January 2005
This portion of the article about the application process for Supplemental Type Certificates was generated by listening to aircraft owners, A&Ps and A&P/IAs across the United States complaining about Flight Standards District Offices (FSDO) no longer approving Field Approvals Form 337 for aircraft.
I don’t know what brought about the modification to FAA Order 8300.10, Volume 2, Chapter 1, Change 15, which listed unacceptable alterations, but after hearing complaints, the FAA was prompted to create Change 16 stating what can be altered under FAA given guidelines.
Form 337 serves two main purposes. One provides aircraft owner(s) and operator(s) with a record of major repairs or alterations made to only one aircraft, an airframe, powerplant, propeller, appliance, or spare parts (indicating the details and approvals). The other provides the FAA with a copy of the form for inclusion in the aircraft records at the FAA Aircraft Registration Branch in Oklahoma City. Form 337 is, basically, a history of the aircraft.
Field Approval is a maintenance performance approval for a major repair or major alteration that is performed by a Flight Standards Service Aviation Safety Inspector. The process is used for one serial-numbered aircraft in accordance with Order 8300.10, Vol. 2, Chapter 1.
The field approval process has been in use for over 40 years and represents a collaboration between the Flight Standards Service, Aircraft Certification Service, designees, and applicants.
Field approvals are not authorized for aircraft operated under 14 CFR part 121. Some major alterations are not eligible for field approval and must be approved under another method such as a Supplemental Type Certificate (STC). These alterations are identified in FAA Order 8300.10, Vol. 2, Chapter 1.
FSDO considers the Field Approval, Form 337, to be a “Poor Man’s STC.” Anyone can fill out a 337. However, an IA or an authorized person must sign the Form 337 to Return to Service. The Form 337 is completed in duplicate (triplicate for extended range fuel tanks). One copy of the Form 337 stays with the aircraft; one copy goes to FSDO; and the third is for the prudent IA/mechanic for his file.
All aircraft modifications and installations require a Form 337, even if there is an STC available.
After completing several of them, I have discovered a systematic way of approaching a Form 337:
• Check the Service Bulletin (SB)/FAA Airworthiness Directive (AD).
• Find out why the change is required. For example, did the aircraft manufacturer put accounting before proper design and manufacturing?
• Is there an existing STC?
• Check with A&Ps, clubs, and organizations to see if other aircraft owners have made this modification or had a similar problem.
After doing the research mentioned above, I proceed directly to FAA Order 8300, Volume 2, Chapter 1, the Major Alteration Job Aid, page 1-15 through 1-26, to see if the modification classification falls into one of these three categories: Supplemental Type Certificate (STC), Evaluation (EVL), or Engineering (ENG).
The categories break down is as follows:
• Items with the letters “STC” require an STC.
• Items with the letters “EVL” may be eligible for approval by means other than an STC, depending on the scope and complexity of the alteration. These items will not automatically qualify for a field approval; they require evaluation and review of guidance to determine if the field approval process may be used.
• Items with the letters “ENG” may be eligible for approval by means other than an STC, but require either supporting Designated Engineering Representative (DER) engineering data or concurrence from the ACO for field approval.
FSDO will not sign off on anything that has to do with flight controls or when the STC says it must be done as a “One Time STC” or “DER Required.”
After this, I call my FSDO contact to see if the modification will reside at the FSDO level, or if the FAA-ACO (Aircraft Certification Office) will be involved. FSDO makes the final decision about who will be involved.
You may ask why the FAA-ACO should be involved. ACO says that the folks at FSDO are not engineers. The only requirement to be a FSDO Inspector is to be a licensed A&P. This is all immaterial, since all we want is the Form 337 approved.
Once I know the level of FAA involvement, this dictates the amount of data required for the Form 337 or indicates if an STC is required. Always remember, the modified aircraft must meet the original certification basis to its design requirements.
I was the project leader on a Form 337 for an electro-mechanical pneumatic system to allow a paraplegic pilot to fly a Schweizer 300CB Helicopter (which I believe was the first one of its kind in the world).
Using this as an example of who will be involved by FSDO, the data required in this Form 337 was:
• Instruction for continued airworthiness
• Aeronautical Engineering Report by a DER
• System safety reliability analyst report
• Addendum to the Flight Manual including: installation drawings and systems diagram weight and balance
• Five flight tests
• Special Airworthiness Certificate
The personnel involved were:
• Applicant
• FSDO
• DER
• FAA-ACO
• AFS 600 & 800
• CAMI (aero medical group)
FSDO had requested assistance from other departments of the FAA before approving the Form 337. The Flight Test division at the FAA-ACO did not want to approve the system because of single point of failure, and we did not have a mechanical backup, which all other areas of the system did.
