Aircraft Structures — Components, Spars, Ribs & the Five Stresses

Major components

Every airplane breaks down into the same principal units.

Exploded view of a light airplane labeling the wings, fuselage, powerplant, landing gear, stabilizers, and flight controls. — **Principal airframe units.** The same groups apply to any fixed-wing airplane, from a trainer to a four-engine jet. Source: FAA Aviation Maintenance Technician Handbook – Airframe (FAA-H-8083-31B), Vol. 1, Ch. 1 “Aircraft Structures”, Figure 1-13. U.S. government work, public domain.

Unit	What it does
Fuselage	Main body. Carries crew, payload, and systems, and ties the wings, tail, and (often) landing gear together.
Wings	Make lift and carry it into the fuselage as bending, shear, and torsion. Often hold fuel and support engines and landing gear.
Empennage / stabilizers	Horizontal and vertical stabilizers give pitch and yaw stability and carry the elevators and rudder.
Flight controls	Primary: ailerons, elevator, rudder. Secondary: flaps, slats, spoilers, trim tabs. Hinge and actuator loads feed into spars and ribs.
Powerplant	Engines, mounts or pylons, and nacelles. Thrust, weight, and gyroscopic loads go through the mounts into the airframe.
Landing gear	Takes ground, braking, and side loads, which are some of the largest concentrated loads the structure sees.

Semimonocoque fuselage

Skin and internal framework share the load.

A monocoque (“single shell”) fuselage relies on the skin alone, like an egg or a can. It is efficient until it gets dented, and then it loses strength fast. Nearly all metal aircraft instead use semimonocoque construction. The skin carries a large share of the load (mainly shear and pressurization hoop tension), and an internal framework stiffens it and keeps it from buckling.

Cutaway fuselage section showing riveted skin over ring-shaped formers and a solid bulkhead at one end. — **Skin, formers, and bulkhead.** Formers hold the cross-section shape; the bulkhead is a heavier, often solid or pressure-carrying frame. Source: FAA Aviation Maintenance Technician Handbook – Airframe (FAA-H-8083-31B), Vol. 1, Ch. 1 “Aircraft Structures”, Figure 1-16 (“An airframe using monocoque construction”). U.S. government work, public domain.

Parts of the shell

Skin: riveted aluminum sheet. Carries shear, torsion, and cabin-pressure hoop stress. Forms the aerodynamic surface.
Formers / frames: rings that set the cross-section shape and break the skin and stringers into short, buckle-resistant panels.
Bulkheads: heavier frames at load points, such as wing and gear attachment, cockpit, and pressure boundaries. A pressure bulkhead closes the pressurized hull.
Stringers: light, numerous longitudinal members (formed or extruded) that stiffen the skin and carry fuselage bending as axial tension and compression.
Longerons: heavier longitudinal members, fewer in number, used where loads concentrate.

Why it works

Fuselage bending (tail loads, payload weight, landing) is carried mostly by the stringers and skin as tension on one side and compression on the other. Frames keep the compression side from buckling. Twisting from rudder loads and asymmetric gust loads is carried as shear flow around the closed skin. With so many redundant load paths, one cracked stringer or frame usually doesn't cause immediate failure. That is the basis of damage-tolerant design, and it's why small cracks have to be found and repaired.

Wing structure

A cantilever beam that also has to hold an airfoil shape.

Two wing cutaways: left shows nose ribs, ribs, front spar and rear spar; right shows stringers running spanwise under the skin and through the ribs. — **Wing structure nomenclature.** Left: nose ribs ahead of the front spar, full ribs between front and rear spars. Right: spanwise stringers stiffening the skin. Source: FAA Aviation Maintenance Technician Handbook – Airframe (FAA-H-8083-31B), Vol. 1, Ch. 1 “Aircraft Structures”, Figure 1-23. U.S. government work, public domain.

