Stringers are axial stiffeners that run lengthwise along the fuselage or span-wise across the wing, fastening the aircraft skin to the underlying skeleton. Attached to the inner face of the skin, these slender members supply additional support, preventing local buckling and shear while accepting axial components of the in-flight bending moment. By serving as continuous links between the skin and the internal framework, stringers distribute aerodynamic surface loads into the adjacent frames and keep those frames correctly spaced, turning the thin shell into a stiff, load-bearing structure.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What are stringers in aviation?
Stringers serve as structural components which run lengthwise along an airplane's fuselage or spanwise of a wing, and they transfer loads and stresses from the aircraft's skin to the formers. Stringers are axial stiffeners and are placed inside the shell. Stringers are alternatively known as longerons or spars in the aircraft industry, and they are used more commonly in commercial aircraft.
What is the main purpose of stringers in aircraft fuselage construction?
The main purpose of stringers in aircraft fuselage construction is to carry the loads due to pressurisation and convert them into tensile stress, while simultaneously assisting the skin withstand longitudinal compressive loads. Stringers increase rigidity by aiding skin and preventing skin buckling, and they carry a portion of the fuselage bending moment through axial loading. By transferring aerodynamic loads from skin onto frames, stringers guarantee structural integrity of the fuselage and help maintain skin shape.
In truss aircraft structures, stringers and formers work together: stringers fill between frames to give structure strength, while formers control the shape of the fuselage. Frames, spaced approximately 20 inches (50.8 cm) apart, define the aerodynamic shape and keep the ‘paper thin’ tube from buckling or collapsing. The fuselage shell is constructed with stringers and frames to stiffen the structure, with stringers keeping frames spaced correctly and giving strength to the skeleton.
Which way do the stringers run in an aircraft?
In the wings, stringers run spanwise, aligned with the wing spar. Top stringers run from the windshield back to the vertical stabilizer, continuing the spanwise path along the fuselage upper contour. On the fuselage skin, direction changes with station: stringers placed on the right side carry odd dash numbers and continue clockwise and are labeled R1, R2, R3 while stringers on the left side carry even dash numbers and run counter-clockwise, marked L1, L2, L3.Thus, stringers travel fore-and-aft on the fuselage and spanwise on the wings, providing continuous stiffening paths that carry axial and bending loads between frames and spars.
What is the design of aircraft stringers?
The design of the stringers is driven by the stress point of view. Longitudinal members are smaller in the stringer system, and every hat-section cross section or I-beam cross section is proportioned so that the flange, not the web, touches the skin; the top web is therefore mounted away from the surface. This orientation is chosen because the skin is a thin layer covering the surface that carries shear, and it must remain aerodynamically clean. All stiffeners are placed on the inside, and the feet of the stringers have to be leveled out so that only a single row of rivets is needed for attachment, otherwise two rows are required, adding weight and drag.
Hat-section extrusions are favoured when compression loads are really high, while double I-beam and plain I-beam shapes are used elsewhere. In the semi-monocoque wing these stringers, together with ribs placed chordwise and frames spaced about 38-51 cm (15-20 in), form a lattice that stabilises the thin skins. The stringers are purposely not aligned width-wise with neighbouring rows so that cracks do not propagate straight across the panel, a design decision that preserves the integrity of the fully stressed-skin structure.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
What materials are aircraft stringers made of?
Aircraft stringers are most often made from aluminum alloys. Wrought aluminum is the most widely used alloy in aircraft construction, and high-strength grades like 2024 and 7075 became standard by the 1960s and 1970s. AA7075, containing 5.1-8.1 wt.% Zn, 1.8-3.4 wt.% Mg and 1.2-2.6 wt.% Cu, is a typical raw material for stringers. Clad variants use a corrosion-resistant surface: an AA7072 layer shields the core alloy, while the core itself delivers high strength. Extruded sections in sizes like 5/16" - 5/8" (7.9 mm - 15.9 mm) or 1" 5/16" (25.4 mm - 7.9 mm) are stocked in 12-foot lengths (3.66 m). For example P/N 03-48800 supplied by Aircraft Spruce arrive in bundles totalling 112 feet (34.14 m) with a finished weight near 15 pounds (6.8 kg).
In earlier airframes, wood offered an ideal balance of strength and weight. Spruce was preferred because of its high strength-to-weight ratio and ready availability. Fabric, usually linen or cotton, was then coated with dope and stretched over wooden stringers. Today composite materials like carbon-fiber, glass-fiber and Kevlar-epoxy reinforced panels are also riveted to stringers, especially where weight savings and fatigue toughness are pivotal, while mild-steel brackets or 4130 steel tubing form local reinforcements at highly loaded joints. Thus, from wood and fabric through aluminum alloys to advanced composites, stringer materials track the wider evolution of aircraft structures in the pursuit of performance, efficiency and safety.
What are the types of stringers used in aircraft?
The types of stringers used in aircraft are listed below.
- Z-section stringer
- T-section stringer
- Hat-section stringer
- T-stringer stringer
What is the difference between an aircraft stringer and a longeron?
The differences between an aircraft stringer and a longeron are given in the table below.
| Feature | Stringer |
|---|---|
| Cross-section | Smaller |
| Spacing laterally | Closer |
| Load Carrying Capacity | Smaller loads |
| Structural Framework | Used with longerons |
| Number in fuselage | Many |
| Lengthwise Position | Run all along fuselage length |
| Function in Wings | Used, similar function |
| Attachment to Formers and Bulkheads | Attach to formers and bulkheads |
| Transfer of Skin Loads to Internal Structure | Contribute but not primary |
| Handling Bending Stresses | Not specifically designed for |
| Handling Axial Loads | Not specifically designed for |
| Designation Based on Number | More numerous |
The differences between an aircraft stringer and a longeron are that stringers are smaller longitudinal members installed at much closer intervals; they reinforce the skin but do not assume the primary load paths managed by the longeron system. Longerons form the main continuous backbone of the fuselage while stringers act as distributed stiffeners between the bulkheads and frames, giving shape and stability to the thin shell.
Longerons are spar-like structures that run lengthwise along the fuselage and across the width of a wing. They are few in number, usually four to eight, and attach to multiple formers, bulkheads, frames or ribs. Because longerons are of larger cross-section than stringers, they carry larger axial loads and bending stresses and help transfer skin loads to the internal structure. They are spaced further apart laterally, with frames spaced 15 to 20 in apart. Thus, the principal difference lies in size, spacing, load intensity and structural role: larger, widely spaced, heavily loaded longerons versus smaller, closely spaced, lightly loaded stringers working together to create an efficient airframe framework.
