Greetings folks- I am the third owner of a stol 750 that was a 2009 kit, and completed in 2012. I know that only a few months after the kit was purchased by the original owner, zenith adjusted future kits going forward in 2010 as well as provided a modification to pre changed kits that did some structural changes to the spar and overhead section, allowing the max gross weight to go from 1320-1440 lbs. However, if you listed the max gross as anything over 1320 in land configuration (versus float), it wouldn’t qualify as an SLSA anymore. All of my paperwork still shows 1320 as the listed max gross weight with the SLSA nomenclature. But I have no idea if the builder took into considerations the 2009/2010 kit modifications that allowed it to structurally be able to max gross out at 1440.

https://www.kitplanes.com/gross-weight-boost-for-eab-zenith-ch-750/

How would I visually confirm the presence or lack thereof this 2009 modification?

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Changes included added gussets to the cabin, a larger diameter spar carry through tube, and a beefed up gusset to the wing spar at the strut attachment. Unfortunately I cannot provide you with any before/after dimensions. 

Perhaps Edition 2 or later 750 STOL's have a larger spar carry-through tube, but the Zenith "Edition 2 Upgrade Kit" which, if all components are installed in an Edition 1 kit, allows the 1440 limit and did not provide or require an increased diameter spar carry-through tube.  I think, however, the upgrade kit's rear spar attach plates are thicker than the original ones.

John

When I did the upgrade I had to purchase a new cabin frame and it had a substantially larger carry through tube.

From what I was told, and someone correct me if I'm wrong, the airframe did not change. They just increased the possible max weight rating based on more strenuous stress resting. If the aircraft was given an airworthiness certificate (in the US) stating 1320 as the gross weight, it will always be 1320. You can't change it. The 1440 number gives you nothing more than some added confidence. But if the aircraft has not yet been given an airworthiness certificate, you can certify it up to a 1440 gross. It will never qualify as an LSA if you do that though.

Bob, you're usually right but in this case you're wrong! LOL!  As has been mentioned, the airframe definitely has many gussets, etc. that are thicker material.  Years ago, I talked to Caleb, the engineer at Zenith and he said the original airframe was "plenty strong" for the 1320 limit, but the upgraded parts were required for the 1440 limit.

BTW, I think you can increase the max gross weight any time you want (at your own risk!).  Of course, that takes it out of LSA compliance and you can never go back to the 1320/LSA compliance.

John

Thanks, John. I wondered if the info I was given was correct.

Interesting question!  My "Edition 1" kit's production corresponds very similarly to what you describe and I completed mine in 2012!  It was delivered to the original owner shortly before the upgrade became available and Zenith provided him with an "Edition 2 upgrade kit."  However, he had already built the wing structures before the upgrade kit became available and did not modify the spars.  I purchased the kit partially completed and installed most of the remainder of the upgrade kit (there's lots of small parts that are just a bit thicker/heavier than the Edition 1 parts).  I didn't care to upgrade the spar as I intended to keep it at 1320/LSA-compliant.  It appears to me the original builder would have ordered or been provided with the "Edition 2" upgrade kit if it is installed in your plane.  I'd call Zenith and see if they have records of whether he purchased or was provided the kit.  Of course, whether the upgrade is installed or not is moot if you're going to keep the max gross at 1320 to keep it LSA-compliant.  At present (but likely to change soon!), any weight above 1320 is not LSA-compliant including the 1440 upgrade.

If Zenith has no record, I suppose you might be able to cut a small hole and use a "snake" flexible video camera to determine if the spar doubler is there or not?  Probably Roger at Zenith could tell you what you're looking for on the spar or perhaps there is some other part easily accessible that you could measure the thickness and determine if it is an original part or upgrade part ... too long ago for me to remember and recommend what parts might be good candidates to check!   Correction:  After posting this I remembered the rear spar attach plates are thicker (see my comment above)  but can't tell you the dimensions - that would be an easily accessible part to check and likely Roger could give you the thickness spec).  Next time I'm over at the hangar I can give you the thickness of mine from the upgrade kit.

BTW, you mention your plane's paperwork has "SLSA nomenclature"?  Although some Zenith 750's and 601's were "factory built" as SLSA's, Zenith has never produced a SLSA kit.  If your plane was built from a kit, it is an EAB that is LSA-compliant.   Of course, if it actually was a SLSA ("factory built"), you can't modify it without permission from the factory - which is no longer in business,  However, I think Zenair Canada provides SLSA support.

John

N750A

Thanks John. I was incorrect in my typing.  My planes paperwork is LSA, but the article I had posted mentioned SLSA....

I'm aware that even if it was installed, the plane is certificated already, with the LSA limitations of 1320, so the only point in this would be for my own comfort and knowing that it could go up to 1440 from a stress test perspective but not from any other perspective.  We all know that planes don't just stop flying when they are 1 lb over their max gross weight (but they do stop running when they need 1 lb more of fuel to get to their destination but don't have it), but that's not what I'm suggesting or looking to do here.  More so this is for my own curiosity, but also for a greater understanding about my performance numbers I'm seeing, i.e. takeoff roll, Vy climb, etc....

Ty,

After all this discussion, I couldn't let it go! LOL!  I suddenly remembered where one of the original rear spar attach plates was stored (I never throw anything away! Ha!) and mic'd it at 0.120".  I ran over to the hangar (I have an airstrip and hangar on my farm) and measured the rear spar attach plate from the Edition 2 Upgrade Kit that is installed on my 750 STOL - it was 0.190".  This is the plate in the upper corner of the fuselage just behind and above your shoulder that is riveted and bolted to the airframe and has a protrusion that the rear spar bolts to.  So, that's an easily accessible part with a distinct difference in original vs upgrade thickness!  Of course, no guarantee that all the other parts were installed, but if it is there, likely they are all there!

Please, no worries about about the 1320 - the engineer at Zenith was very reassuring!

John

Thank you John!  This is helpful and I'll give it a look.  Now...I only wish I just had to go out to the back 40 to take a look at my on-properly grass strip and hangar.  I'm surprised my computer still works after drooling so much.  I'm jealous.

I think I found the rear spar attach plate you've described  Is it this?

https://homebuilthelp.com/CH750/fuse750Part2.htm

Yes, that's it!

John

THANK YOU!!!!

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