Weight and balance issue - experts check in

[Clayton Harper]

Years ago when there were "field approvals" in Alaska, there was a form that had to be filled out for over size tires. I saw it once, and I believe it had a complicated weight and balance calculation. I had the form at one time, but I don't think I have it now.

[TxAgfisher]

Anyone have it?

[Steve Pierce]

I take it you don't have a flight manual suppliment? If not I would call Univair and get an official copy, I believe it will have your N number on it.

[Jeff J]

I guess I don’t see the problem other than a very thorough annual inspection would ground you for not having the required flight manual supplement. If all you need is a new w&b sheet or revision then make one based off the one you have from the initial big tire installation. If you have questions about arms then level the airplane, get a plumb bob or two and take some measurements. The airplane must be leveled if it is to be weighed anyway. The third option would be to either get some over the should supervision or turn the w&b over to someone trained to do it (although I have fixed plenty of their mistakes too as I am sure some have had to fix mine).

[Steve Pierce]

I don't check flight manual supplements at annual inspection. I do check that the airplane meets the TCDS or has paperwork for modifications. How do you legally fly the airplane if you don't know what the CG limits are? This particular W&B used the firewall as the datum instead of the wing leading edge and some Piper paperwork uses 60" forward of the wing leading edge. In reality I know that it is hard to get a Short Wing airplane out of CG but you still have to prove that it is within limits. I am assuming the person that converted this used the firewall because of the STC for the tailwheel conversion called it out but not having a LPC Flight Manual Supplement in front of me I do not know that.

[Jeff J]

I don't check flight manual supplements at annual inspection. I do check that the airplane meets the TCDS or has paperwork for modifications.

This is contradictory. If a modification requires a FMS then that IS part of the the paperwork for the modification, if not the modification itself, and the mod is not legal if the paperwork is not complete.

I have no experience with bush wheels. Do they alter the CG envelope for the airplane? If that answer is "no" then all that needs to be done is w&b update same as any other mod from removing a radio to an engine upgrade to installing floats. If the STC moved the datum to the firewall (I can't fathom why) then the update should use the firewall. Finding the CG envelope for a moved datum is a simple math problem... easier than the w&b formulas by far. Just adding and subtracting, no division nor multiplication. Or a person can always level the airplane and take measurements.

Believe me, I know about the discrepancies in the Piper paperwork. I was one of the first people to get flamed hard for suggesting we should be using the WLE as the datum because that is what the TCDS called out.

[Steve Pierce]

If an IA signs a 337 form that includes a flight manual supplement I am going to assume that he put the supplement in the AFM as well. I am not going to read the STC and go back and check that he did everything IAW the STC correctly either unless I see a blatant error. I could spend two weeks on every annual going back on every single nut, bolt, screw etc. If someone signed it off at some point I have to take their word for it. In this particular case the tailwheel conversion has a flight manual supplement not the Bushwheels.

Maybe I should have gone back and checked that the flight manual supplements were all included from all previous modifications when I did the annual for the previous owner on this particular airplane as should have all the inspectors since 2006. I guess I should ut this item on my inspection checklist.

[Steve Pierce]

Believe me, I know about the discrepancies in the Piper paperwork. I was one of the first people to get flamed hard for suggesting we should be using the WLE as the datum because that is what the TCDS called out.

I don't remember you getting flamed. I always use the wing leading edge because that is what is on the TCDS. Some flight manuals show the 60" forward but I put the CG limits off the TCDS in with the W&B. I think they did that to keep people from working negative numbers. If you can't work negative numbers you might not should be flying.

[Gilbert Pierce]

My Clipper factory W&B used 60" forward for the calculations but then subtracted the 60 back out when listing the CG. That puts it back at the LE. Go figure why they did that? Guess they didn't like negative number math.

[Jeff J]

I don't remember you getting flamed. I always use the wing leading edge because that is what is on the TCDS. Some flight manuals show the 60" forward but I put the CG limits off the TCDS in with the W&B. I think they did that to keep people from working negative numbers. If you can't work negative numbers you might not should be flying.

I don't recall much from this forum but back in the SWPC forum and the even older email list I used to catch hell. Especially from JohnW and a few of his peers(?) who were very adamant about following flight manuals from the fifties that listed the datum in front of the airplane then referenced everything to the WLE instead of the TCDS WLE datum from the sixties through the nineties. Some of it was because of the way I express myself in print. I don't recall you ever saying anything on the subject one way or the other in those threads but you can find one of JohnW's rants in this forum. I think I offended him because I questioned his position (made light of it actually) and indirectly referred to him as an "old timer".

In one of the shortwing forums I had someone actually tell me you could not use "zero" as an arm for computing weight and balance. Maybe the author of the bushwheel mod had similar thoughts if he moved the datum to the firewall?