Our flight manual became an addendum to the airplane flight manual, and the FAA-ACO handed the responsibility back to FSDO, who considered the fact that the unit was removable like a GPS and approved the Form 337.
The process took only three months from start to finish. The FSDO Inspector and I attended a reliability seminar, at which single point of failure was mentioned. The speaker gave us a definition of single point of failure. We looked at each other with amazement—we both determined at the same time that single point of failure was the pilot!
If the FSDO Inspector does not feel comfortable with the request, one can always look for another FSDO. (This is known as “FSDO shopping.”) Ask around until you find someone who will listen to you and take on your project.
However, once you find a FSDO person who is willing to take on the task, the A&P and A&P/IA will have to be in the same region. For my last modification, the aircraft owner had his plane based at El Monte, Calif. under the Los Angeles FSDO. This contact did not feel comfortable with it, so I contacted another Inspector at Long Beach, who did. To keep it simple, I had the modification completed by Ces-Air in Long Beach.
Personally, I use Form 337 either for gathering data during the modification for an STC or when amending the STC for an additional aircraft requiring a conformity installation. While waiting for the STC (or the amended STC) to be issued, and after the modification is done, the aircraft must be flyable.
If this is an amended STC, I reference the STC number in block 8 of the Form 337. Since all my data is on file with the FAA-ACO, the FSDO will only have to confirm this before approving the 337. In block 8, the description of work is a scaled-down description from the data package that references the installation instructions and engineering drawings. This informs FSDO that you are going for a Multi-STC instead of a single aircraft installation.
I recommend having 337 approval before starting work. If FSDO does not approve it, the aircraft is grounded and has to be returned to original condition. This makes the alteration even more expensive.
Filling out a Form 337
Completing a Form 337 is reasonably easy. Now, having it approved—this is another story. AC 43.9-1E, instruction for completion of FAA Form 337, provides excellent guidance material.
However, the following is an overview:
Blocks 1 and 2 pertain to information on the aircraft and owner of the aircraft.
Block 3 is reserved for the FSDO stamp and the signature of an FAA inspector when the field approval is granted. This is supposed to be issued after the paperwork is submitted to FSDO. If ACO is involved, you need concurrent approval before the work is accomplished. When the inspector determines that design change complies with the applicable regulations, FAA approval is granted for the modification or installation by completing FAA Form 337.
Approval by FSDO is specified in one of the following ways:
• Approval by examination of data only
• Approval by physical inspection demonstration, testing, etc., of the data and aircraft
• Approval by examination of data only—duplication on identical aircraft. However, the Form 337 is non-transferable, and FSDO cannot approve data for use on multiple aircraft.
FSDO will give a go-ahead on approved data with OEM specification. The Form 337 is not accepted by the FAA-ACO as data, if you are thinking you can use this for an STC. Field Approval is used by the FSDO to document the approved technical data that is used for a major repair or major alteration. Minor changes and some major changes can be field-approved. This means that FSDO or an authorized designee inspects both modification and supporting data.
A simple way of looking at a major alteration and approved data is:
• If there is approved data, submit FAA Form 337 to FSDO.
• If there is no approved data, this requires a Field Approval and Block 3 is stamped.
To avoid the confusing FSDO or FAA-ACO, this is what you can do:
• No approval stamp is required in Block 3 when you have a STC or minor change.
• If a DER issues a Form 8110-3, then a field approval is not necessary. However, DER data is not a field approval; the 8110-3 is the approved data stated in Block 8 and attached to the Form 337 when submitted to FSDO. (Side note here: if a DER happens to be an IA, he/she can approve the data and Return to Service on the Form 337. However, this is considered, in appearance, a conflict of interest.)
• If an STC exists, but the Aircraft Master Eligibility List (AML) does not have that aircraft listed, the STC may not be used. You must have an approval letter from the STC holder and include the copy of the letter with the copy of the STC when you submit the Form 337 to FSDO. However, you must still obtain a Field Approval.
Blocks 4 and 5 pertain to Unit Identification and Type of Repair or Alteration.
Blocks 6 and 7 relate to information regarding the repair station, A&P, and IA. The Approval for Return to Service identifies the person or agency making the airworthiness inspection.
Block 8 is the Description of Work Accomplished. “Tell a story,” one FSDO inspector explained when filling out Block 8. This is a clear, concise, and legible statement describing the work accomplished. It is important that the location of the repair or alteration, relative to the aircraft or component, be described.
The approved data is used as the basis for approving the major repair or alteration for return to service and should be identified and described in this area. Instead of using a lot of descriptive words to define the location and alteration, I go by the old saying, “One picture is worth a thousand words.”
