How to Fill Out FAA Form 337 (w/Examples) + FAQs

FAA Form 337 is the official Major Repair or Alteration form used to document significant repairs and modifications to an aircraft, and it must be submitted to the FAA. According to a 2023 FAA compliance report, over 30% of aircraft owners and mechanics file FAA Form 337 late or incorrectly, risking fines of up to $1,000 for each missing or incorrect form. Filing this form properly is not just bureaucracy – it’s essential to keep your aircraft airworthy and in legal compliance with federal regulations.

• ✈️ What FAA Form 337 is and why it’s critical: Understand the purpose of Form 337 and when the FAA requires it, so you don’t risk flying with undocumented major fixes or upgrades.
• 📝 Step-by-step how to fill out each section: Get a breakdown of every block on Form 337 – from aircraft info to the sign-offs – ensuring you complete each part accurately.
• 💡 Real-world examples and scenarios: See practical examples (with tables) of common situations requiring a Form 337, like installing a new STC kit or performing a major repair, and how they’re handled.
• 🚫 Mistakes to avoid and pro tips: Learn the biggest pitfalls A&P mechanics and owners face (like missing data or wrong signatures) and how to avoid these common FAA Form 337 errors.
• 📋 Form 337 vs. other documents: Know how Form 337 compares to other aviation paperwork (8130-3 tags, 8110-3 approvals, logbook entries), plus a glossary of FAA terms, key offices, and what each means.

FAA Form 337 Overview: What It Is and Why It’s Required

FAA Form 337 (titled “Major Repair and Alteration”) is a federal form used to record any major repair or major alteration done on a certificated aircraft, engine, propeller, or appliance. In plain terms, it’s the document that tells the FAA and future owners what significant non-routine work was done on the aircraft. The form becomes part of the aircraft’s permanent record on file with the FAA in Oklahoma City, as well as part of the aircraft’s maintenance logs.

Why is Form 337 required? Federal aviation regulations (notably 14 CFR Part 43 and Part 91) mandate that whenever you perform a major repair or major alteration, you must document it on Form 337 and submit a copy to the FAA. This requirement ensures there’s an official, traceable record of any big changes affecting the aircraft’s airworthiness. For example, if you modify an airplane’s wing or install a new type of avionics under an STC, the FAA wants that on record. Failing to file the form can not only lead to legal penalties, but it can also leave the aircraft technically unairworthy (since its paperwork isn’t in order).

Major vs. minor – the big distinction: A major alteration is generally any change that might appreciably affect the aircraft’s performance, weight & balance, structural strength, or other airworthiness factors, and isn’t done according to standard practices or specifications. A major repair is a fix that, if done wrong, might affect those same factors, or that isn’t just a simple replacement or adjustment. Everything not fitting those definitions is considered “minor” and does not require Form 337; minor repairs and alterations are simply logged in the aircraft’s records by the mechanic. The FAA provides detailed criteria (see 14 CFR Part 43 Appendix A) for what counts as major; if you’re in doubt, consult a mechanic with Inspection Authorization (IA) or ask the FAA – because doing a major change without filing a 337 is a big no-no.

Uniform Federal Rules (No State-Specific Differences)

Aircraft maintenance is regulated at the federal level, so Form 337 requirements are the same in every state. There are no separate state aviation forms for major repairs or alterations – it’s all under FAA rules.

However, how you interact with the FAA can vary by region. A major alteration that isn’t covered by an STC or standard approved data might require a field approval from your local FSDO, and different FSDOs have slightly different procedures or levels of strictness for approvals. Still, the baseline requirements (fill out the form, get the proper signatures, submit it) remain the same everywhere – whether you’re in California or Kansas, you follow the same Form 337 process under federal law.

How to Fill Out FAA Form 337 (Step-by-Step Guide)

Filling out Form 337 may look daunting, but it’s quite straightforward if you tackle it block by block. The form has several numbered sections (blocks) to capture all the necessary information. Here’s a breakdown of each part of the form and tips for completing them correctly:

Block 1: Aircraft Information

This section identifies the aircraft. You need to enter the Nationality and Registration Mark (for U.S.-registered aircraft, that’s the N-number like N12345), the aircraft Make, Model, Serial Number, and any specific Series or model variant. Make sure these entries exactly match what’s on the aircraft’s registration and data plate – double-check the N-number and serial, because a typo here could misfile your form in the FAA’s records.

