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Friday, September 22, 2023

Doug's Domain

Doug Vetter, ATP/CFI

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FAA/PMA parts, STCs, and 337's.

(Image: top of a FAA Form 337)

With over $2500 invested in vacuum-driven gyros over the last several years, the owner of an aircraft I fly on a regular basis and I began to think about how to protect that investment.

We happened upon a solution in the form of an inline filter that at first glance appeared to have FAA/PMA approval, but no STC. After speaking with the manufacturer, we ultimately learned that the part was indeed STC'd for our aircraft, but in the interim, I opened a dialog with my avionics shop owner to ask if he could install this "PMA-approved but non-STC'd" filter.

I quickly learned that just because a part has a FAA/PMA stamp on it doesn't mean it can be legally installed in a certified airplane.. If you just came back from Sun 'N Fun or Oshkosh with the latest whiz-bang upgrade for your airplane that the manufacturer says is "FAA/PMA" approved and can be legally installed on your airplane, this article is a must-read.

Good Intentions

While most aircraft owners know they have to change their vacuum system inlet filter and regulator "garter" filter on a regular basis to reduce the strain that can lead to premature pump failure, many ignore the very real threat of contamination of the entire system that results when a vacuum pump fails.

(Image: Closeup of clearview filter)What contamination, you may ask? The vanes of a dry vacuum pump are made of a graphite material that works very well until it breaks, at which point the high-pressure in the engine bay combines with the low-pressure in the vacuum system upstream of the pump to force the resulting particles backward through the regulator, and into the gyros. I know this because I've seen the results of a pump failure firsthand -- our regulator was crammed with the stuff.

Therefore, when a vacuum pump fails, fixing the system is not as simple as replacing the pump. You should expect to remove and blow out all lines, clean/replace the regulator, and send the gyros out for a cleanup or overhaul as well. If you don't do this, those particles are bound to find their way back into the new pump, which may lead to premature pump failure.

If you hadn't guessed, the best way to prevent all of this hassle is to install a sacrificial inline filter between the pump and the regulator. This kind of inline filter is most often used on pressure based systems (where the vacuum pump connections are reversed and the pump blows air toward the gyros instead of pulling it through them), but they can be legally installed on vacuum systems for this very purpose.

(Image: Clearview filter installed)

There are two basic types of inline filters for this application, primarily differing in the materials used to form the case -- one is metal, and the other clear plastic. We chose the clear plastic type for the very simple reason that the case design allows us to gauge how dirty the filter is during every preflight (our '71 Cessna has a nice large cowl door that facilitiates access to the engine bay). This allows us the freedom to replace it on condition, rather that at some set interval. It's only $45, but money is money.

This filter is marketed under the "Clearview" name and is available from Aircraft Spruce among other aircraft parts suppliers. It comes in either kit form ($79), or just the filter ($45). The first time you buy the filter, you need to buy the kit, which contains a copy of the STC that must be submitted by your mechanic along with the completed FAA Major Alteration form (337) to make the installation legal on a certified aircraft. When it comes time for replacement, however, you need only purchase the filter and make a logbook entry.

Not so fast...

You may be asking, "why does a simple thing like an inline filter require an STC or a 337? That's hardly a "major" alteration of the airframe...it's not like I'm rebuilding the wings or anything".

I asked the very same question of my avionics shop when I originally asked them to install this filter, before I knew that the filter came with an STC, because they balked at installing it unless there was an appropriate engineering paper trail to support the modification. I thought FAA PMA (Parts Manufacturing Approval) was sufficient to install this or any other after-market upgrade, but that's simply not the case. Unfortunately, many parts manufacturers tout the FAA-PMA approval stamp on their products, but fail to mention that this does NOT mean, by definition, that you can legally install the part on your aircraft.

I then asked, "well, you installed an alternate static valve on the airplane without submitting a 337...why doesn't this apply in this case?". I further reasoned, "why can't I just submit a 337 signed by a mechanic to document and approve the installation?" The response I received from the shop owner explains why (paraphrased as necessary):

As far as 337's go, I think you may be missing the point on who has authority to approve an alteration. Any modification to an aircraft has to been done in accordance with "Approved" data. This is most often in the form of an STC, or a DER (Designated Engineering Representative) approval, or an approved airframe manufacture repair manual.

The only person allowed to make an engineering call on a modification (that is not part of the aircraft original type certificate, or in the manufacturer's approved maintenance manual) is a DER or the FAA in OK City. Our local FSDO (ABE), FAA certified mechanics, or IA mechanics, cannot generate approved data, or approve a alteration without "Approved" or "Acceptable" data.

"Approved data" is FAA-approved data that details a specific alteration to a specific airframe or engine. "Appropriate" data is FAA-approved data that details an alteration, but not to your aircraft or engine, or the alteration you are performing varies in detail from the alteration described in the "Acceptable" data. FAA AC41.13 (the maintenance bible) is "Acceptable" data. The FAA has approved AC413.13, but the procedures are often guides and not procedures specific to the actual repair/alteration being made.