Instead of spending hours writing the description, show the modification in a drawing or several drawings. Therefore, I always use engineering drawings instead of words and photographs to depict the alteration with clarity. People tend to understand things better when viewing a drawing than they do by trying to comprehend a bunch of words, so I try to be very helpful. This goes a long way in gaining brownie points to have the paperwork approved by the FAA.
There are two types of data: Approved Data and Acceptable Data.
Approved Data is substantiating and descriptive technical data that is approved by the Administrator for use when making a major repair or alteration (In a nutshell, the FAA is looking for certified traceability whether in a Field Approval or STC).
The following list, although not all-inclusive, contains sources of approved data:
• Type Certificate Data Sheet (TCDS)
• Aircraft specifications
• Supplemental Type Certificate (STC)
• Airworthiness Directive (AD)
• Manufacturer’s FAA-Approved Data (DOA) is also Acceptable Data
• Form 337 is Acceptable Data, within certain limitations, and can be used as data for an identical Form 337 of the identical make, model aircraft for an identical alteration by the original modifier; 337s approved in 1955 and before may be used as Approved Data.
• Service Repair Manual (SRM) is also Acceptable Data
• Designated Engineering Representative (DER)
• Designated Alteration Station (DAS)
• Technical Standard Order (TSO)
• Special Federal Aviation Regulation (SFAR)
• Parts Manufacturer Approval (PMA)
• Service Bulletin (SB)
• Appliance manufacturer’s manuals (excluding Installation Instructions)
Acceptable Data, as noted above, and AC 43.13-1B, Acceptable Methods, Techniques, and Practices, may be used as a basis for developing approved data to substantiate repairs or alterations. Approved data is also listed in AC65-19G.
Normally, I use certified aircraft parts and material on modifications. However, on an oil cooler swap, the oil cooler manufacturer, Stewart-Warner (well known by the FAA), did not have an STC, PMA or TSO on the unit. The oil cooler came with a Certificate of Compliance, which does not carry the same weight as the STC, PMA, or TSO.
This oil cooler had been installed on other aircraft because the aircraft manufacturer bought the oil cooler from Stewart-Warner. They installed the oil cooler under their Type Certificate. This did not require FAA approval.
FSDO stated that if I could find a similar oil cooler used on a similar aircraft and engines—known as Comparative Analysis—we would be allowed to continue with the alteration. I was able to accomplish this task by locating a similar aircraft, engine, and oil cooler with the generous assistance of Pacific Oil Cooler, El Monte, Calif.
However, the oil cooler swap fell into the engineering category of FAA Order 8300.10, Volume 2, Chapter 1, so the FSDO requested FAA-ACO assistance. With this in mind, I listed the installation procedures on the back of the Form 337 in Block 8, and included an Engineering Report containing the following information:
• Scope
• Certification basis
• Description
• Compliance data
• Mechanical installation
• Compliance with FARs
• Compliance with FAA Order 8300.10, Volume 2, Chapter 1
• Load analysis
• Weight and balance data
• Compliance with Instructions for Continued Airworthiness
• Instructions for operations
• Conclusions
• Reference
• Appendix
A five-page report with 32 pages of substantiating data may be an overstatement. In addition, there were a few obscure questions from the FAA-ACO. What is the aerodynamic loading on the oil cooler? (No comment is required here.) Where does the exhaust air from the oil cooler go? (I will give you hint—through the cowl flap doors).
My all-time favorite was, “the applicant needs to address the effect of inlet air duct flow chocking .”
Well, my response was somewhat modest: “I was unable to find the engineering definition of chocking, however, I am familiar with the term choked flow. The duct is designed as straight tube from the engine cowling going back to a semi-fan out an opening the size of the oil cooler face, at which point the highest speed obtained at this location is 106 mph. In this configuration, it would be impossible to obtain sonic velocity. The other condition for choke or stone wall point is in dealing with dynamic compression, which does not apply to this condition.”
After two months and more questions, the larger oil cooler was approved for installation on the Cessna 177RG. Patience and a good sense of humor can be a virtue when working with the FAA.
Some wording that might be included in Block 8 as follows:
• Installed: one widget. Describe the unit with part number installed per STC number. Provide drawing number and revision letter (your engineering drawing data); give the date in accordance with installation instruction manual, revision letter, date and procedures outlined in AC43.13-1B.
• State that the installation conforms to Acceptable Methods, techniques, and practices contained in AC 43.13-1B and AC43.13-2A. Include weight and balance computations in Block 8. If you cannot, attach the weight and balance computations to a separate sheet and attach to the Form 337. Weight and Balance computations are not required if the weight change is less than one pound.