Block 2: Owner Information

Here you input the aircraft owner’s complete name and address, exactly as shown on the FAA registration certificate (Form 8050-3). If the aircraft is in the middle of a sale and the registration update is still in progress, you can use the new owner’s name and address as on the registration application. The key is that the owner info on the Form 337 should match FAA records to avoid confusion.

Block 3: For FAA Use Only (Approval Office Identification)

Block 3 is labeled “For FAA Use Only” – and indeed, in almost all cases you leave it blank. It’s there for scenarios where the FAA itself must approve the data. For example, if you seek a field approval for a custom modification, an FAA inspector will sign and date Block 3 to officially approve the alteration. If you’re using pre-approved data (like an STC or manufacturer’s approved repair), nothing goes in Block 3 – think of it as the FAA’s signature line only when they need to be directly involved.

Block 4: Type of Work (Repair or Alteration)

Here you indicate what kind of action you’re documenting, using the checkboxes for “Major Repair” or “Major Alteration” and marking the category it applies to (Airframe, Powerplant, Propeller, or Appliance). For example, installing a new avionics system under an STC would be a Major Alteration affecting the aircraft’s systems (you’d tick “Alteration” and likely “Airframe” or “Appliance,” as appropriate). If you happen to do both a repair and an alteration at once, you generally fill out separate forms for each, since one form is used for one major action.

Block 5: Unit Identification

This section identifies the specific component or unit that was repaired or altered, if it’s not just the entire aircraft. Usually, if the work was on the aircraft itself (like an airframe modification), you’d simply repeat the aircraft’s make, model, and serial number here under the appropriate category. If the work was on a particular component – say an engine, propeller, or avionics unit – you list that here (e.g., “Engine: Lycoming O-360, serial #XYZ123”). Block 5 basically ties the Form 337 to the exact item that was worked on.

If a part was repaired or altered off the aircraft (for example, an engine overhauled on a workbench before reinstallation), a Form 337 can be filled out for that part and travel with it. When that part is later installed in an aircraft and returned to service, the installer then fills in Blocks 1 and 2 (the aircraft info) and submits the form to the FAA. In most cases, you won’t encounter this scenario – Block 5 will just contain the identification of whatever unit you repaired or modified on the aircraft.

Block 6: Conformity Statement (Who Performed the Work)

Block 6 is where the person or agency that performed the repair/alteration certifies that the work conforms to FAA regulations. It’s essentially the mechanic’s statement of compliance, and this block has several parts:

• 6A (Agency’s Name and Address): If you’re an A&P mechanic or IA doing the work, enter your name and address here. If it was done at a repair station or other certified facility, they would enter the business name and address as listed on their FAA certificate.
• 6B (Kind of Agency): Check the box that describes who did the work – options include Individual Mechanic, Repair Station, Manufacturer, or Other (e.g., a foreign repair station or Canadian AMO). Most often for general aviation it’ll be “Mechanic” (for an A&P/IA) or “Repair Station.”
• 6C (Certificate Number): Enter the certificate number that corresponds to the agency. If an A&P mechanic did it, this is the mechanic’s certificate number. If a repair station did it, it’s the repair station’s Air Agency certificate number. Manufacturers would put their Production Certificate or STC number, if applicable.
• 6D (Conformity Statement): The person who oversaw or performed the work signs here, attesting that the repair or alteration was done in accordance with the applicable regulations and data. They also enter the date the work was completed (the sign-off date). By signing, the mechanic is essentially saying “I did this work and I certify it complies with the requirements of Part 43.” Note that this signature is not the final approval for return to service – that comes in Block 7 – but it’s an important confirmation that the work was performed properly per the approved data.

