Keel Beam is a crucial structural component in an aircraft's fuselage and serves as the backbone or spine of the aircraft structure. Running longitudinally along the bottom part of the fuselage and positioned underneath the center wing box, this massive beam bridges the cutout aft of the wing box while absorbing compression loads along that section. The keel beam assembly includes a pair of lower chords disposed underneath the pair of keel attach chords, extending forward and aft through the cutout and spanning the fuselage from an aft bulkhead to a forward bulkhead, thereby ensuring continuous structural integrity beneath the aircraft.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is the definition of the keel beam in an aircraft?
The keel beam is a fundamental structural element in an aircraft's fuselage, providing essential support, rigidity, and load-bearing capabilities. It runs longitudinally along the bottom part of the fuselage and serves as the backbone or spine of the aircraft structure.
The keel beam is the primary longitudinal girder that runs along the fuselage centerline, resembling the keel of a ship. It is a metallic structure whose lower chord extends longitudinally between an aft wheel well bulkhead and a forward bulkhead of the aircraft, thereby covering the fuselage cutout that houses the wing center section. Mirrored left and right about the centre line, the beam comprises a symmetrical pair of keel attach chords disposed above a corresponding pair of lower chords. A pair of forward webs form sidewalls that connect each keel attach chord to its underlying lower chord. These webs include a cutback frontside extending diagonally downward and aft toward the lower chords, a shape that reduces undesirable stress concentration and provides increased space for installing aircraft systems such as environmental control systems. Outer reinforcement is arranged outside the closed section of the beam and connects the lateral walls of the body, while a metallic splice plate possesses a second portion outward of the beam that provides a support surface for the lower panel of the central wing box. This second portion is connected to the lower panel by connection means. By distributing significant loads that originate from the wings, landing gear, and tail, the keel beam ensures structural stability and affords flexibility that enables increased independent movement between the wings and the fuselage pressure barrier formed by the forward and aft bulkheads.
What is the purpose of the keel beam in an aircraft?
A massive beam called the keel beam bridges the cutout aft of the wing box, runs along the fuselage centerline through the wing center-section area, and absorbs compression loads along that section. It distributes loads along the fuselage structure, bears stresses experienced during flight, absorbs payload stress, and ensures structural stability by providing essential support. During landing, the keel beam absorbs landing loads and vertical shear loads that come from the landing gear, while in flight it absorbs effects of bending and torsion. In a wheels-up emergency landing, the keel beam assembly protects the fuselage by absorbing crash forces. Frames resist force, and the cutback configuration reduces undesirable stress concentration while creating increased open area for improved installation of aircraft systems such as environmental control systems.
What is the structure of the keel beam in an aircraft?
The keel beam box structure is located below the center wing tank and runs longitudinally along the bottom part of the fuselage, extending from approximately STA 985 to STA 1480 and from left buttock line 9.0 to right buttock line 9.0. The assembly begins with a U-shaped body closed by a cover whose curved edges are flattened against the inner surface of the lateral walls, creating a closed-section torsion-resisting box. Each anchoring point on both sides of the beam incorporates a metallic splice plate whose first portion connects to the lateral wall of the body, while an outer reinforcement links those same lateral walls externally. A second portion of the plate projects outward from the beam to provide a support surface for the lower panel of the central wing box, the connection being completed by fasteners that still allow slight deformations around axes parallel to the longitudinal direction.
Frames affixed to the keel beam tie the front structure to the rear structure, and the belly skin of Section 42 receives longitudinal loads through the beam, as well as through underwing drag-splice fittings positioned where the wing lower surface intersects the fuselage skin.
