attach to. Longerons and stringers together stop tension and compression from bending the fuselage.
• Frames, bulkheads and formers hold the cross-section shape. The handbook notes that manufacturers use different names, and there's often little difference between a ring, a frame and a former.
• Web members are extra bracing that can be installed vertically or diagonally between longerons and stringers.

Materials are primarily aluminum and magnesium alloys, with steel and titanium sometimes used where temperatures are high. None of the parts is strong enough alone to carry flight and landing loads. Together, joined with gussets, rivets, nuts and bolts, screws and in some cases friction stir welding, they form a strong, rigid framework. A gusset is a bracket that adds strength at a joint.

The payoff is in how it fails. Because loads are spread through the skin and the substructure, no single piece is failure critical. The handbook says a stressed-skin semimonocoque fuselage can take considerable damage and still hold together, and the skin thickness can vary with the load at each location.

Fuselages are normally built in sections and joined afterward. Small aircraft are usually made in two or three, and larger ones can have six or more.

Composite Fuselages

The FAA dates composites in aircraft to the Second World War, when soft fiberglass insulation went into B-29 fuselages. By the late 1950s European high-performance sailplane makers were using fiberglass for primary structure. In 1965 the FAA type certified the first all-fiberglass aircraft in the normal category, a Swiss sailplane called the Diamant HBV, and four years later it certified a four-seat, single-engine Windecker Eagle. By 2005, the handbook says, over 35 percent of new aircraft were built of composite materials.

A composite is a fiber-reinforced matrix. The matrix is the glue that holds the fibers together and gives the part its shape when it cures, and the fibers carry most of the load. The most common matrix in aircraft is epoxy resin, a thermosetting plastic. The two most common fibers are fiberglass and carbon fiber.

The advantages the handbook lists go beyond weight, and it warns that building in composites doesn't automatically make an airframe lighter. The more important gain is drag. A very smooth, compound-curved composite shape reduces it, which the FAA gives as the main reason sailplane designers moved from metal and wood to composites in the 1960s. It credits the same effect for the performance of the Cirrus and Columbia production aircraft despite their fixed landing gear. Composites also help hide the radar signature of stealth designs such as the B-2 and F-22.

Two more advantages: corrosion and fatigue. The handbook describes the all-composite fuselage of the Boeing 787 as designed for a higher cabin pressure differential and more humidity than earlier airliners, because engineers worry less about moisture condensing on hidden skin behind the insulation. And in flexing parts such as helicopter rotor blades, composites don't suffer metal fatigue and crack growth the way metals do.

The hidden damage problem

The FAA calls the lack of visual proof of damage the most important disadvantage. If a car backs into an aluminum fuselage, it leaves a dent you can see. A composite can behave differently:

Anytime you suspect an impact, even a minor one, the handbook says to have an inspector familiar with composites look at the structure. If an impact leaves delaminations, crushing or a puncture, repair is mandatory. Many composite parts are thin skins over a honeycomb core, which is stiff but easy for water to get into. A piece of speed tape over a puncture keeps rain out, but it isn't a structural repair, and a paste filler is a cosmetic fix only.

The Five Stresses

All aircraft structures are subject to five major stresses. Designers mostly arrange structural members to carry tension or compression rather than bending.

Pressurization

Pressurized aircraft pump air into the cabin after takeoff so passengers can breathe normally and move around at high altitude without special equipment. The pressure difference between the cabin and the outside air is regulated and maintained, and it puts significant stress on the fuselage and adds complexity to the design.

There's a second load on top of the pressure itself. Cycling from unpressurized to pressurized and back on every flight causes metal fatigue. Nearly all pressurized aircraft are semimonocoque to cope with both, and their structures get extensive periodic inspections so damage is found and repaired. Repeated weakness in one area may mean that section of the fuselage has to be modified or redesigned.

Where Things Are on a Fuselage

With thousands of similar-looking frames and stringers, technicians need a way to point at one. The maintenance handbook describes a system of stations measured in inches from a reference datum, an imaginary vertical plane at or near the nose.

• Fuselage station (FS): the distance back from the datum along a line through the center of the fuselage. A blueprint that reads "fuselage frame station 137" points to a frame 137 inches behind the zero station. Some manufacturers call it a body station.
• Buttock line (BL): a vertical reference plane down the center, used to measure left and right.
• Water line (WL): height in inches from a horizontal reference plane, usually the ground, the cabin floor or another easy reference.

The numbering systems aren't the same across manufacturers, so the handbook says to check the one that applies before trying to locate a structural member.

Where the Engines Go

On a single-engine aircraft the fuselage houses the powerplant. On multiengine aircraft the engines may be inside the fuselage, attached to it or hung from the wing. For the engine types themselves, see our guide to aircraft engines. The empennage usually includes a relatively light tail cone that closes and streamlines the rear fuselage. The tail is covered in our guide to tail structures, and flaps and other high-lift devices are in the guide to flaps.

Frequently Asked Questions

What are the main types of aircraft fuselage?

The FAA names truss and monocoque as the two general types. Monocoque divides into monocoque, semimonocoque and reinforced shell classes, and most modern aircraft are semimonocoque.

What's a semimonocoque fuselage?

A stressed skin supported by longerons, stringers, frames and bulkheads. The structure spreads loads between the skin and the substructure, so no single piece is failure critical.

What's the difference between a longeron and a stringer?

Both run lengthwise. Longerons are heavier and help the skin carry primary bending loads. Stringers are more numerous and lighter, and mainly shape the fuselage and give the skin something to attach to.

What's a truss fuselage made of?

Usually welded steel tubing arranged so every member can carry tension and compression, covered with fabric. Some light aircraft use riveted or bolted aluminum alloy with rod or tube bracing.

Why are pressurized aircraft semimonocoque?

Pressurization stresses the fuselage, and cycling between unpressurized and pressurized on each flight causes metal fatigue. The FAA says nearly all pressurized aircraft are semimonocoque to cope with those stresses.

Is composite damage always visible?

No. A low energy impact such as a dropped tool may leave no mark on the surface while causing delamination underneath, so the FAA says to have an inspector familiar with composites check any suspected impact.

Related guides: Types of Aircraft Flaps: Plain, Split, Slotted and Fowler, Types of Aircraft Engines: Piston, Turboprop, Turbofan and More.