Block 7: Approval for Return to Service

This is the sign-off that actually approves the aircraft for return to service after the major repair or alteration – essentially the final okay to fly. Only authorized individuals can sign Block 7. Typically, this will be:

• An FAA-certificated mechanic with Inspection Authorization (IA) – this is the case for most privately operated general aviation aircraft.
• Or an authorized individual from a certified repair station – if the work was done under a Part 145 repair station, their appropriately certificated inspector signs off.
• In some cases, an FAA inspector or designated airworthiness representative (DAR) might sign here (for example, during a field approval or in a testing scenario, though that’s uncommon for routine work).

Block 7 has its own checkboxes to indicate who is making the approval (Inspector, DAR, IA, etc., or “Other” if it’s a different authorized person such as an organizational designee). The signer prints their name, signs, enters their certificate or designation number, and the date. By signing here, the IA or other authorized person is certifying that they have inspected the work, found it to comply with the regulations and approved data, and that the aircraft is safe to fly (airworthy) with this repair/alteration. No flight is legal until Block 7 is signed for a major repair or alteration – this signature is the aircraft’s release back to service.

Block 8: Description of Work Accomplished

This is usually the longest part of your Form 337 – the narrative description of what you did. You have a large space (often the entire back side of the form) to detail the work. In Block 8, you should:

• Describe the work in detail: Explain what was repaired or altered, and where. Include part numbers, locations, and what exactly was done. For example, you might write “Installed Auxiliary Fuel Tank Kit per STC SA01234XYZ: Added 18-gallon wing tip tanks to both wings, including new fuel lines, selector valve, and wiring for fuel gauges.”
• Reference the data or specifications used: This is crucial. Mention any approved data that the work was based on. If it’s an STC, cite the STC number and title. If it’s a major repair using the manufacturer’s Structural Repair Manual or an Advisory Circular (e.g. AC 43.13-1B), say so (e.g., “Repair accomplished per Piper Structural Repair Manual, Figure 4-15, and AC 43.13-1B guidance”).
• Note any changes in weight and balance or operating limitations: If the alteration affects weight and balance, list the new weight & balance or note that an updated W&B sheet is attached. For example: “Aircraft empty weight increased by 15 lbs; new empty CG at 40.2 inches aft of datum (see attached revised weight & balance report).” Also mention if there’s any change to operating limitations (rare, but if so, note it).
• Attach supporting documents if needed: If you have engineering sketches, test reports, photographs, or ran out of space and need an extra sheet, indicate “See attached __ pages” and attach them. All attachments (like a DER’s 8110-3 form or weight and balance sheets) should be referenced in the description so it’s clear they are part of the record.
• Sign off in the narrative (if applicable): Block 8 itself doesn’t have a signature line (the signatures are in Blocks 6 and 7), but if you use additional pages, it’s good practice for the mechanic/IA to sign and date at the bottom of those continuation sheets. This way, there’s no doubt those pages are part of the official submission.

Clarity is key in Block 8. You want someone reading it years later (another mechanic, an FAA inspector, or a potential buyer) to understand exactly what was done and that it was done with proper approved methods. Avoid vague statements. It’s better to write something detailed like “Installed Jones Aviation shoulder harness kit, part #XYZ, under STC SA01234WI, per installation manual rev. 2” rather than a vague line like “Installed new shoulder harness – per STC.” In short, details matter here.

Submitting the form: After everything is filled out and signed, you must give a signed copy of the form to the aircraft owner and send another copy to the FAA (Aircraft Registration Branch in Oklahoma City) within 48 hours of the return-to-service approval. Many mechanics mail the form – or now often submit it electronically – immediately after signing to meet this deadline. The owner’s copy goes into the aircraft’s logbooks/maintenance file. This way, there’s a record on file federally and with the plane itself.