Detailed metal wing cutaway labeling leading edge, trailing edge, rows of rivets, skin, lightening holes, spar webs, spar caps, spanwise stringers, and ribs. — **Stressed-skin wing cutaway.** Rows of rivets fasten the skin to spar caps, stringers, and rib flanges; the ribs carry flanged lightening holes. Source: FAA artwork (credited “wingstructure © FAA, Public Domain”) as labeled in J. Gordon Leishman, *Introduction to Aerospace Flight Vehicles*, Ch. 10 “Aerospace Structures” (Embry-Riddle Aeronautical University, doi:10.15394/eaglepub.2022.1066). Public domain per the book’s Media Attributions.

Wing cross-section showing skin, an I-section main spar with web and top and bottom spar caps, a C-section rear spar, a rib with lightening holes, and the dashed neutral bending axis. — **Wing section: main spar, rear spar, and the neutral axis.** Spar caps sit as far from the neutral bending axis as the airfoil allows, where they do the most good in bending. Source: © J. Gordon Leishman, J. Gordon Leishman, *Introduction to Aerospace Flight Vehicles*, Ch. 10 “Aerospace Structures” (Embry-Riddle Aeronautical University, doi:10.15394/eaglepub.2022.1066). Licensed CC BY-NC-ND 4.0; reproduced unmodified for non-commercial training use.

Spars

The main spanwise beams, running from the root to the tip. Most metal wings have a front (main) spar and a rear spar. Larger aircraft may add more. The spars carry most of the wing bending and vertical shear into the fuselage attachment.

Spar caps

The top and bottom flanges of the spar. They carry bending as axial load. In normal positive-g flight the top cap is in compression and the bottom cap is in tension. Putting them at the top and bottom of the airfoil, far from the neutral axis, gives the most bending stiffness for the weight.

Spar webs

The vertical plate joining the caps. It carries shear and holds the caps apart. Webs are often thin, so vertical stiffeners are added to stop them from shear-buckling.

Ribs

Chordwise members that give the wing its airfoil shape. They support the skin and stringers, carry air loads from the skin into the spars, and break the stringers into shorter, stronger columns. Heavier ribs are used at the root, at engine, gear, and flap fittings, and at fuel-tank ends.

Nose ribs

Short ribs ahead of the front spar that shape and support the curved leading-edge skin. They take the high local air pressure near the leading edge.

Stringers

Spanwise stiffeners riveted to the inside of the skin. They help carry bending, especially compression in the upper skin, and break the skin into small panels that resist buckling.

Skin

In a stressed-skin wing the skin is structure, not just a covering. With the spars and ribs it forms a closed torque box that carries twisting as shear flow, and the skin-stringer panels carry part of the bending.

Lightening holes

Holes cut where a web carries little load, to save weight. They are usually flanged (edge-formed), which stiffens the web against buckling so most of the lost strength comes back. They also make room for wiring, tubing, fuel flow, and access. Treat them as engineered features: you can't add, enlarge, or move one without engineering approval.

Spar construction types

Different ways to build the same beam: caps far apart, joined by a web.

Four metal spar types: extruded I-beam spar with lightening holes, built-up riveted I-beam box spar, tubular spar with welded extruded tube caps, and built-up I-beam spar with web stiffeners and T-section caps. — **Metal spar construction types.** Extruded I-beam, built-up box, tubular (welded), and built-up I-beam with web stiffeners and T-section caps. Source: © J. Gordon Leishman, J. Gordon Leishman, *Introduction to Aerospace Flight Vehicles*, Ch. 10 “Aerospace Structures” (Embry-Riddle Aeronautical University, doi:10.15394/eaglepub.2022.1066). Licensed CC BY-NC-ND 4.0; reproduced unmodified for non-commercial training use.

