A wing spar is a primary structural member running spanwise at right angles to the fuselage, extending from root to tip. In transport-category aircraft, two C-style or I-style spars are standard. The front spar is usually located near the leading edge and the rear spar is provided so that trailing-edge flaps and aileron surfaces can be attached to the main wing structure. On each wing, these two spars occupy the same chordwise positions relative to the local airfoil. When only one spar is used per wing, it is placed at the thickest part of the section, although the exact chord location may differ between left and right wings.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
Where is the wing spar located?
The main wing spar is located at or near 25% chord location, the aerodynamic centre situated about one quarter of the chord from the leading edge. One spar is located at the leading edge at approximately 12% of the local airfoil chord. The other spar is located at the trailing edge at approximately 71% of local airfoil chord, two thirds of the distance toward the wing's trailing edge. Single spars are generally located at the thickest part of the wing airfoil. Commercial aircraft spars are located at the trailing edge. Spars in each wing are at the same location chordwise, and each spar is attached to the fuselage by wing fittings.
What is the front spar in an aircraft?
The front spar is the main structural member of the wing, located near the front of the wing and running spanwise at right angles to the fuselage. It carries flight loads and the weight of the wings while on the ground, and spar caps prevent buckling under applied loads.
The front spar is a main load-bearing unit of the wing structure, positioned near the quarter-chord where aerodynamic lift is concentrated. It forms a shear web that serves as the front wall of the wing box and runs lengthwise from the wing root to the wingtip, parallel to the lateral axis of the aircraft. Forged caps of the front spar extend from the airplane centerline and outboard of the fuselage, providing the primary attachment for wing fittings that connect the wing to the fuselage.
Structurally, the front spar is taller than it is wide to resist bending in the up-and-down direction while aiding both bending and shearing forces experienced during flight. It serves as the attachment point for ribs and stringers, which stabilize the wing and hold the skin in place. The region between the front spar and the leading edge accommodates systems and control linkages, and in some designs functions as a fuel bay. When combined with wing skin panels, the front spar contributes to forming an integrated fuel tank within the wing box.
What is the rear spar of a wing?
The rear spar is the main structural member of the wing, and it runs spanwise at right angles to the fuselage. The spar carries flight loads and the weight of the wings while on the ground, and ribs may be attached to spars. More than one spar may be in a wing, and the main spar carries most of the force.
The rear spar is the shear web forming the aft wall of the wing box. Running lengthwise from wing root to wingtip, it is a main load-bearing unit of the wing structure. In addition to carrying flight and ground loads, the rear spar serves as an attachment point for ribs and stringers, while ribs maintain the aerodynamic profile and hold the skin in place.
Which control surfaces are hinged to the wing's rear spar?
Ailerons and flaps are the control surfaces hinged to the wing's rear spar. On each wing the rear spar carries the two principal roll-control devices. Ailerons are the primary control surfaces; they are hinged to the trailing edge close to the wingtip and ride on brackets fixed to that rear spar. Working beside them, flaps are hinged surfaces on the trailing edge that also anchor to the same spar and lower when deployed.
What is the structure of a wing spar?
The structure of a wing spar is a long C-type primary structure that consists of spar caps and a spar web, forming either an I-beam or C-type cross-section. The spar web maintains fixed spacing between the upper and lower spar caps, which serve as the main load-bearing flanges. This arrangement creates a built-up box beam when combined with wing skins, with the entire cross-section designed to resist both bending and torsional loads. The internal structure of the wing incorporates this spar system extending from one side to the other, perpendicular to the fuselage, as part of a semi-monocoque approach where stressed skin construction shares loads with the primary structural members.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
A wing spar bolt is the fastening hardware used to secure spar components together or attach the spar to other aircraft structures. The front spar cap on a wing refers to the upper and lower flange elements of the forward-most spar that carry primary bending loads. A wing spar splice is a structural joint where two spar sections are connected, typically through bolted or bonded connections that maintain continuous load transfer across the length. Wing spar reinforcement involves additional material or structural elements added to stress areas, like the root attachment points or engine mount locations, where concentrated loads from the fuselage, landing gear, or powerplants are introduced into the primary structure.
What is the function of the wing rear spar flap cover?
The function of the wing rear spar flap cover is to serve as a structural fairing that seals the aft spar region where the flap tracks and actuators are attached. Fairings are aerodynamic covers placed over the flap control actuators; in this location they form a smooth tapered shell that bridges the gap between the fixed rear spar and the moving flap panels. By creating an aerodynamic envelope, the cover shields the internal flap track mechanism from airborne debris and excess airflow, so the encapsulation prevents the flap track mechanism from being damaged during the flight. At the same time it streamlines the local airflow, helping fairings reduce drag and permitting the aircraft to operate at lower flight speeds for landing and takeoff.