Examples of FAA Form 337 in Action

To make things more concrete, let’s look at a few real-world scenarios where you’d use Form 337. Below are examples of different situations and how FAA Form 337 is applied in each case:

Scenario	How FAA Form 337 is Used (Explanation)
Installing a Supplemental Type Certificate (STC) mod (adding an approved upgrade)	You’ve decided to install a long-range fuel tank STC kit on your Cessna. The STC provides pre-approved data for this mod, but you still must document the installation. An A&P with IA follows the STC instructions to install the kit, then fills out Form 337 describing the work (citing the STC number and kit details) and signs it off. A copy of the completed 337 is sent to the FAA to officially record that your aircraft now has this alteration in its records.
One-time field approval for a unique alteration (custom mod with FAA sign-off)	You want to use a propeller from a different aircraft model on your vintage plane, but no STC exists for this swap. Your mechanic works with the local FSDO to get a one-time field approval. You provide engineering data (and even enlist a DER for calculations) to support the change. An FAA inspector evaluates the data and, if satisfied, signs Block 3 of Form 337 to approve this specific alteration. The signed Form 337 (with the inspector’s approval and your mechanic’s details) is then filed with the FAA, documenting the custom modification.
Major repair after damage (using approved repair data)	Your aircraft suffered a hard landing that bent the nose gear and cracked the engine mount – clearly a major repair situation. An A&P with IA replaces the damaged engine mount and repairs related airframe sections using the manufacturer’s repair manual and AC 43.13-1B as approved data. All the work is described on Form 337, and after inspecting the repair, the IA signs Block 7 to return the aircraft to service. The completed Form 337 is sent to the FAA so the major repair is officially recorded in the aircraft’s file.
Repair station alteration on a charter plane (Part 135)	A charter company’s twin-engine airplane gets a new interior configuration (removing seats and adding cabinets), which is a major alteration affecting weight & balance. A Part 145 repair station performs the work using manufacturer-approved data and documents the changes on Form 337. The repair station’s chief inspector signs off the conformity and return-to-service on the form, and the company submits it to the FAA. Even for a commercial Part 135 aircraft, this major alteration is recorded via Form 337 in the official aircraft records.

(Each example above highlights a different approval path: one uses an STC, one uses an FAA field approval, one uses existing approved repair data, and one is done under a repair station. In all cases, Form 337 is the common method of reporting the major repair or alteration.)

Common Mistakes to Avoid with Form 337

Even experienced mechanics can flub a Form 337 if they’re not careful. Here are some frequent mistakes and pitfalls – avoid these to keep your paperwork airtight:

• Not filing the form on time: Remember, you must submit the FAA copy of the 337 within 48 hours of the work being approved for return to service. Forgetting to send it (or procrastinating) is a violation. If the FAA has no record of your major repair/alteration, you’re asking for trouble during an inspection or investigation.
• Incomplete or vague descriptions: Simply writing “Fixed wing damage” or “Installed new radio” in Block 8 is not good enough. The FAA expects detail. If the description is too sparse, you haven’t shown that the work was done per proper standards. Always provide enough information so a future reader understands the scope of the work and how it was done.
• Missing references to approved data: A huge red flag is a Form 337 that doesn’t mention any supporting data. For major repairs and alterations, you’re supposed to base the work on approved or accepted data (such as STCs, service bulletins, engineering drawings, or AC 43.13 for some fixes). If you don’t cite any data, an FAA inspector will question how you know the fix is airworthy. Always include references (STC numbers, drawing numbers, etc.) to show the work was done using proper approved data.
• Wrong signatures or no IA sign-off: Only certain people can sign Block 7. A common mistake is a regular A&P (without an IA) signing a logbook for a major repair and thinking that’s enough – it’s not. For an aircraft, an IA or other authorized person must sign Block 7 to approve a major repair/alteration. Similarly, if a repair station did the work, their authorized inspector needs to sign the form with the repair station’s certificate number. If Block 7 isn’t properly signed, the aircraft isn’t legally returned to service.
• Forgetting to give the owner a copy: By regulation, the person who performs the work has to give a signed copy of the Form 337 to the aircraft owner. If you only send a copy to the FAA and don’t provide one to the owner, the aircraft’s maintenance records will be incomplete. This can cause headaches later (during an annual inspection or when selling the aircraft, the owner might not have proof of the work). Always give the owner their copy and advise them to keep it with the logbooks.
• Misjudging what needs a 337 (or doesn’t): Some folks file a Form 337 for a change that’s actually minor – that’s unnecessary paperwork. Others assume a big mod (or even an STC install) is “minor” and skip the 337 – a dangerous mistake. Make sure you know the threshold: if it’s minor, log it; if it’s major, you must file it. When in doubt, err on the side of caution and file (or ask the FAA). Filing an extra 337 isn’t going to get you in trouble, but skipping one when it’s required definitely will.
• Typos and admin errors: It sounds trivial, but small mistakes like an incorrect tail number or a transposed digit in the serial number can cause issues. The FAA might not match the form to the right aircraft if the N-number is wrong. Double-check all the details in Blocks 1 and 2. Also ensure the form is legible; if your handwriting is messy, use the fillable PDF or the FAA’s online e-Form 337 system to avoid errors.