Advisory Circular 43.13-B – contains methods, techniques, and practices acceptable to the Administrator for the inspection and repair of nonpressurized areas of civil aircraft, only when there are no manufacturer repair or maintenance instructions. This data generally pertains to minor repairs. The repairs identified in this AC may only be used as a basis for FAA approval for major repairs.
The repair data may also be used as approved data, and the AC chapter, page, and paragraph is listed in block 8 of FAA form 337 when:
• The user has determined that it is appropriate to the product being repaired;
• It is directly applicable to the repair being made, and
• It is not contrary to manufacturer’s data.
Include Instructions for Continued Airworthiness (ICA)—this documentation details “how to” maintain the aircraft, engine, or propeller. This describes any maintenance requirements necessary to maintain aircraft airworthiness, provided in accordance with the applicable FAR regulations. In my case, this was FAR 23.1529 and Appendix G and FAR 43.
Airworthiness—the FAA definition per AC120-77 is that the aircraft must conform to its Type Certificate (TC). Conformity to type design is considered attained when the aircraft configuration and the components installed are consistent with the drawings, specifications, and other data that are part of the TC. This would include any STC and field approval alterations incorporated into the aircraft.
The aircraft must be in a condition for safe operation. The condition of the aircraft relative to wear and deterioration (e.g., skin corrosion, window delaminating/crazing, fluid leaks, tire wear, etc.) must be acceptable.
Include ICA Check List. (If a subject is not applicable, then the title would be filled in with “N/A” next to the heading):
• Introduction
• Description
• Control, operation information
• Servicing information
• Maintenance instruction
• Troubleshooting information
• Removal and replacement information
• Diagrams
• Special inspection requirements
• Application of protective treatments
• Data • List of special tools
• For commuter category
• Recommended overhaul periods (for aircraft operating under FAR part 91, this recommended overhaul time is not mandatory)
• Airworthiness limitation section
• Revision
• Implementation and record keeping
For a major alteration performed in accordance with FAA Field Approval policy, the owner/operator is responsible for insuring that the ICA is made part of the applicable section 91.409 inspection program for their aircraft.
This is accomplished by a maintenance entry, in accordance with section FAR 43.9, that will be made in the aircraft’s maintenance record to record the major alteration and identifying the original ICA location. For example, Block 8 of the FAA Form 337 might be dated 5/28/98. A statement must be included that the ICA is now part of the aircraft inspection/maintenance requirements.
What is FSDO looking for?
Here is what the FSDO Inspectors will review and inspect.
Some of this information may or may not be required, depending the complexity of the alteration or repair:
• Engineering Report
• Proposed Flight Manual Supplement
• FAA Form 8110-3
• Description of the proposed alteration or repair to ensure that it correctly and accurately describes the alteration or repair
• Methods, sketches, drawings, stress analyses, photographs, electrical load analyses, etc., to ensure that the operator has considered all applicable design standards and has analysis to substantiate the findings in this regard.
The inspector must consider at least the following:
• The certification basis, including special conditions (fail-safe, damage tolerance, etc.)
• The structural requirements that may be affected by the alteration or repair
• Any hazards that may affect the aircraft occupants
• Weight and balance computations
• Operating limitations
• Any other factors affecting safety of airworthiness
• All ground and flight tests and operational checks that meet applicable certification requirements to substantiate the alteration or repair.
• Instructions for Continued Airworthiness
FSDO will evaluate the proposal and determine if the applicant has conducted a conformity evaluation to ensure that the proposed alteration will not impact the airworthiness of the aircraft. The applicant will provide verification that he/she has inspected the aircraft and reviewed aircraft records to ensure compatibility of this alteration or repair with previously approved modifications.
FSDO may require or have the FAA-ACO inspect the alteration on completion of the alteration or repair.
Conclusion
After working with the FAA for a number of years, I do know one thing: if you turn in the paperwork and think you’re finished, you are wrong. Follow up with phone calls or e-mails on a regular basis to find out the status of your paperwork. This is required to fulfill your obligations.
Form 337 can be complicated or relatively easy. This depends on how you want to approach the process and what you would like to accomplish.
While FSDO may not be engineers, they are putting their signature on your Form 337, and they do not want an accident to come back to haunt them. The FAA’s biggest concern is safety; and they will not approve an alteration that is unsafe. However, safety is relative to any alteration, and at times, it can never be overstated.
Norm Ellis is an instrument-rated private pilot, who has flown 23 different General Aviation aircraft types. He holds three Multi-STCs on five different Type Certificate aircraft that are certified on 88 aircraft with 30 more pending.