What material is used for a wing spar?
Most manufactured aircraft have wing spars made of solid extruded aluminum or of aluminum extrusions riveted together. The high-strength aluminum-zinc alloy 7075-T6 is selected when maximum strength and fatigue resistance are required. A typical metal spar in a general-aviation aircraft therefore consists of a sheet aluminium spar web with aluminium spar caps riveted or welded to the top and bottom. Carbon-fibre and other composite wings also need a spar. In these designs the spar caps are made of high-modulus carbon unidirectional fibers that carry the wing-bending loads, while the spar web is often biased toward 45 woven composite carbon or glass to transfer the vertical shear loads. The same web is stabilized against shear buckling with a honeycomb or foam core, or with bonded or fastened stiffeners.
Wooden spars laminated from spruce or Douglas fir are used in light aircraft like the Robin DR400, and titanium alloy 6Al-4V is chosen for machining load-bearing spar fittings where the higher cost is justified. Whichever material is selected, the spar-web thickness in light aircraft usually falls between 0.1 mm (0.0039 in) and 1 mm (0.039 in), while the upper and lower wing skins are built considerably thicker, typically 0.15-0.4 in (about 4-10 mm), to carry local aerodynamic and fuel pressures.
What causes wing spar damage?
Wing spar damage is caused by different reasons. Wing spar damage incidents are the major cause of compression cracks and other spar damage in low horsepower and lightweight airplanes. Excessive loading from hard landings and side loading from ground loops distort the spar, start cracks, and cause existing cracks to propagate. Corrosion of the main wing spar, especially in areas not easily accessible for inspection, causes the main wing spar to fail with consequent loss of control of the airplane. Dents at the wings' main wing spar cap aft flange caused by incorrectly installed jack pads pose a structural risk, and cold bending a metal wing spar to create a dihedral creates residual stress that significantly reduces the fatigue life of the spar bolt holes and creates the risk of future fatigue and failure.
Fatigue cracks often initiate from corrosion pits. The 2018 fatal accident involving a Piper Model PA 28R 201 was caused by fatigue cracks in the lower main wing spar cap which required a one time inspection of certain lower spar cap bolt holes using non-destructive crack detection tools like the dye penetrant method and eddy current testing to aid in the search for cracks. The same fatigue cracks were found in two additional PA 28 accidents in 1987 and 1993, illustrating the ongoing risk.
More than 21,000 Piper aircraft will have new wing spar longevity limits and recurring inspections required under a pair of proposed airworthiness directives. Piper and the agency emphasized the significance of following prescribed procedures precisely to avoid damaging the wing spar assembly during inspection, because damaging the wing spar assembly requires spar replacement, and the FAA estimates replacing the wing spar is less expensive than installation of a reinforcement kit. Repetitive inspections cause further damage if not done precisely.
What are the types of wing spars?
The types of wing spars are listed below.
- Mono-spar wing with false spars or light shear webs
- Web and caps wing spars
- Plate web type spar
- Plate web wing spar with vertical stiffeners
- Box-section spars
- Truss type spar
- Metal spars like Xenos spars
- Laminated spars
- Sine wave web type spar
- C-section wing spars
- Truss wing spar
- Compression glued spars
- Light shear web spar
- Sine wave wing spar
- Shear web spar
The types of wing spars depend on the construction of web and caps. The shear web spar, built from a thin plate, is the arrangement that is widely adopted in the modern wing. For a truss wing spar, the depth-to-mass penalty makes it less efficient than the shear web. When the web is a plate, vertical stiffeners stop the web from buckling and create a fail-safe spar that keeps the riveted spar web crack from spreading. The spar assumes an I-beam, C-beam, or box-beam form because the caps or flanges carry the bending moment while the web carries each shear load. Box spar is a segment term that consists of metal plates, composite plies, built-up balsa, or cedar, so the box-section spar is either a metal spar or a wooden spar. Composite spars look like I-beams.
Wooden spar types have been used where the classic box-section spar is built from two plywood webs and four wooden caps. In a laminated spar, alternate spruce and birch plies are laid up in a jig and a compression glue line holds the dihedral angle. Wooden spar types include box, I-section, or C-section lay-ups. In typical mono-spar wing the single main spar is supplemented by false spars that support each aileron or flap, the design is modified by addition of light shear webs that run along the trailing edge. Multi-spar wing incorporates more than one main longitudinal member; multiple spars share bending moment, save rib height, and spread bending loads. Externally braced wings - called semicantilever wings - have wires or struts to hold the wing beam. Full cantilever wings are aided internally with no external bracing and use a torsion box that doubles as fuel tank.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air