FAA Form 337 vs Other Aviation Documents

FAA Form 337 is just one piece of the puzzle in aircraft maintenance documentation. It’s helpful to understand how it differs from or relates to other forms and records you might encounter:

Document	Purpose & Usage
FAA Form 337 (Major Repair/Alteration)	Records any major change to the aircraft. It’s filled out by the mechanic/IA and filed with the FAA (and kept in the aircraft’s records). It does not replace the logbook entry – it’s an additional report specifically for significant work. Required whenever a repair or mod is beyond “minor” in scope.
FAA Form 8130-3 (Airworthiness Approval Tag)	A completely different form – basically an airworthiness tag for parts and components. An 8130-3 tag is issued by manufacturers or repair stations to certify that a new or repaired part is airworthy and conforms to its approved design. For example, an overhauled magneto or a new altimeter might come with an 8130-3 tag. This form travels with the part; it’s not used to document modifications on the aircraft itself.
FAA Form 8110-3 (DER Technical Approval)	A form used by a Designated Engineering Representative (DER) to approve technical data for a major repair or alteration. If you have a custom fix or mod that needs engineering analysis, a DER can review your design and sign an 8110-3 to signify the data complies with regulations. That signed 8110-3 becomes “approved data” you attach to your Form 337. In other words, 8110-3 is about the engineering approval, whereas 337 is about documenting the actual work on the aircraft.
Aircraft Logbook Entry	Every maintenance action, whether minor or major, is recorded in the aircraft’s logbooks (airframe log, engine log, etc.). The logbook entry is written by the mechanic and includes a brief description of the work, the date, and the mechanic’s signature and certificate number. For a major repair/alteration, the logbook entry will usually note “Form 337 filed” as a reference. Logbook entries stay with the aircraft and are not sent to the FAA. Think of the logbook as the day-to-day record of maintenance, and Form 337 as the special report for the big changes.

Approved Data & Field Approvals: Ensuring Compliance

One phrase that comes up repeatedly with Form 337 is “approved data.” The FAA doesn’t want major repairs or mods done arbitrarily – they need to be based on data that shows the change will meet the applicable standards. Here’s how to make sure you have the right approvals in place:

