Definition
A type of structural truss used in early aircraft fuselage and wing construction, consisting of vertical members that carry compression loads and diagonal members (typically wires or rods) that carry tension loads, all joined to horizontal top and bottom chords (longerons in a fuselage).
Plain English
A framework where the upright pieces handle the squeezing forces and the diagonal pieces handle the pulling forces, giving the airframe its strength and shape.
Context Anchor
Seen in airframe structure discussions, especially when identifying or inspecting truss-type fuselage or support structures.
Derivation
Named after Thomas Pratt and his brother Caleb, American engineers who patented this truss design in 1844 for railroad bridges. The pattern was later adopted in early aircraft because it efficiently separates compression and tension loads into different members.
Why Pilots Care
Maintenance technicians inspecting older aircraft must understand which members carry which loads. A loose tension wire or a bent vertical compression member affects the airframe's structural integrity in different ways and requires different repair approaches.
Analogy
Think of a simple bridge made from repeated triangles. In a Pratt truss, the slanted pieces are arranged in a particular direction so the frame handles pulling and pushing forces in a planned way.
Intuition Check
Do not read Pratt truss as just a fancy name for any braced frame. The key point is the specific arrangement of vertical and diagonal pieces and how they carry load.
Example Sentence 1
The fuselage of the early biplane used a Pratt truss design, with steel tubes serving as compression members and tensioned wires as the diagonals.
Example Sentence 2
During inspection the technician checked each member of the Pratt truss for corrosion and fatigue cracks.