Hardware Is Not the Approval Basis
An aircraft can carry equipment that is not part of its original type design: avionics, connectivity systems, winglets, auxiliary fuel, interior changes, or other alterations. Finding non-stock hardware does not, by itself, tell you whether it was installed under a Supplemental Type Certificate (STC), another approved design change, acceptable data, or a maintenance action that did not require an STC.
Asking “Where are the Form 337, ICA, and AFMS?” skips a step. The first question is:
What is the classification and approval basis for this specific change on this specific aircraft?
That answer determines which records should exist. Treating every modification as though it requires the same package creates false findings. Treating installed hardware as self-proving creates the opposite problem: an unsupported configuration that cannot be reconciled to the aircraft's approved design and maintenance records.
What an STC Does
An STC is FAA approval of a major change in type design. The certificate identifies the approved change and the product to which it applies. The installation still must be accomplished, inspected, recorded, and approved for return to service under the rules applicable to the work.
A useful STC installation file includes:
The applicable STC and approved installation data.
Evidence that the STC applies to the aircraft model and configuration.
FAA Form 337 when required for the major alteration.
The maintenance-record entry and return-to-service approval.
Instructions for Continued Airworthiness (ICA) applicable to the installation.
An Airplane or Rotorcraft Flight Manual Supplement when required.
Weight-and-balance, equipment-list, wiring, placard, and configuration changes.
Work-order pages, conformity evidence, and inspection results.
Not every item applies in every case. An AFMS is relevant when the approved change affects operating limitations, procedures, performance, or other flight-manual content. The required ICA comes from the approved design package and applicable rules. Form 337 requirements turn on the work and the regulatory pathway. An aftermarket look proves nothing.
Separate Four Questions
A defensible review separates four questions rather than collapsing every missing page into “the modification is illegal”:
Is the installed configuration identified?
Record the equipment, part and serial numbers where applicable, location, aircraft effectivity, placards, controls, and interfaces. Photographs establish what is physically present, but they are not approved data.
Is the design approval identified?
Find the STC, amended type certificate, field approval, manufacturer data, or other basis used. Confirm that the approval applies to the aircraft and installed configuration.
Was the work properly accomplished and recorded?
Locate the maintenance entry, Form 337 when required, work order, inspection evidence, and return-to-service approval. The approval of design data and the approval of completed work are related but different.
Are continuing obligations controlled?
Identify applicable ICA tasks, limitations, inspections, airworthiness limitations, manual supplements, placards, and weight-and-balance changes. Confirm they are reflected in the active maintenance and operating systems.
An exception in any one area deserves investigation. The consequence depends on what is missing and what can be verified; it is not an automatic percentage loss in value.
How the Gap Appears During Diligence
Consider a hypothetical auxiliary-fuel installation. The aircraft log references an STC, but the operator's file does not contain the supporting package. The correct response is not to declare the aircraft unairworthy from the desk or to invent a replacement sign-off.
The reviewer should:
1. Confirm the installed hardware and aircraft effectivity. 2. Verify the STC number and approval holder. 3. Search the aircraft's FAA records for submitted Form 337 material. 4. Request the installation work package from the installer or prior operator. 5. Obtain the applicable approved ICA and flight-manual material from an authorized source. 6. Have appropriately qualified maintenance and engineering personnel compare the aircraft to the approved configuration. 7. Ask the FAA or authorized representatives for direction when the regulatory path is not clear.
Technical acceptance, operation, and closing decisions should be based on that review, not on a generic statement that every missing STC attachment has the same legal effect.
Do Not “Paper Over” Prior Work
A missing original is not cured by creating an unsupported backdated record. Nor should a current technician sign as though they personally performed or observed work they did not perform or observe.
Depending on the facts, legitimate resolutions include:
Recovering an existing FAA-filed form or installer work package.
Obtaining controlled copies of approved data.
Inspecting the installation against the approved configuration.
Correcting a current record through an authorized process.
Accomplishing new work needed to establish conformity.
Obtaining engineering or FAA approval.
Removing or replacing the modification.
The appropriate path must be determined by people with authority and aircraft-specific evidence. A transaction deadline does not change that standard.
What the Exception Can Cost
There is no defensible universal “20 to 50 percent” discount for an undocumented modification. The financial effect depends on:
Whether the approval and source records can be recovered.
Whether the installed configuration conforms.
Whether required continuing instructions are available.
Inspection, engineering, correction, removal, and downtime costs.
Buyer and lender acceptance.
Market demand for the modified aircraft.
The time available before closing.
Possible outcomes include no material value change after verification, a defined cure cost, an escrow or closing condition, a price adjustment, removal of the equipment, rejection of the aircraft, or a delayed transaction. Label any modeled outcome as a scenario, not an industry rule.
Build a Modification Register Before the Pre-Buy
A modification register turns the search into a controlled process. For each change, record:
Description and installed location.
Approval basis and effectivity.
Form 337 or maintenance-entry reference, as applicable.
ICA and active maintenance tasks.
Flight-manual supplement or operating material, as applicable.
Weight-and-balance and equipment-list effect.
Work-order and return-to-service references.
Open evidence gaps and named owner.
Then reconcile the register to the aircraft, current manuals, logbooks, FAA file, and maintenance tracking system. The goal is proof that the current configuration is identified, approved, recorded, and maintained under the requirements that actually apply. Collecting three documents mechanically proves none of that.
Sources and Further Reading
cert/designapprovals/stc/stc_app">FAA, Supplemental Type Certificate Process
cert/designapprovals/stc/stc_responsibilities">FAA, STC Holder Responsibilities
14 CFR Part 43 Appendix B, Recording of Major Repairs and Alterations
Radar can help inventory and connect the evidence before a modification exception becomes a closing-day discovery.