• Using existing approved data (STCs, manuals, etc.): The easiest path is when your planned repair or alteration is already covered by something the FAA has approved. The classic example is an STC (Supplemental Type Certificate). An STC is basically the FAA’s pre-approval for a specific modification on a specific aircraft model. If you install an STC kit, the engineering is already vetted – you just follow the instructions, then fill out the 337 to record it. Other sources of approved data include FAA-approved service bulletins or repair instructions from the aircraft manufacturer, or data in the manufacturer’s maintenance/repair manuals that the FAA has accepted. Advisory Circular AC 43.13-1B is even considered “approved data” for certain repairs on older aircraft when no other data exists (the regulations allow use of AC 43.13 for that purpose in specific cases).
• Field approvals (one-time FAA approvals): If you want to do something truly unique that isn’t covered by existing data, you can seek a field approval. This means you propose the repair or mod to your FSDO and provide supporting info (like calculations, drawings, maybe even test results). An FAA inspector can then approve it for your specific aircraft by signing Block 3 of Form 337. Field approvals are essentially one-of-a-kind approvals for a particular aircraft, allowing you to do a custom mod legally without a full-blown STC. However, not all FSDOs are eager to grant field approvals for complex mods – they might require you to get a DER’s sign-off or additional proof – so it’s wise to approach the FAA early and discuss your plans.
• Designated Engineering Representative (DER) involvement: DERs are individuals (usually aerospace engineers) authorized by the FAA to approve technical data. If you have a major repair or alteration that needs custom engineering, a DER can review your design and issue an FAA Form 8110-3 to certify the data meets the required standards. That 8110-3 makes your data “approved,” and you attach it to your Form 337 as proof. Sometimes the DER route is combined with a field approval – the DER approves the engineering, and then an FAA inspector signs the 337 for the actual installation on the aircraft.
• Manufacturer or STC-holder help: If your project involves the original manufacturer of the aircraft or component, they might be able to provide a repair scheme or service bulletin that is FAA-approved. Likewise, if you need a slight deviation on an STC, working with the STC holder can help – they might obtain an STC amendment or provide written permission/data for your case. Large manufacturers and STC companies often have in-house designees (engineering representatives) who can approve data. In short, involving the manufacturer or STC provider can streamline getting the proper approved data for your 337.
• Paper trail matters: Whichever route you use, attach copies of all the approvals (STC certificates, DER 8110-3 forms, field approval letters, etc.) to your Form 337, and reference them in Block 8. The FAA will archive those attachments with the form. This way, the record clearly shows that not only was the work done, but the engineering behind it was vetted and approved. If anyone questions the alteration later, the approved data is on file to back it up.

Real Consequences: Why Proper 337 Documentation Matters

Failing to use Form 337 when required – or messing up the paperwork – isn’t just a harmless oversight. There have been FAA enforcement cases and NTSB findings that highlight the importance of proper 337 usage:

• Enforcement actions against mechanics: Mechanics (and shops) have faced fines and certificate suspensions for not filing Form 337 after performing major repairs or alterations. If an FAA inspector discovers you did a major mod without the paperwork, you can expect enforcement action. For instance, there have been cases where an IA’s authorization was revoked because they signed off major repairs but never submitted the 337s – a clear breach of the rules.
• Accidents with undocumented mods: The NTSB has noted in accident investigations that undocumented modifications can hinder the investigation and even contribute to accidents. Imagine an accident where a modified fuel system is a factor, but there’s no record of that modification because the Form 337 wasn’t filed. This raises serious questions about whether the mod was done correctly and evaluated for safety. Proper documentation ensures everyone is aware of changes to the aircraft and that those changes have been vetted.
• Airworthiness and legal liability: From a legal standpoint, an aircraft that has a major alteration with no Form 337 could be deemed not airworthy. That means if something goes wrong, insurance might deny coverage, and the owner/operator could face liability issues. FAA rules (like 14 CFR 91.403) put the onus on the owner/operator to maintain airworthiness – which includes having all required paperwork in order. There have been cases of owners in trouble for unknowingly operating planes with undocumented major mods.
• FAA directives and reminders: There have even been Airworthiness Directives and FAA policy memos emphasizing the need to document certain repairs with Form 337. In one instance, after a series of incidents, the FAA reminded mechanics that even if a manufacturer’s Service Bulletin is “recommended” (not mandatory), if it involves a major alteration it still requires a 337. Some folks were incorporating SB modifications without filing the form, which the FAA had to correct through these reminders.

The bottom line: Proper Form 337 usage isn’t just bureaucratic red tape – it’s considered an integral part of safety and regulatory compliance in aviation maintenance. Skipping or mishandling it can ground your aircraft and jeopardize your certificates.

