Confusion often surrounds STCs (Supplemental Type Certificates), FAA Field Approvals, and 337s (FAA Form) as they relate to aircraft modifications.  When are they used?  How are they used?  How can you tell the difference between them?

As Stearman owners, we need to understand these methods of making approved modifications to our airplanes.  Believe it or not, there are many Stearmans flying at this moment that are not legal!  Why not make sure, as an informed owner/operator, that YOUR airplane has legal approvals for existing modifications.

 First, let’s start with some definitions:

 What is an STC?

A supplemental type certificate (STC) is a certificate issued when an applicant has received FAA approval to modify an aircraft, propeller, or appliance from its original design. The STC, which incorporates by reference the related type certificate, approves not only the modification but also how that modification affects the original design.

 What is an FAA Field Approval?

A Field Approval is the granting, by an FAA airworthiness inspector, of FAA “approval” for a major repair or major alteration. The approval is given only after conducting a physical inspection and/or after reviewing data.

 What is a Form 337?

The FAA Form 337 is required anytime you have performed a major repair or major alteration.

 Confused?  Don’t be.  Here’s an explanation that will help.

When we speak of an STC we are speaking about a modified item such as an airframe, engine or component. The paperwork for this change has gone through a number of agencies in order to obtain approval. The approval is based on existing Federal regulations and approval criteria used by the FAA to determine the airworthiness of the modification.

 When a particular approval is used, the accompanying paperwork i.e., drawings, descriptions and related manuals, MUST be retained with the aircraft permanent records. This is to insure the proper installation and inspection upon completion and that it meets the required criteria on subsequent annual inspections.

 Upon obtaining an STC, the owner of the approval will often request documentation regarding the make, model, and serial number of the aircraft on which the STC will be installed.  It is important to note that the person who obtains the approval owns the STC and its related information–all drawings, data, specifications -. This person has the legal right to sell his information on deny the use of it all together. You must contact the STC holder to seek written permission. The FAA will not release this information without authorization from the owner.

 We can’t always find the owner of the STC! If this is the situation, the most common practice is to obtain a copy of the approval from another aircraft. This procedure is generally accepted unless the approval incorporates the use of parts that are manufactured by the STC holder.  No one but the STC holder can manufacture parts for this approval without the permission of the approval holder.

 A FAA form 337 must accompany all STC installations. This form is used to document the existence of either a major alteration or a major repair to an airframe, power plant or component. When an STC’d item is installed it is a major alteration to the area in which it is installed. Any 337 along with the STC documentation must be a part of the aircraft’s permanent records.

 Now that you understand what an STC is, what is a Field Approval?  Remember, we said a Field Approval is the granting, by an FAA airworthiness inspector, of an approval of a major repair or alteration.  This approval is given only after conducting a physical inspection and/or after reviewing data.

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

 Ø EXAMINATION of data only: This is the most common form of Field Approval. The mechanic 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.

 Ø 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.

 Ø 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. You can submit the data to be approved along with a request that the data approval be extended to other identical aircraft.

 When attempting to obtain a field approval the following criteria should be followed.

 Make sure you follow these procedures prior to doing any work on the aircraft.

(1) All criteria regarding the alteration should be submitted with the 337.

(2) The 337 form should be submitted to the FAA by a licensed mechanic holding an inspection authorization who will be responsible for its execution.  The form should not be dated or signed at this time.

(3)    Remember, all information submitted must have some approval basis in order to meet the requirement for further approval. This could be installation on other aircraft of similar type; engineering data from a DER (Designated Engineering Representative) or a similar STC.  If an STC is used, make sure it covers Standard Category Aircraft.

 Is Your Stearman Restricted Category or Standard Category?

 There are numerous Stearmans out there that have been converted from agricultural aircraft and are accompanied by a 337 with an STC number.  Many of these STCs are for Restricted Category only and may not be used when the aircraft is returned to Standard Category.  Check your paperwork!

 If all of the criteria as mentioned above are met, there is still no guarantee that a field approval will be forthcoming. Since the agent involved is responsible for the safety and airworthiness of this change, he or she may be hesitant to put their name on the dotted line. So, the more substantiating information the better.