Type	How it is built	Notes
Extruded I-beam	One extrusion with caps and web in a single piece, sometimes machined to taper.	Simple, with few fasteners. Common on smaller aircraft and secondary spars.
Built-up I-beam / box	Separate cap extrusions riveted to a sheet or plate web. Two webs make a box spar.	Thickness can change along the span. The riveted joints also stop cracks from running, which suits fail-safe design.
Tubular	Extruded tube caps joined by welded web members.	Mostly found on light aircraft and in truss-style structure.
Built-up with web stiffeners	Thin web with vertical stiffeners and T-section caps.	Stiffeners stop the thin web from shear-buckling, so the web can be lighter. Typical of transport-category wings.

The FAA Airframe handbook also describes wood spars, truss spars, plate-web spars, sine-wave spars, and fail-safe spars with riveted webs.

The five stresses

Tension, compression, shear, torsion, and bending. How each one shows up in the wing and fuselage.

Five cylinders illustrating compression (pushed together), tension (pulled apart), shear (ends sliding opposite ways), torsion (twisted), and bending (curved). — **The five basic stresses.** Compression, tension, shear, torsion, and bending. Real structure almost always sees several at once. Source: © J. Gordon Leishman/ERAU, J. Gordon Leishman, *Introduction to Aerospace Flight Vehicles*, Ch. 10 “Aerospace Structures” (Embry-Riddle Aeronautical University, doi:10.15394/eaglepub.2022.1066). Licensed CC BY-NC-ND 4.0; reproduced unmodified for non-commercial training use.

Stress	What it is	Where you see it
Tension	Pulling apart. The part tends to stretch.	Lower wing skin, stringers, and the bottom spar cap in flight. Pressurized fuselage skin (hoop and longitudinal tension). Tension is where fatigue cracks grow, which is why lower wing surfaces get close inspection.
Compression	Pushing together. The part tends to shorten or buckle.	Upper wing skin, stringers, and the top spar cap in flight. On the ground the wing hangs on its supports, and the upper side can go into tension while the lower side is compressed. Compression members usually fail by buckling, not crushing, so stringer and rib spacing matters.
Shear	Layers sliding past each other.	Spar webs carrying the wing's vertical load. Skin panels in a torque box. Rivets in a lap joint. Rivets are designed to be loaded mostly in shear.
Torsion	Twisting. It produces shear stress inside the material.	Wing twist from aerodynamic pitching moment, aileron and flap loads, and engines mounted ahead of the spar. Fuselage twist from rudder and asymmetric tail loads. Carried by the closed skin-and-spar box.
Bending	A mix of tension and compression across a section, with zero stress at the neutral axis.	Lift bends the wing upward: compression on top, tension on the bottom. The bending moment peaks at the root. The fuselage bends between the wing and the tail. Down-loads and ground conditions reverse the signs.

Shop-floor takeaway

Scratches, tool marks, mis-drilled or elongated holes, and wrong-temper material all raise stress, and they do the most harm on tension-loaded parts like lower skins and bottom spar caps. That is where fatigue cracks start. Disposition goes through the SRM or engineering, never your own judgment.

Image credits & licenses

Principal airframe units; skin/former/bulkhead; wing structure nomenclature: FAA Aviation Maintenance Technician Handbook – Airframe (FAA-H-8083-31B), Vol. 1, Ch. 1 “Aircraft Structures”, Figures 1-13, 1-16, 1-23. Federal Aviation Administration, U.S. government work, public domain.
Stressed-skin wing cutaway: FAA artwork, public domain, as labeled in J. Gordon Leishman, Introduction to Aerospace Flight Vehicles, Ch. 10 “Aerospace Structures” (Embry-Riddle Aeronautical University, doi:10.15394/eaglepub.2022.1066).
Wing spar section; spar types; five stresses: © J. Gordon Leishman / ERAU, J. Gordon Leishman, Introduction to Aerospace Flight Vehicles, Ch. 10 “Aerospace Structures” (Embry-Riddle Aeronautical University, doi:10.15394/eaglepub.2022.1066). CC BY-NC-ND 4.0. Shown unmodified, with attribution, for non-commercial educational use.

The page text is original general-aerospace summary written for this site. No Boeing, SRM, AMM, or E-3/E-6-specific data is reproduced.