In a fixed-wing aircraft, the spar is often the main structural member of the wing, running spanwise at right angles (or thereabouts depending on wing sweep) to the fuselage. The spar carries flight loads and the weight of the wings while on the ground. Other structural and forming members such as ribs may be attached to the spar or spars, with stressed skin construction also sharing the loads where it is used. There may be more than one spar in a wing or none at all. Where a single spar carries most of the force, it is known as the main spar.

Spars are also used in other aircraft aerofoil surfaces such as the tailplane and fin and serve a similar function, although the loads transmitted may be different from those of a wing spar.

The wing spar provides the majority of the weight support and dynamic load integrity of cantilever monoplanes, often coupled with the strength of the wing 'D' box itself. Together, these two structural components collectively provide the wing rigidity needed to enable the aircraft to fly safely. Biplanes employing flying wires have much of the flight loads transmitted through the wires and interplane struts enabling smaller section and thus lighter spars to be used at the cost of increasing drag.

Some of the forces acting on a wing spar are:

Many of these loads are reversed abruptly in flight with an aircraft such as the Extra 300 when performing extreme aerobatic manoeuvers; the spars of these aircraft are designed to safely withstand great load factors.

Early aircraft used spars often carved from solid spruce or ash. Several different wooden spar types have been used and experimented with such as spars that are box-section in form; and laminated spars laid up in a jig, and compression glued to retain the wing dihedral. Wooden spars are still being used in light aircraft such as the Robin DR400 and its relatives. A disadvantage of the wooden spar is the deteriorating effect that atmospheric conditions, both dry and wet, and biological threats such as wood-boring insect infestation and fungal attack can have on the component; consequently regular inspections are often mandated to maintain airworthiness.

Wood wing spars of multipiece construction usually consist of upper and lower members, called spar caps, and vertical sheet wood members, known as shear webs or more simply webs, that span the distance between the spar caps.

Even in modern times, "homebuilt replica aircraft" such as the replica Spitfires use laminated wooden spars. These spars are laminated usually from spruce or douglas fir (by clamping and glueing). A number of enthusiasts build "replica" Spitfires that will actually fly using a variety of engines relative to the size of the aircraft.