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Form 337

Updated April 23, 2006

Form 337s are one of the more confusing areas of FAA regulations. We've assembled information from the FAA and Flight Safety District Offices to help you understand more about this process. If you have any Form 337s for your aircraft, please contact us! We want to start accumulating a database of these forms that others can use for a starting point to get 337s for their own aircraft. (This is expecially true for older classic airplanes.) Our email address is feedback@PopularAviation.com.

We do have a collection of Luscombe 337s available by clicking here.

The FAA Form 337 is required anytime you have performed a MAJOR REPAIR and/or MAJOR ALTERATION. If you are not sure whether the maintenance that is being performed will be considered a Major Repair/Alteration, look at Appendix A of FAR Part 43 and the definitions of Major Repair/Alteration in FAR Part 1.

A common misconception is that FAA Form 337 replaces the required maintenance record entry. This is not true. FAA Form 337 does not replace the maintenance record entry required by FAR Part 43. The last paragraph of Section 43.9(a)(4) states "In addition to the entry required by this paragraph, major repairs and major alterations shall be entered on a form, and the form disposed of, in the manner prescribed in appendix B, by the person performing the work." So, according to the regulations, a maintenance record entry is required even after performing a major repair or major alteration.

Another misconception is that a work order can be used in place of FAA Form 337. According to FAR Part 43, Appendix B, FAA certificated repair stations may use their work order in place of FAA Form 337 for major repairs only.

If you have any questions concerning FAA Form 337, contact your local FSDO.

Approved vs. Acceptable Data
Getting 337s thru the system has become a real nightmare for aircraft owners, in part because FAA FSDOs want "approved data". 337 approvals dated before October 1, 1955 are treated just like information on the TCDS ("approved data"). This extremely important difference in the definitions between "acceptable data" and "approved data" is what makes early 337 forms so useful and important to us today.

Four regulations, FAR 65.95, FAR 121.378, FAR 135.437, and FAR 145.51 all require “approved” data for major repairs and major alterations.

The difference is laid out in Order 8300.10, Vol. 2, Chapter 1, Section 1, (5)(A)(4)(f):

Approved data can be Type Certificate Data Sheets (TCDS), Airworthiness Directives (AD), Designated Engineering Representative (DER) data, Designated Alteration Station (DAS) data, FAA-Approved data, FAA-Approved Manufacturer’s data, Supplemental Type Certificates (STC) and appliance manufacturers manuals, and Form 337s dated before October 1, 1955.

If you can’t find approved data of this type, your only chance for getting data approved is to apply to the FAA for an STC or to ask a local FAA inspector for a Field Approval.

There are three different kinds of Field Approvals for which the local FAA inspector can sign off:

  1. EXAMINATION of data only: This is the most common form of Field Approval. The mechanic or repairman submits “acceptable” data to the local FAA office for approval. The “approved data” can be used to perform a major repair or major alteration. Once the data has been approved under this procedure it can be used only for that one aircraft (described in Block 1 of FAA Form 337). However, if you want to do the exact same repair or alteration to another like make or model aircraft you can use the original Form 337 as the basis for obtaining a new Field Approval for the second aircraft.

  2. PHYSICAL INSPECTION, demonstration or testing of the repair or alteration: This is rarely done except in cases where technicians find unapproved engine or components installed on aircraft, which apparently have been installed for some time. Since the aircraft has flown successfully for many hours, and FAA inspector can, if satisfied with the installation, approve the installation. He does so by signing a new Form 337.

  3. EXAMINATION of data only for duplication on identical make and model aircraft by the original modifier: This is a procedure that saves the maintenance technician and the FAA a lot of time. For example, one technician wants to install duplicate avionics packages on as many Cessna 501s as possible; or maybe he wants to install duplicate installations of tundra tires on Beech 18s. The technician can submit the data to be approved along with a request that the data approval be extended to other identical aircraft. The FAA inspector, if satisfied, signs Block 3 that grants duplication of the data for the original Form 337. When the technician finishes a duplicate alteration on other aircraft, he sends the FAA a regular FAA Form 337 properly filled out listing the “approved data” on the back and making references to the Field Approval. To avoid problems, attach a duplicate copy of the original Field Approval From 337.

