Definition
The load-bearing components of an aircraft that carry flight, ground, and pressurization loads, and whose failure would jeopardize the safety of the aircraft. This typically includes the wing spars, fuselage frames and longerons, main wing attach fittings, engine mounts, and the control surface attachment points.
Plain English
The parts of the aircraft that hold everything together and keep it flying. If one of these parts breaks, the aircraft can't safely stay in the air.
Context Anchor
Seen in maintenance and preventive maintenance discussions, especially when deciding whether a task is simple owner-performed maintenance or work that needs a qualified aircraft mechanic.
Derivation
Primary' comes from the Latin primus, meaning 'first' or 'most important.' In maintenance terms, primary structure is first in importance — it's what keeps the aircraft intact under load.
Why Pilots Care
Work on primary structure is usually restricted to certified mechanics because damage here directly affects airworthiness.
Grounding Statement
If a part helps carry the forces of flight, landing, or aircraft weight, it may be part of the primary structure.
Intuition Check
Primary structure does not mean the first part you notice or any large aircraft part. It means the main load-carrying structure the aircraft depends on to remain strong and controllable.
Example Sentence 1
The mechanic explained that the wing spar is primary structure, so the cracked area would require an FAA-approved repair, not a field fix.
Example Sentence 2
Any crack found in the primary structure must be reported and repaired by a certified mechanic.