Every time an aircraft is assembled — whether fresh from the manufacturer's production line, returned from a major repair, or reassembled after an annual inspection — an Aviation Maintenance Technician (AMT) must follow a deliberate, structured sequence of steps. This is not simply a matter of bolting parts together in a convenient order. The assembly sequence ensures that underlying structure is correct before it is covered, that control systems are properly rigged and travel is within limits, and that the finished aircraft conforms to its approved design data. A conformity inspection is the formal verification that every component, dimension, and configuration matches the type design or approved data. Together, assembly sequencing and conformity inspection form the backbone of airworthiness assurance at the maintenance level.
Understanding these concepts is critical for the AMT Airframe knowledge test, and even more critical in the shop. An error caught during a ground conformity check costs time; the same error discovered in flight can be catastrophic. This article walks through both the logical sequence of assembly and the documentation-driven process of conformity verification, grounded in FAA guidance from the Aviation Maintenance Handbooks and 14 CFR.
The Logic Behind Assembly Sequence
The fundamental principle of aircraft assembly sequence is inside-out and bottom-up: you complete, inspect, and verify primary structure before attaching secondary structure, and you complete secondary structure before installing systems and fairings that would hide it. This logic exists because once a wing skin is riveted closed or a fuselage panel is fastened down, inspecting the interior becomes extremely difficult or impossible without disassembly.
A typical airframe assembly sequence follows this general progression:
- Primary structure first: Fuselage frames, longerons, bulkheads, and wing spars are assembled and inspected. Every attachment fitting, bolt hole alignment, and joint is verified before skin panels are attached.
- Secondary structure: Ribs, stringers, formers, and doublers are added to the primary skeleton. Again, inspection occurs before covering.
- Skin attachment: Skins are riveted, bonded, or fastened. Each panel is inspected for correct fastener type, spacing, edge distance, and installed torque before moving on.
- Control surface installation: Ailerons, elevators, rudder, flaps, and tabs are hung and their hinges verified for correct fit, freedom of movement, and absence of binding.
- Systems installation: Flight control cables, pushrods, bell cranks, hydraulic lines, and electrical wiring are routed and connected. This occurs after structure is complete so routing is not disturbed by later structural work.
- Rigging and adjustment: Control surfaces are rigged to manufacturer-specified neutral positions. Travel limits — both up and down — are measured and adjusted to match the approved data.
- Fairings and fillets: Fairings, access panels, and inspection plates close out the assembly. These are the last items installed because they cover everything underneath.
- Final conformity inspection and functional checks: The entire assembly is walked through against approved data before the aircraft is released.
In practice, an AMT working from a manufacturer's maintenance manual (MM), Structural Repair Manual (SRM), or an FAA-approved repair specification will find detailed assembly instructions specific to the aircraft type. Those documents are the controlling authority; this general sequence reflects the logic underlying them.
Conformity Inspection Defined
A conformity inspection is a systematic, documented comparison of the assembled aircraft — or a specific portion of it — against its approved design data. Approved design data includes the type certificate (TC) data sheet, aircraft specifications, applicable Airworthiness Directives (ADs), the manufacturer's maintenance and parts manuals, FAA-approved engineering orders, and, for field repairs, the data approved by an FAA Designated Engineering Representative (DER) or the Aircraft Certification Office (ACO).
The goal is to answer one question with documented evidence: Does this aircraft, as assembled, conform to its approved type design? If it does, it is eligible to return to service. If it does not, the discrepancy must be corrected before any return-to-service signature is made.
What a Conformity Inspection Covers
A thorough conformity inspection addresses several categories simultaneously:
- Part identification and traceability: Every structural component should be identified against the illustrated parts catalog (IPC). Critical parts — spars, fittings, control horns — require traceability paperwork (8130-3 FAA Authorized Release Certificate, manufacturer's Certificate of Conformance, or approved equivalent). Unapproved parts are an airworthiness violation.
