This Q&A is now closed and replaced with a new Q&A based on the latest upgrade drawings at

To all 601XL and 650 builders and aircraft owners. If you have a specific question regarding the upgrade, post it here and I will consult with Chris Heintz so that we can give you the best possible answer. Please be specific about your questions.

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Zenith Aircraft is upgrading its 601 XL, so a lot of questions are answered on the step-by-step Blog Posts on this main page.

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Replies to This Discussion

Please define stiffening. I'm working on the elevator right now. Should I hold off on installing any of the trim parts?

Thanks, Rich
Yes, hold off until you have the drawings. We are just adding 2 "L" angles on either side of the trim servo motor so that the motor does not flex the skin. This can be done from the access panel. We are doing the same for the aileron trim servo.
We have heard conflicting reports on how 6-ZU-1-2 fits inside the center spar without changing the spacing of the front and rear portions of the spar. Can you comment on this?

-- Craig Payne, QBK
Yes, 6-ZU-1-2 is bolted to the wing spar root and slides into the centre spar. What makes this possible is by removing the front wing attachment uprights 6-W-13-1. Once these are removed, the front spar caps are quite flexible and makes it possible to slide the wing into the centre spar very easily. The new front spar uprights (extrusions) are fitted once the wing is on the fuselage. This is part number 6-ZU-2-4. This all sounds rather confusing but Zenith will be doing this installation on their demo aircraft very soon where they will have a detailed procedure.
In the wing spars if we replace the solid rivets with bolts and seal up the wings how will we check the torque on the bolts in the future?
We will be recommending a high torque SL nut, such as the AN310, to be torqued 50-70 inch-pounds. This will not loosen over time. Many certified aircraft use internal bolts that cannot be inspected and have not had problems with this. We are still evaluating the exact type of bolts to be used. Our first recommendation will be to replace the solid rivets with new solid rivets of the same diameter if the hole size is not to large. Zenith will see how hard that is to do in the next few days.
Suggestion for consideration. Might want to have an alternate to new solid rivets. Installation of the bolts would seem more straight forward than bucking the rivets. Why not use an AN365 as apposed to the AN310 castle nut?
Correction to the 310 nut. We will be recommending the MS20365 nuts to be torqued 50-70 inch-pounds.
Have you looked into using Hi-lok or Hi-lite fasteners for the rivet replacement. Hi-lock being titanium might be overkill but the hi-lite being steel might fit the bill. Being that they are a close tolerance when installed properly I would think they would be superior to the bolts. They also might eleviate the concerns builders have with the bolts and nuts seeing the nut only hold the pin in the hole and normally you have to tap them with a hammer to seat the head before installing the collar or nut wichever method found to be fit for securing them. Also people would probably be pleased to know that they hold the wing spars together in the CRJ 200, 700 and 900 airliners. Anyway just an idea I came up with from my experiance with larger aircraft maintenance.
I have the Duel stick system I already have 2 spar uprights on the front spar box to support the flight controls. Do I now need the other 4 uprights on thr front with a total of 6 or just 2 more? If just 2, what locatons/ spacing ? as drawaing 6_-ZU-2 does not show Duel Stick mod stuff shown
chris Sinfield
Yes, you need to add the spar uprights 6-ZU-2-3, front and rear. Top and bottom are bolted, with rivets into the spar web.
I, like many other builders, have already drilled the center wing spar to the cabin floor. By making the spar thicker, the front row of holes is offset by 0.063". I made a spar mock up today where I drilled holes for A5 rivets in two matching peices of Al sheet. I then offset the parts by 0.063". Using a small step drill I drilled out each pair of holes to 0.1875". The 1/8" drill at the end of the step drill acts as a pilot to keep the drill from wandering. The holes were then reamed to #11 or 0.191". The holes are not perfect but well within the maximum hole size for an A6 rivet as shown in the Zenith construction manual.
This process would eliminate the alleged problem of forcing the wing spar into the center wing spar.

Herb Heaton - Plans Builder


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