mainspar

The mainspar is the main structural beam running spanwise through an aircraft wing, typically positioned between 20% and 40% of the chord depth from the leading edge. It carries the majority of the bending moment created by aerodynamic lift and fuel weight, making it the single most critical structural element in wing design. Most modern transport aircraft have two spars per wing, the mainspar and an aft spar, with the mainspar bearing roughly 70 to 85% of the total vertical load.

Constructionally, the mainspar consists of a vertical web, usually made of 7075-T6 or 2024-T4 aluminum alloy sheet, reinforced by horizontal flanges or boom sections that run the full span. The flanges work in tension and compression as the wing bends; the web resists shear stress. Access to the interior is gained through inspection panels and fastener rows along the web, which are subject to rigorous borescope and eddy current inspection throughout the aircraft's service life.

Inspection and Failure Modes

Fatigue cracking in the mainspar web initiates most often at fastener holes or along weld seams where older aircraft were repaired. Disbonding of doublers, which reinforce the spar at high-stress stations, is also common in aged structures. Corrosion pitting, particularly in areas of trapped moisture around wing-fuselage attach points, reduces the thickness available to carry bending loads and can trigger unexpected failures at routine operating stresses. Inspection intervals for the mainspar are typically driven by the aircraft type's damage tolerance limit or crack growth analysis, with mandatory overhaul or retirement at a specified flight hour threshold.

The term mainspar derives from its role as the primary spar in the load path; it is distinguished from the aft spar, which is smaller and carries secondary bending and torsion. In military fighter aircraft and high-performance types, spars are sometimes designed with variable cross-section or composite overwrapping to optimize weight and stiffness; in large transport aircraft, the mainspar may span more than 60 feet and weigh several tons per wing.