What Cannot be Approved:
Some repairs and alterations are so complex they are actually design changes and require an STC. The following alterations are examples of alterations that can't be Field Approved.

  1. Increase in gross weight and/or changes in center of gravity range

  2. Installation, changes, or relocation of equipment and systems that may adversely affect the structural integrity, flight, or ground handling characteristics of the aircraft

  3. Any change (alteration) of movable control surfaces that may adversely disturb the dynamic and static balance, alter the contour, or make any difference (plus or minus) in the weight distribution

  4. Change in control surface travel outside approved limits, control system mechanical advantage, location of control system component parts, or direction of motion of controls

  5. Changes in basic dimensions or external configuration of the aircraft, such as wing and tail platform or incidence angles, canopy, cowlings, contour or radii, or location of wing and tail fairings

  6. Changes to landing gear, such as internal parts of shock struts, length, geometry of members, or brakes and brake systems

  7. Any change to manifolding, engine cowling, and/or baffling that may adversely affect the flow of cooling air

  8. Changes to primary structure that may adversely affect strength or flutter and vibration characteristics or damage the tolerance design philosophy

  9. Changes to systems that may adversely affect aircraft airworthiness, such as:
    • Relocation of exterior fuel vents
    • Use of new type or different hydraulic components
    • Tube material and fittings not previously approved

  10. Changes to oil and fuel lines or systems that may adversely affect their operation, such as:
    • New types of hose and/or hose fittings
    • Changes in fuel dump valves
    • New fuel cell sealants
    • New fuel or oil line materials
    • New fuel or oil system components

  11. Any change to the basic engine or propeller design controls, operating limitations, and/or unapproved changes to engine adjustments and settings having an affect on power output

  12. Changes in a fixed fire extinguisher or detector system that may adversely affect the system effectiveness or reliability, such as:
    • Relocation of discharge nozzle or detector units
    • Use of new or different detector components in new circuit arrangements
    • Decreasing amount or different type of extinguishing agent

    li> Changes that do not meet the minimum standards established in a Technical Standard Order (TSO) under which a particular aircraft component or appliance is manufactured

  13. Modifications to approved type (TSO) radio communications and navigational equipment that may adversely affect reliability or airworthiness, such as:
    • Changes that deviate from the vacuum tube or semiconductor manufacturer's operating limitations
    • Any changes to IF frequency
    • Extension of receiver frequency range above or below the manufacturer's extreme design limits
    • Major changes to the basic design of low approach aids
    • Changes that deviate from the design environmental performance

  14. Changes to aircraft structure or cabin interior of aircraft that may adversely affect evacuation of occupants in any manner

Helpful Hints for Field Approvals
First, do not cut metal, splice wire or install equipment until you receive the approval. The only thing worse than not getting a Field Approval is finding out that the expensive equipment you installed in your aircraft has to be removed.

Determine if the repair or alteration is major as defined by FAR 1. If it is major, 337s are applicable.

Do not set unreasonable goals. Allow a reasonable time, at least 30 days for the Field Approval.

Research all sources for “approved data” to make the repair or alterations. Find out what kind of data the inspector wants to see. Then assemble it in a reasonable and understandable format. The data must be current, accurate and must support as well as describe the alteration or repair. Data can be in the form of drawings, sketches or photographs. References to AC 43.13-1B and 2A. manufacturer’s maintenance manuals, kits, bulletins, and service letters may be helpful.

A cover letter for the Form 337 describing in detail how you are going to accomplish the repair or alteration is also helpful. Vague or useless technical references are unprofessional and should be avoided because it destroys your credibility.

With your research completed, send the FAA inspector duplicate copies of the Form 337 along with the data you want approved.

If you did your homework carefully and followed these helpful hints, you will have an excellent chance of getting your repair or alteration approved on the first attempt. If you do not, find out what is wrong and try again.

You can download a PDF version of Form 337 by clicking here. Adobe's Acrobat reader can be downloaded at www.adobe.com/products/acrobat/readstep2.html.

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