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Thread: PA-18-105 to 1750# Then to 2000#?

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    PA-18-105 to 1750# Then to 2000#?

    I think I’ve read most of the discussion of the requirements and STCs required to take a 105 to a 150 with a 1750# GW and I’ve tried reading specific STCs to answer this question: Can you get a 105 to a 2000# GW through a paperwork 2-step? E.g. the Wipaire STC on top of the UnivaireSTC? Wouldn’t a new fuselage from Airframes or Dakota Cub meet the technical requirements?


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    Quote Originally Posted by CWilson View Post
    I think I’ve read most of the discussion of the requirements and STCs required to take a 105 to a 150 with a 1750# GW and I’ve tried reading specific STCs to answer this question: Can you get a 105 to a 2000# GW through a paperwork 2-step? E.g. the Wipaire STC on top of the UnivaireSTC? Wouldn’t a new fuselage from Airframes or Dakota Cub meet the technical requirements?


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    The Wipaire STC specifically allows the use of early airframes if modified by 1750 lb. gross weight STC. The Wip STC lists several 1750 GW STC's that are "compatible" and does require at minimum a 150 hp engine. Your problem is the Wipaire STC is only approved for PA18-150's. Using an Airframes or Dakota Cub frame approved for PA18-150's would definitely meet the standards of the STC. But your model is not on the approved model list so you would need to get a field approval. Wip does not appear to have any data to allow the use of the early model 13 rib wings. I currently have a field approval request to install the Wip STC in a PA18A-135 but that airplane has been configured as equivalent to a PA18A-150 with a 1750 GW STC, O-360 engine, and 16 rib wings. Part of the field approval requirement was to write new Flight Manual Supplements as the FMS's supplied in the Wip STC are specific for PA18(A)-150's. Hope to get the approval soon. The request has been at the FSDO for about five months.
    Last edited by bubb2; 11-06-2021 at 08:08 PM.

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    Crash, Jr.'s Avatar
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    The Airframes fuselage STC includes PA-18-105 models and those fuselages include the additional structure required to meet the requirements of the Univair STC. From there the Wipaire STC should allow you to go up to 2000lbs.

    This is all dependent on your IA's discretion. The Airframes fuselage STC (and most if not all other STC's that I've read) include the line "compatibility of this design with previously approved modifications must be determined by the installer". This means the IA is the sole person responsible for determining if the two gross weight increase STC's are compatible or conflict with one another. Have you asked your A&P/IA who does your work this question about STC compatibility?

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    Quote Originally Posted by Crash, Jr. View Post
    The Airframes fuselage STC includes PA-18-105 models and those fuselages include the additional structure required to meet the requirements of the Univair STC. From there the Wipaire STC should allow you to go up to 2000lbs.

    This is all dependent on your IA's discretion. The Airframes fuselage STC (and most if not all other STC's that I've read) include the line "compatibility of this design with previously approved modifications must be determined by the installer". This means the IA is the sole person responsible for determining if the two gross weight increase STC's are compatible or conflict with one another. Have you asked your A&P/IA who does your work this question about STC compatibility?
    This is not up to the IAs discretion! An IA has to ensure he has Approved Data applicable to the aircraft the alteration is being installed on. STC SA00997CH is ONLY applicable to PA-18-150, PA-18A-150, PA-18S-150, and PA-18AS-150 as indicated on the “original product” section of the STC. It may be “acceptable data” for other models, but it is not “approved data” by itself. It would need further approval either as a Field Approval, or as a DER approval. The Main issue would be the flight manual supplement as many of the limitations for the PA-18 are different than the PA-18-150. This is further complicated by the fact that the AFS-300 job aid says gross weight increases can only be done by STC, so it is likely that they would tell you you need to apply for an STC. All that said, I believe there was discussion about this in a previous thread and there were comments that the instructions for the STC indicated it could be applied to any PA-18 that has a GW of 1750. The problem is that an IA is bound by the limitations of the actual STC certificate. Best solution would be convince Wip to submit an amendment to the STC to include all the PA-18 series in the “original product” section,and a limitation stating it is only applicable to aircraft that have had the gross weight increased to 1750 lbs.


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    Quote Originally Posted by dgapilot View Post
    This is not up to the IAs discretion! An IA has to ensure he has Approved Data applicable to the aircraft the alteration is being installed on. STC SA00997CH is ONLY applicable to PA-18-150, PA-18A-150, PA-18S-150, and PA-18AS-150 as indicated on the “original product” section of the STC. It may be “acceptable data” for other models, but it is not “approved data” by itself. It would need further approval either as a Field Approval, or as a DER approval.
    This is the center of my question. A new fuselage and wings either by PMA or STC eligible to replace the original 105 parts that also meet the requirements for the 2000# GW would not be eligible for that STC.

