Following are copies of posts regarding installation and approvals for use of Vernier Engine Controls in Short Wing Piper Aircraft.  Also posted on this site are letters from ASC Products Company, manufacture of Vernier Controls that explain the difference between their certified product and non certified product.

Andy


Vernier Controls - Post by H.L.
The vernier control issue has been at issue for years, and when discussed, it usually yields everything from good success in getting it approved to horror stories. In addition to what John outlines above is a category of part referred to as a PMA'd part. PMA means Parts Manufacturing Approval. PMA parts are replacement aircraft parts made by someone other than the original equipment manufacturer (OEM). Referencing to the 337 issue, the reason you are going that route and first to the FAA is that you have a non-PMA'd part. (such as the ACS brand of controls that Aircraft Spruce sells). The first step in that process was to submit DATA to the FAA to substantiate that your desired part is airworthy and is fit for the specific purpose for which it is intended. That is what the FAA is signing off on when he signs Block 3. He is concurring with you that the DATA proves the part to be airworthy. When you have a PMA'd part, the FAA system for granting PMA to a manufacturer has already done all of the inspections and testing and documentation and everything else to buy off on the suitability of the part as manufactured for use in aircraft. Thus Block 3 FAA signature is not needed. In fact, if the replacement is not a Major Modification, no 337 is needed at all, just a log book entry. So, if your mechanic were to find a PMA'd vernier control, he would be authorized to put it in your airplane. (I will dispense with starting the argument as to even whether a 337 is needed, I don't think it would but others would disagree.) There have been PMA'd vernier mixture controls available for our planes, but they cost about twice as much as a non-PMA'd control. In the WAG-Aero catalog you will find them. These same issues are applicable to a vernier throttle cable. So, your choice is to pay the price for a PMA'd vernier control and have a quick and painless installation, or save money and jump through all of the hoops necessary to have an FSDO Data approval on a 337 and subsequent installation on that 337. If you note the paperwork on this WEB site, there is a statement from the manufacturer stating pull strength and equivalence of manufacture. That may be all you need, however if the FSDO wants more, the manufacturer is the best source for more information, remember, you have leverage, he wants to sell his part, so the onus of assistance to you should be on him. Here is one thing about this process that you should hear, and it is straight from an associate of mine in the FAA. He said, "If you want to put a non-PMA'd part in the plane, and want me to sign off on it's suitability, don't just show up in my office with someone elses soggy old 337 and expect me to grab my pen and sign off your Data." He said it is up to the mechanic to do his homework and determine airworthiness, and submit to the FAA the Data to defend his determination. Good Luck, and inspite of all this imposing verbosity, the process by and large works pretty well. 

Vernier Controls - Additional Post by H.L.

At the risk of "here we go again" , well, here we go again. I discussed this issue with several OEM connected persons, and we all agreed on one concensis. The letter that is on this WEB site is insufficient data about the ACS controls, and if that is the only thing we have to use to justify our belief that it is airworthy, we have a weak position. I then contacted the ACS folks, and discussed it with them, and lo-and-behold they furnished me a letter that has what to my pea brained engineering mentality feels is exactly what we need. It begins with the magic words "conformity statement". Now, contrary to the belief that there is no PMA'd throttle or vernier cable for our planes, ACS has two PMA'd controls(they are in the Aircraft Spruce Catalog.) Part Number A-800-12 (throttle) and A-790-12 (Vernier Mixture). They are 53 inches from the nut to the end of the cable. A point of interest here is that I could not find anything that specifies how long the control cables in a PA-22 needs to be. Mine measure 49 inches, so I am of the belief that a 53 inch cable is an acceptable length so these part numbers will work. However, they are twice the cost of the non-PMA'd A-790 and A-800 cables that are avaiilable in exactly 48 inch and 53 inch lengths. The conformity statement I have says that the A-790, and A-800 cables are exactly the same as the A-800-12, and A-790-12 PMA'd cables in manufacture, testing, and it lists the use of the identical parts, and states that they are drawn from the same lots. These statements plus some other verbiage should in my mind overcome any FSDO belief that there is insufficient data available to defend their airworthiness. So, we need someone to try out this letter and see how it works. I am going to try it, but my schedule is a Zoo these days and I may not get to it in a timely manner, so if anyone wants a copy of these let me know, or perhaps Mr. Anderson would agree to post them on his WEB site. The only proviso I would like to see is that whoever uses them the first time keep the membership appraised of how the process is working and provides any information from their FSDO that may make it easier for the next person. Now my next question is relative to Steve P and John W belief that they can be the determination of the acceptability (PMA or not) for a simple cable. Here goes: In the PA22 parts manual, I see the Throttle and Mixture cables, but I see no specifications for them. That being the case then doesn't that put them in the same category as a basic drawing item marked PURCH, meaning that it is such "garden variety" that detailed specifications are not necessary and that an A/P can indeed be the determining authority for the airworthiness for a suitable replacement ? With that in mind, I think I could find dozens of things on a PAxx that are visible in parts breakdowns but are not specified, nor PMA'd parts. Wouldn't a simple cable assembly be within the same category ? In Kevin Chambers case, he was caught between a rock and a hard place, the inspector was saying he had an illegal part and Kevin did not have the time to protest the determination or get some clarification. It really does seems like this is a general subject that maybe should be introduced for "guidance" or "clarification" from on-high.


