The International Cessna^® 170 Association

I appologize but I don't have my parts manual with me but I had a question/concern. I purchased a Rapco vacuum system filter from spruce (1J7-1) as it was one of only two available FAA/PMA approved filters. I don't remember seeing a vacuum filter in the parts manual but a C170 is nowhere to be found in Rapco's list of approved aircraft for installation. My thought is that this is because Cessna never had a filter installed in their 170's. I also purchased the Rapco vacuum regulator RA2H3-12. What are most of you using for modern gyro's?

The original AN-gyro's had individual filters mounted upon their cases and did not use a "central" filter such as modern installations commonly do. The earliest systems also were operated by venturiis and did not use a regulator as the later systems did (subsequent to SN 25373) and those we now see in modern installations, instead a Pesco valve was used to reduce excessive vacuum. See the two illustrations below for pre and post-25373.

Keep in mind that early-style AN-gyro's only req'd 3.5" of vacuum, while modern "pictorial" 3-1/8" gyros require 4.5 to 5" of vacuum. For that reason very few who convert to "modern" gyros find any need for a regulator as the venturiis typically only produce 4" to 5" in-flight.

If you have "modern" 3-1/8" gyros you will need to power them with sufficient vacuum. This usually requires a "super venturi" installation, which is a larger venturi that produces much more vacuum, ...typically 8" or more...or it requires conversion to a vacuum-pump (Booo-Hisss).. and both of those require a vacuum regulator/relief-valve to reduce the vacuum so as to avoid overspeeding the gyros (which not only wears them out faster but also causes them to precess excessively.)

The alteration of your gyro system will require a basis of approval. The super venturi is a "standard part" and is a minor alteration and therefore needs no further approval than a logbook entry. The regulators and central filters are FAA-PMA replacement parts for OEM part-numbers and may be used as such. If you are creating a new installation then your instrument/avionics shop (the presumed installer) will probably want to document the installation on a Form 337 as a major alteration*....in accordance with Cessna dwgs. for later aircraft. *(Depending upon your FSDO, it may or may not require Form 337 block 3 signoff. (field approval) I personally view it as a minor alteration, and my opinion is worth what you paid for it.)

My own (post 25373) system (with AN-gyros) uses original dual-venturi installations according to the 170-B IPC, and item 403 of the TCDS, Cessna dwg 0500006, which uses the "relief valve" (regulator.) (The Type Cert. Data Sheet item is the basis of approval.) The central filter was installed as a "minor alteration" requiring only a logbook entry.