Glossary of Key Terms & Entities for Form 337

Here’s a quick-reference glossary of important terms, FAA offices, and concepts related to the Form 337 process, with notes on how each fits in:

Term / Entity	Definition & Relevance
A&P Mechanic	A mechanic certificated under FAA Part 65 (Airframe & Powerplant). Can perform maintenance and alterations on aircraft. For major repairs/alterations, an A&P can do the work and fill out the 337, but an IA must inspect and sign off the return to service.
Inspection Authorization (IA)	An additional qualification for an experienced A&P mechanic, allowing them to approve aircraft for return to service after major repairs or alterations and to perform annual inspections. The IA is usually the person signing Block 7 of Form 337 for general aviation aircraft, giving the final approval that the major repair/alteration is airworthy.
Major Repair	A repair that might significantly affect the aircraft’s airworthiness if done improperly, or that isn’t done by simple routine operations. Examples include splicing a wing spar or repairing significant structural damage. Major repairs require approved data and must be documented on Form 337.
Major Alteration	An alteration (modification) not listed in the aircraft’s original type design that could appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, etc. Examples: converting an aircraft to floats, installing a turbocharger kit, adding winglets. Must be performed per approved data and documented via Form 337.
Minor Repair/Alteration	Any repair or alteration that is not “major.” These do not require Form 337 – they’re simply logged in the aircraft’s logbooks by the mechanic. (For example, replacing a seat cover or installing minor avionics that don’t significantly alter the aircraft.)
FSDO (Flight Standards District Office)	The local FAA field office that oversees aviation safety in a region. FSDO inspectors handle things like field approvals and can advise on maintenance questions. If you need an FAA signature on a Form 337 (field approval), you’ll be dealing with a FSDO inspector.
FAA Aircraft Registry	The branch of the FAA (in Oklahoma City) that maintains ownership and airworthiness records for U.S.-registered aircraft. When you submit a Form 337, it gets filed in that aircraft’s record at the registry. Anyone researching the history of an aircraft (such as a buyer or investigator) can request the file to see all Form 337s on record.
Approved Data	Technical information that the FAA accepts as meeting required standards for a repair or alteration. Approved data can come from many sources: STCs, ADs, factory service manuals or service bulletins that are FAA-approved, FAA field approvals, or DER-approved engineering (8110-3). A major repair/alteration must be backed by approved data.
STC (Supplemental Type Certificate)	A certificate issued by the FAA that approves a design modification on a type-certificated aircraft. An STC authorizes you to make a specific alteration (e.g., engine upgrade, avionics retrofit) and serves as the approved data for that mod. You still file a Form 337 to document that you installed the STC on your aircraft.
Field Approval	A one-time approval granted by the FAA (typically by a FSDO inspector’s signature on Form 337) for a major repair or alteration. Field approvals are specific to one aircraft and one particular change. They allow unique or innovative mods to be done legally without an STC, but they require coordination with the FAA and solid data to support the change.
DER (Designated Engineering Representative)	An FAA-authorized engineer who can approve or recommend approval of technical data for repairs and alterations. If you come up with a custom fix that isn’t covered by existing data, a DER can review your design and issue a Form 8110-3 to make it approved data. DERs often work in conjunction with mechanics to support field approvals or unique repairs.
FAA Form 8110-3	The form used by a DER to document approval of engineering data. If attached to a Form 337, an 8110-3 shows that the repair/alteration data has been reviewed and found compliant with regulations. In essence, it’s the DER’s “OK” on the engineering aspects.
FAA Form 8130-3	Also known as an Airworthiness Approval Tag. This form is issued for parts/components (not for mods on the aircraft itself). It certifies that a part is airworthy and conforms to approved design, often used when shipping aircraft parts or returning components to service. It’s related to maintenance records but separate from Form 337.
Logbook	The aircraft’s maintenance logs (airframe log, engine log, propeller log, etc.) where all inspections, repairs, and alterations are recorded. Major repairs and alterations get a log entry noting what was done and usually that a Form 337 was filed. The logbooks must be kept by the owner and are reviewed during inspections, sales, etc. Form 337 information is typically referenced in the logs, linking the detailed report to the day-to-day record.
Airworthiness	The condition of an aircraft being safe to operate and in compliance with its type design (or properly altered condition). Properly filing Form 337 for any major changes is part of maintaining airworthiness from a documentation standpoint. An aircraft could be deemed unairworthy if a required Form 337 is missing for a major mod, even if it’s mechanically sound.