General Post on Form 337 from J.W.
Ray; The Form 337 is used TWO DIFFERENT WAYS. The Primary use is to record a Major Repair or Alteration that is done in accordance with (iaw) APPROVED Data. This is most often done with a Supplemental Type Certificate (STC). An example of this would be recovering an airplane. You (specifically, the person who will be signing for the work) would use an STC that tells you what materials and procedures you need to use to cover the airplane, and this information is APPROVED by the FAA. The work is done, the Form 337 is filled out, the statement that the work was done iaw the STC mentioned above is made, and an "appropriately rated person" (an IA) returns the aircraft to service. No other special approvals are required. 

The OTHER use of a 337 is to record a Major Repair or Alteration that does NOT have "APPROVED" data (no STC or other approved data exists). This allows for a way to repair or alter an airplane by doing something that may well have not ever been done before (but if it HAS, the Approval is for THAT airplane ONLY). THAT requires that the job be done iaw "acceptable" data (practices, methods, techniques) and that the work be done "in an airworthy manner". This also requires an "APPROVAL" made by the FAA for that repair or alteration, and that approval is once again limited to the ONE AIRPLANE listed on the Form 337. That is what is known as a "Field Approval (on Form 337)". This approval can only be made for alterations if if doesn't require (in FAA's opinion) addressing several important issues that affect airworthiness. Structural changes in the basic airframe, for example, would require Engineering Evaluation to see if they adversely affect the airworthiness of the airplane that is so modified. Once you get into THAT realm, the FAA requires an STC, even if for just that one airplane! There are other examples of "Approved Data", and other examples of specific requirements; but this discusses your question specifically. If you are "applying for" a Major Repair or Alteration that does NOT have Approved data, it is very common for applicants to use a PREVIOUSLY APPROVED Form 337 (Field Approved for a different airplane) as a BASIS for getting Field Approval. THAT is a Form 337 with a signed stamp in Block 3. 

The tipoff (if you do not know whether the work was done iaw Approved Data, or if it will require a Field Approval) is on the front of the Form 337. IF there is NOTHING in Block 3 of the Form 337, the repair/alt may be "presumed" to be for work done iaw APPROVED data (the "presumed" part is because if it WASN'T done iaw Approved data and REQUIRES Field Approval, the Form 337 is actually bogus, probably NOT on file with FAA, and meaningless)(There are SEVERAL EXAMPLES of this on "our" 337 listing). If there IS a statement there (remember, Block 3 is titled "For FAA Use Only" and the statement is almost always in the form of a Rubber Stamp), then you have in your hand an Approval made on a one-time basis for the airplane listed in Blocks 1 (Field Approval). Only. But that info MAY be used as "acceptable data" to GAIN AND APPROVAL on another airplane. "Acceptable". Do not confise that with "approved"! See the distinction? The "presence" of a previous Field Approval is not absolutely garanteed to get you another one for YOUR airplane... but using the vernier as an example... there is an excellant chance that your local FSDO would grant such an Approval for that particular alteration. The decision for all of this is the responsibilty of a Certified Mechanic (A&P when we are talking U.S. Registered airplanes. Remember that an IA also has to hold an A&P certificate). So HERE COMES the "real answer" to your "real question".

The FIRST place to go to discuss ANY kind of repair or alteration is MINIMALLY your favorite A&P. Ideally, the IA that signs off your Annual Inspections is even better, since he will have to "pass on" whatever Major work you are doing at some point anyway. He (She) will have to sign the Return to Service (Block 7 on the 337) before the airplane flies again anyway. So I highly recommend the old "Bag o' Donuts and Cups of Coffee" and a friendly sitdown with the IA who will eventually be licensing your airplane. Even if you just bought your airplane with a fresh annual, you actually want to develop a good working relationship with your IA NOW. It would be in your best interest to have him do a "light duty inspection" (or more) on your New Baby even if the ink hasn't dried on the fresh Annual it came with. If nothing else, have him do a "Conformancy Inspection" (verifies everything is "in order" with your paperwork, and that you have a "real", "legal" airplane). You COULD BE surprised to know that it is very possible that is not the case. You should know that NOW, not next year! So the "coorect way to proceed" is to go introduce yourself (and your airplane) to your new friend, the IA.

I hope you are actually LESS confused that you were a while ago! Also, this is NOT meant to say you shouldn't come "here" with questions like this. Just the opposite! You should feel free to come here specifically FOR questions like this! There is no such thing as a Stupid Question (well, there MAY actually be, but we don't see them much!). Welcome! And, ask away! John