- Dimensions and alignment: Wing incidence, dihedral, washout (twist), and sweep must be within the tolerances published in the manufacturer's data. Fuselage straightness (no twist or warp) is verified by measuring diagonal distances from reference points. Any structure outside tolerance can affect flight characteristics and structural loads.
- Fastener compliance: The correct AN/MS/NAS fastener must be installed in each location. Bolt grip length, washer type, nut type, and torque values must match the manufacturer's table. Missing cotter pins, improperly torqued castellated nuts, or substituted fastener grades are common discrepancy items.
- Control surface rigging: Every movable surface is verified at neutral and at both travel limits. Measurements are made with a rigging board, inclinometer, or protractor against manufacturer-specified degree values. Cables are checked for correct tension using a tensiometer, with tension corrected for ambient temperature if the manual specifies a temperature-correction table.
- Safety wiring and locking devices: Control system turnbuckles, critical fasteners, and hydraulic fittings are verified for correct safety wire installation or use of approved locking devices.
- AD compliance: Any applicable Airworthiness Directive affecting the areas worked must be verified as complied with and documented.
Why Assembly Sequence and Conformity Inspection Matter
From a safety standpoint, the stakes are straightforward: an airframe assembled out of sequence or released without conformity verification can fail structurally or lose control authority. A wing spar splice installed with the wrong bolt grade may appear airworthy but carry reduced load capacity. A control cable routed through an incorrect fairlead may bind under load. An elevator rigged two degrees from neutral will result in out-of-trim forces the pilot must fight throughout flight.
From a regulatory standpoint, 14 CFR Part 43 requires that maintenance, preventive maintenance, and alterations be performed in accordance with approved data, and that the person approving the work for return to service ensure the work was done correctly. The AMT's signature on the maintenance record is a legal certification that the work conforms to approved data. Deliberately falsifying conformity records, or making a fraudulent or intentionally false entry, is a violation of federal aviation regulations and constitutes falsification of maintenance records under 14 CFR 43.12; merely skipping a step, while a serious maintenance discrepancy, is not automatically a 43.12 violation unless fraud or intentional false entry is involved.
For major repairs and major alterations, the standard is even higher. FAA Form 337 (Major Repair and Alteration) must be completed, describing the work and the approved data used. A conformity inspection, often conducted by an FAA airworthiness inspector or DER before closing out the repair, is commonly required for these categories of work. The assembly sequence becomes part of the work record, showing that each stage was inspected in the correct order.
Key Numbers and Rules
- 14 CFR 43.13: Requires performance of maintenance using methods, techniques, and practices acceptable to the Administrator — which means approved data and manufacturer's methods govern assembly and rigging.
- 14 CFR 43.9 and 43.11: Require entries in maintenance records that describe the work performed, date, signature, and certificate number. These records are the documentary trail of a conformity inspection.
- FAA Form 337: Required for major repairs and major alterations. Must identify approved data and be submitted to the FAA Aircraft Registry for permanent record.
- AC 43.13-1B and AC 43.13-2B: Advisory Circulars providing acceptable methods and practices for structural repair and aircraft alterations — a primary reference for AMTs performing repairs where manufacturer data is absent or incomplete.
- Control surface travel: Always measured in degrees against manufacturer data, not estimated visually. There is no single FAA-mandated tolerance range; the acceptable tolerance for travel and rigging is aircraft-specific and defined by the manufacturer's maintenance or rigging manual.
- Cable tension: Must be corrected for temperature per the manufacturer's tension-temperature chart. As temperature rises, cables expand and tension decreases, so a cable tensioned correctly at 50°F will be undertensioned at 100°F if the temperature correction is not applied.
Common Test Traps
- Out-of-sequence inspection: Test questions may describe closing a panel before checking cable routing, or riveting a skin before inspecting the interior. Recognize that structure must always be inspected before it is covered.
- Approved data vs. field judgment: The AMT does not have the authority to decide that a substituted fastener is