A field approval is a process in which "Acceptable" data is approved by the FSDO. The field approval in that case is like a one-time STC for your aircraft. You submit "Acceptable" data as the basis of the field approval and the local FSDO approves the alteration based on the fact that the "Acceptable" data is close enough to the alteration you are trying to do that they authorize it.

This is how your Garmin 430 was granted a field approval. There is no "Approved" data to install a Garmin GNS-430 in a Cessna 172. Garmin has an approved STC for a Mooney which they provided to us. You do not own a Mooney, but the installation in your Cessna is close enough to the Mooney, that I submit the Mooney STC as "acceptable" data to form the basis of the field approval. The FSDO looks at the data and says OK, the Mooney and Cessna are close enough, and we will approved the Cessna installation based on the Mooney. If Garmin had no STC for any aircraft, but the GNS-430 was PMA'd, I could not submit a 337 to the FAA asking for them to OK this installation in your Cessna. I have no data for them to approve, and they will not be put in the position to make an "engineering" call on the suitability of an alteration.

The same holds true for a previously approved 337. It is not just some random mechanic I'm trusting. I'm trusting the FAA approval process. A legitimate 337 could not be approved by the FAA unless some other mechanic went through the trouble off supplying acceptable data, or hiring a DER to approve the alteration. So, if such a 337 exists it is saying someone jumped through [the hoops necessary for approval], and I can use that as a basis of a field approval. I would submit a 337 to our FSDO with a copy of the other approved 337 and ask for a field approval based on the approved 337. The FSDO would check the authenticity of the 337 and if it looks good, they would grant a field approval based on that 337.

To wrap things up on the approvals, [if the STC specifically calls out your engine/airframe on the approvals sections], then we do not need a field approval, but we still need to submit a 337. The 337 will reference the STC, a copy of the STC will be attached and the approval basis is that the alteration we performed is same as called out on the STC, which lists your engine/airframe.

So.... if you have an STC or approved 337 showing this filter alteration has been installed in your make/model aircraft, the approval process is simply a matter of filing out a 337 and sending a copy to the FAA. If you have an STC for the filter alteration that calls out some other airframe/engine, then we need a field approval and I will need to call Allentown to see if they would grant one. If all you have is an filter that has a PMA approval, then that's a battle I would rather not spend time on.

As far as I know this approval process has not changed in the last 15 years."

In the case of the alternate static source valve, it turns out that the 1971 Cessna 172 type certificate mentions an alternate static valve as an option. Our aircraft obviously didn't have the option leaving the factory, but its installation (with either an OEM or PMA part) was covered by the original type certificate, thus they didn't need to request a field approval for the installation.

Show me the STC!

The moral of the story is simple -- next time you're talking to a parts manufacturer and they're touting how easy (and legal) it is to install their "FAA/PMA approved" part on your certified aircraft, you need to look at two possible scenarios:

  1. If the part is supposed to be an exact replacement (both in form and function) for an OEM part, you need to ask the manufacturer if the PMA part is approved to subsititute for the specific model and serial number OEM part. If it is approved for the application, your mechanic need only install the part and make a logbook entry to that effect. This is very often the case for "generic" parts such as various MIL-SPEC hoses and spark plugs.
  2. If the part represents a modification of your aircraft that was not part of the original type certificate (like something as simple as adding an inline filter to a vacuum system that didn't have it in the first place, or as radical as swapping that underpowered engine for a firebreathing, turbocharged model, the first words out of your mouth should be SHOW ME THE STC (stated with the same intonation as that classic movie line).

If the installation requires a STC and either the STC doesn't exist or it doesn't apply to your specific make/model/serial number aircraft or engine, this is not to say that you couldn't obtain the necessary engineering data to have the installation approved, but you may find this prohibitively expensive.

If the modification is known by the manufacturer to require a STC, the manufacturer usually jumps through the hoops necessary to obtain a STC. The wider the scope of the installation, the less likely the manufacturer is to invest in the STC process for the very simple reason that the testing and approvals required to match each design application may be too expensive. In this case, the manufacturer may tout PMA approval and sell the part(s) to you assuming you will invest the $$$ necessary to gain field approval (in effect, a one-time STC). Practically speaking, unless you have a large sum of money to throw at such an installation, it is best to avoid this scenario.

Complicating matters are the simple fact that some FSDOs still view existing field approvals (signed 337 forms) as "private" data between the FAA and the aircraft owner, and may thus not accept an existing 337 as appropriate data for the approval. This means that you may need to acquire either the original STC or applicable engineering data on your own in order to submit it for approval.

This in part explains why FAA/PMA approved parts cost more than their uncertified cousins and why a STC'd part or modification might cost even more. In short, it takes time and lots of money to prove to the FAA that any given installation will not detrimentally affect the airworthiness of a certified aircraft, and the cost of that approval process is passed on to the customer.