    Seems one could build an airplane that started as a 105 using new PMA’d parts and STCs that is physically and functionally identical to one that started as a 150 but never be legal from a paperwork perspective. I’d prefer to avoid relying on the prospect of a Field Approval or DER approval.

    Am I tracking this properly?


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    You guys are bringing up good points.
    I have seen a number of early PA-18-95s with flaps and 150/160 hp, and am aware of one with Wip 2000# kit and 180 hp engine. Afraid to look at the paperwork, but most of it after the conversion to 150 configuration was done at CC.

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    Quote Originally Posted by CWilson View Post
    This is the center of my question. A new fuselage and wings either by PMA or STC eligible to replace the original 105 parts that also meet the requirements for the 2000# GW would not be eligible for that STC.

    Seems one could build an airplane that started as a 105 using new PMA’d parts and STCs that is physically and functionally identical to one that started as a 150 but never be legal from a paperwork perspective. I’d prefer to avoid relying on the prospect of a Field Approval or DER approval.

    Am I tracking this properly?


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    The issue is the fact that the aircraft is a PA-18, and not a PA-18-150. Yes, it has all the required parts. But unless you actually go through the paperwork to change the model ( and that can only be done if the serial number is listed as eligible in the PA-18-150 section of the TCDS), the 2000 lb gross wt STC would not be applicable. If your serial number is applicable, you could do a model change with approved data and a 337. It requires a supplemental data plate, an amended airworthiness certificate and a replacement registration.


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    Quote Originally Posted by CWilson View Post
    This is the center of my question. A new fuselage and wings either by PMA or STC eligible to replace the original 105 parts that also meet the requirements for the 2000# GW would not be eligible for that STC.

    Seems one could build an airplane that started as a 105 using new PMA’d parts and STCs that is physically and functionally identical to one that started as a 150 but never be legal from a paperwork perspective. I’d prefer to avoid relying on the prospect of a Field Approval or DER approval.

    Am I tracking this properly?



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    You are spot on. Even though the aircraft would be identical to a PA18-150 with the same modifications the data tag would still indicate an early model aircraft not on the Approved Model List of the STC. The logical way to move forward would be a model change. It is my understanding (as explained by my PMI) that whoever owns the Type Certificate would have to agree with the model change and produce an additional data tag for the aircraft. At that point the FAA would issue a new Airworthiness Certificate indicating the new model designation. I think your much more likely to win the lottery than that to occur.
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    Quote Originally Posted by dgapilot View Post
    The issue is the fact that the aircraft is a PA-18, and not a PA-18-150. Yes, it has all the required parts. But unless you actually go through the paperwork to change the model ( and that can only be done if the serial number is listed as eligible in the PA-18-150 section of the TCDS), the 2000 lb gross wt STC would not be applicable. If your serial number is applicable, you could do a model change with approved data and a 337. It requires a supplemental data plate, an amended airworthiness certificate and a replacement registration.


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    I have been told different opinions on the supplemental data plate with the latest opinion from my PMI that it must be produced (at least for Pipers) from the Type Certificate owner. Is this true?
    Last edited by bubb2; 11-07-2021 at 11:48 PM.

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    PA-18-105 to 1750# Then to 2000#?

    I’ve done a couple model change approvals as a DER (J3-F to J3-C) all with the OK from the ACO. The supplemental data plate is an “owner produced part” made to the specification in the approved data. Never talked to the TC holder. I’ve also done several amended airworthiness certificates for model changes. It requires a new application, conformity inspection, and a copy of the 337 that documented the model change. Once all the airworthiness documents are filed, the registry is supposed to send the revised registration. That usually doesn’t happen and it takes several calls to get them to do their jobs.


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    Last edited by dgapilot; 11-07-2021 at 12:18 PM.
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  11. #11

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    Thanks all - the depth of knowledge in this group is extraordinary.


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  12. #12

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    Never forget that the PMIs are people just like us, and often do not have any more knowledge than we do.
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    Quote Originally Posted by bob turner View Post
    Never forget that the PMIs are people just like us, and often do not have any more knowledge than we do.
    In many cases less! I get calls quite often from different FSDOs and from Oklahoma City asking my opinion on how to handle certain situations, mostly certification related on old airplanes.


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