Pros and Cons of Using FAA Form 337

Like any regulatory procedure, the Form 337 process has upsides and downsides. Here’s a quick look at the pros and cons:

Pros	Cons
Ensures Safety & Compliance: By requiring approved data and documentation, the Form 337 process helps make sure major changes are done safely and meet standards. It creates an official paper trail for significant maintenance.	Additional Paperwork: It does add paperwork for mechanics and owners. Filling out the form and managing submissions can be seen as a hassle, especially for simpler mods that still qualify as “major” and trigger the 337 requirement.
Permanent Record of Modifications: The form becomes part of the aircraft’s permanent file, which promotes transparency. Anyone examining the aircraft’s history can see what modifications or big repairs were done and how they were approved.	Can Be Confusing: Determining what needs a Form 337 (versus what can just be a log entry) isn’t always straightforward. The form itself has to be filled out correctly, and misunderstandings can lead to errors or delays.
Enables Custom Approvals: The 337 mechanism (with field approvals and DER data) provides a way to get unique, one-off modifications legally approved without going through a full STC process. This flexibility encourages innovation and custom solutions in general aviation.	Requires FAA Buy-In: A field approval needs an FAA inspector’s cooperation, which can introduce uncertainty or delay. If your local FSDO is very strict or busy, obtaining that Block 3 signature for something new can be challenging or time-consuming.
Protects Aircraft Value: An aircraft with well-documented major repairs and alterations (all properly filed) often retains value better. Buyers and insurers have more confidence when they see that all major work was done by the book and recorded.	Penalties for Skipping: The flip side of being mandatory is that if you don’t comply, there are consequences. Failing to file a required Form 337 can lead to fines, certificate actions, and the aircraft being grounded until corrected. In other words, there’s an enforcement “stick” to ensure compliance.

FAA Form 337 FAQs

Q: Do all aircraft modifications require an FAA Form 337?
A: No. Only major repairs or alterations need a Form 337. Minor changes are documented with logbook entries only – no FAA form or field approval is necessary for those.

Q: Is a Form 337 required for installing an STC on my plane?
A: Yes. Even though an STC modification is pre-approved, you still file a Form 337 to record that the STC was installed on your specific aircraft (including the STC number and details of the kit/work).

Q: Can I submit FAA Form 337 online instead of mailing it?
A: Yes. The FAA offers an electronic Form 337 filing system (accessible through an online portal via your FSDO account). You can also still mail the paper form to the FAA Aircraft Registry if you prefer.

Q: Does an engine overhaul need a Form 337?
A: No. A standard engine overhaul per the manufacturer’s specifications is considered maintenance, not a major repair. Only if the overhaul includes a major repair (like welding a cracked crankcase) would you need to file a Form 337.

Q: Can an A&P mechanic without an IA sign off a Form 337?
A: No. A regular A&P can perform the work and fill out most of the form, but only an IA (or other authorized inspector, like a repair station’s chief inspector or an FAA official) can sign Block 7 to approve a major repair/alteration for return to service.

Q: Can I get in trouble for not filing a required Form 337?
A: Yes. Failing to file a Form 337 when it’s required can lead to FAA penalties, such as fines or suspension of your mechanic’s certificate. Plus, your aircraft could be deemed unairworthy until the paperwork is corrected.

Q: Are FAA Form 337s public record?
A: Yes. Anyone can request an aircraft’s FAA file (which includes all its submitted Form 337s) from the Aircraft Registry. It’s not posted online for casual viewing, but the records are available by submitting a request to the FAA (usually with a small fee).

Q: Do experimental aircraft need Form 337 for major changes?
A: No. Experimental-category aircraft (like homebuilts) do not use Form 337. Major changes on an experimental are handled via amendments to the operating limitations and additional flight testing, not by filing a 337 with the FAA.

Q: Do I need to carry a copy of the Form 337 in the aircraft?
A: No. You don’t need to carry the actual Form 337 on board the plane. The owner has to keep it with the aircraft’s maintenance records, and it should be available to inspectors or buyers if needed.