 Many aircraft in the field have modifications that have no substantiating approvals. This can be a problem at annual time. The IA, that is an A&P mechanic holding an inspection authorization, is responsible for the aircraft meeting the approved criteria of its type data sheet. If there is some modification to the aircraft it must be backed up by some sort of documentation. Most of the time it will be in the form of a 337. With aircraft that have been highly modified with no substantiating documentation it is possible to expect a revocation of its airworthiness certificate. Re-issuance of the certificate after this happens is not easy and can be extremely costly.

 In summary, if we are dealing with an STC, we know that it has been previously approved. Make sure that your make; model and serial number is on the STC eligibility list. Make sure that a form 337 is submitted prior to the aircraft being returned to service.  It is the owner/operator’s responsibility to make sure that all documentation is filed with the FAA!

 What about the engine on your Stearman?

Changes such as an engine substitution may or may not require a 337 or a field approval.  If, for instance, you are running a Continental in your Stearman and you want to install a 300 HP Lycoming, you can accomplish this by using the information stated in the aircraft’s type data sheet. There is no STC required for this particular installation.  A log book entry is the only paperwork required (no 337). Of course, a new weight and balance, along with a supplement to the operations manual for the aircraft will have to be accomplished.  Make sure that any A.D. (Airworthiness Directive) pertaining to this particular engine is also on record in the engine log.  Remember you should have a separate log book for the airframe, engine, and propeller.  Each one will be stamped with the total time of the components and the date and total time at the annual or 100 hr inspection.

Common questions regarding STCs and Field Approvals  (Portions of the following information have been obtained from the FAA website –

If the mail you sent was returned “undeliverable” and there is no phone listing for the holder, contact the issuing FAA office. STCs are approved and issued through FAA aircraft certification offices (ACOs) which serve the geographic area of the STC owner’s residence.

For information regarding the STC application process, you may contact the ACO in your geographic area. You may also find it helpful to review related orders and advisory circulars, in particular, Order 8110.4B, “Type Certification Process,” and AC 21-40, “Application Guide for Obtaining a Supplemental Type Certificate”.

All 337’s are mailed to FAA’s Aircraft Registration’s branch and filed in individual folders by registration number. You may visit FAA aircraft registry’s website at or contact them at:

Aircraft Registration Branch, AFS-750
P O Box 25504
Oklahoma City OK 73125
(Phone: 405/954-3116)

 Helpful Hints for Field Approvals

 Do not cut metal, splice wire or install equipment until you receive the approval.

  • 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.
  • Assemble data 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. For field approval of  major alterations you must submit Instructions for Continued Airworthiness; These may be submitted on the back of a Form 337 or as a separate sheet attached to your Form 337.  A cover letter for the Form 337 describing in detail how you are going to accomplish the repair or alteration is also helpful.

· Last, but not least: When submitting your Stearman for an airworthiness certificate (from Restricted to Standard), the mechanic holding the inspection authorization must sign off on an annual inspection.  He is stating that the aircraft conforms to its type data sheet and any modifications are done in accordance with approved criteria (STC or Field Approval) for standard category aircraft, not restricted or is not followed, whoever signs off is placing himself or herself in jeopardy.  The FAA has a tendency to frown on fraudulent documentation!  Also keep in mind that because some FAA inspection issued as a standard airworthiness certificate it does not mean that all items on the aircraft have been approved.  The signer is depending on the IA who signed the annual inspection.  When the aircraft is due for its next annual, the IA signing it at that time is responsible for its legal condition and cannot fall back on the signer of the airworthiness certificate.


 Many people have a tendency to think that if the particular aircraft they are flying is “in annual” that they have no responsibility.  Not so!  AD notes and modifications/alterations that you think are no big deal really ARE a big deal.  As the owner and/or operator of the aircraft, you are responsible for complying with these notes and making sure changes are legal.  Last but not least, remember that preflight inspections are required, and if you ignore items that render the airplane un-airworthy, YOU are responsible.


Sounds complex, but really the ins and outs of staying legal in your Stearman and still having the performance and looks that you want are very possible.  After reading this article you will know more than many IA’s .  Find a knowledgeable A&P/IA who has spent time working on Stearmans and other aircraft of similar make.  These people will help you decipher the rules and paperwork