Super Cub Weight and Balance Report

The Type Data Certificate lists the wing leading edge as the Datum but the Piper Weight and Balance Report attached to the Airplane Flight Manual for both the PA18-135 and PA18-150 shows the datum 60" forward of the Wing Leading Edge. I realize this was to eliminate negative numbers for the engine components but I'm confused about this inconsistency in compiling a new weight and balance report to attach to the Airplane Flight Manual after an extensive rebuild of a L21B (PA18-135) with installation of a Lycoming 0320 engine. Should I use this Phantom Datum (60" forward of the Wing Leading Edge) or the real Datum at the Wing Leading Edge and deal with the negative numbers?
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Use the real Wing Leading Edge datum for computing your empty weight W&B. Watch your numbers carefully. It is EXTREAMLY easy to induce mistakes here.

CAREFULLY create a different form using the 60" forward datum (prop flange) for your everyday computations.

Piper tried to help you, but got everybody confused with these datum locations on the same page.

John Scott

Thanks John for your reply.

I'm still confused however. My airplanes is an L21B (PA18-135) but we have installed a Lycoming 0320-A2B engine and several mods including the 1750 GW increase (Borer STC). We have recovered the airplane and made other mods that dictate the actual weighing of the airplane and construction of a new Weight & Balance Report. The Borer STC changes the CG envelope to that of the PA18-150 airplane. Its still a PA18-135 however and I feel like I need to start with the PA18-135 Airplane Flight Manual. The Airplane Flight Manual for both the PA18-135 & the PA18-150 shows a Phantom Datum at the 60" forward of the Wing Leading Edge which is contrary to the Datum location of the Wing Leading Edge in the Type Certificate Data Sheet. Because of the installation of the Lycoming 0320 engine and Borer STC for increasing the GW to 1750 lbs, I must modify the Airplane Flight Manual. So for the Airplane Flight Manual which must be carried in the airplane, do I follow the example in the PA18-135 & PA18-150 Airplane Flight Manuals and use this Phantom Datum of 60" forward of the Wing Leading Edge or should I modify the Equipment List by recalculating all the locations of equipment based on the Datum being the Wing Leading Edge? I have to construct a new Equipment List because the one attached to the PA18-135 Flight Manual has practically none of the modifications (additions of new equipment). I have to satisfy my IA (and possibly the FAA) who will do a new Annual Inspection on the airplane when its finished. He will scrutinize all the paperwork because of the many changes to the airplane. I'm not looking for more work or reinventing the wheel but want to do it right the first time. So which method do most Super Cub owners use for their airplanes?

There are two things being talked about here.

One is the aircraft Empty Weight Center of Gravity, which you can think of as the MECHANICS weight and balance for adding and removing equipment from the airframe; this uses the wing leading edge as the datum. Your mechanic is supposed to be smart enough to work with negative numbers (don't hold your breath). No offense to the Cub savy mechanics on here! All the distances in the TCDS are referenced from the wing leading edge.

The other is the Actual Weight and Balance which you can think of as the PILOTS weight and balance for everyday calculations; Piper tried to help by moving the datum 60" forward of the wing leading edge so us stupid pilots didn't have to work with negative numbers. The pilot, baggage, fuel, etc. arms in the "Flight Manual" use this datum.

When you weigh your airplane as Piper did there will be a scale under each main wheel and one under the tailpost.

The formula to give you the Empty Weight Center of Gravity (MECHANICS number) is:

2.25 + ((weight at tailpost X 186) / total weight))= EWCG in inches aft of datum (wing leading edge). This is your MECHANICS number.

Add 60 to it and you now have the PILOTS number for the Actual Weight and Balance aircraft CG.

Hope this helps,

John Scott

Great explanation John !!!

S2D said:

Great explanation John !!!

Click to expand...

ditto!!!!!

Longwinglover said:

There are two things being talked about here.

One is the aircraft Empty Weight Center of Gravity, which you can think of as the MECHANICS weight and balance for adding and removing equipment from the airframe; this uses the wing leading edge as the datum. Your mechanic is supposed to be smart enough to work with negative numbers (don't hold your breath). No offense to the Cub savy mechanics on here! All the distances in the TCDS are referenced from the wing leading edge.

The other is the Actual Weight and Balance which you can think of as the PILOTS weight and balance for everyday calculations; Piper tried to help by moving the datum 60" forward of the wing leading edge so us stupid pilots didn't have to work with negative numbers. The pilot, baggage, fuel, ect arms in the "Flight Manual" use this datum.

When you weigh your airplane as Piper did there will be a scale under each main wheel and one under the tailpost.

The formula to give you the Empty Weight Center of Gravity (MECHANICS number) is:

2.25 + ((weight at tailpost X 186) / total weight))= EWCG in inches aft of datum (wing leading edge). This is your MECHANICS number.

Add 60 to it and you now have the PILOTS number for the Actual Weight and Balance aircraft CG.

Hope this helps,

John Scott

Click to expand...

great job !!!

Another tip.
Most mechs weight the tail with the scale under the wheel, not the tail post. So, instead of (Weight at tailpost X 186), use Weight under tailwheel X 200. The 200" is from Piper Report 834, page 7, Tailwheels. All tailwheels are listed as +200".
Hope I didn't add to any confusion!:roll:

T.J. said:

Another tip.
Most mechs weight the tail with the scale under the wheel, not the tail post. So, instead of (Weight at tailpost X 186), use Weight under tailwheel X 200. The 200" is from Piper Report 834, page 7, Tailwheels. All tailwheels are listed as +200".
Hope I didn't add to any confusion!:roll:

Click to expand...

It would be a good idea to actually measure the distance from the datum to the tail wheel as the TC sometimes is in error. I weighed an airplane once and used the TC numbers. They were wrong. The tail wheel was a few inches different than it was supposed to be.

Weighing is so much fun. It is legal to compute, although many will argue that covering requires a re-weigh. I personally have never seen a Super Cub without significant errors in weight and balance, most often with mixed arms, but once in a while with incorrect formula after weighing. Weighing always adds significant weight to the Cub, but I note that Citabrias can be weighed without a giant increase. Strange . . .

Opinion.

Also, important to choose the right location to weigh a plane since the force of gravity apparently varies quite a bit. For instance, it is believed that South Saint Paul, MN, is a sweet spot with low gravity compared to Fergus Falls, MN. This is only circumstantial based on floats seeming to weigh more in Fergus Falls than in South Saint Paul.

The 200" I mentioned above is not from the TC. It is from an original Piper Report 834, "Actual Weigh and Balance". Page 7 is the "Equipment List". That number is listed as "Arm About W. L.E.".

Longwinglover said:

When you weigh your airplane as Piper did there will be a scale under each main wheel and one under the tailpost.


Hope this helps,

John Scott

Click to expand...

When you weight the plane it is level(as in set for rigging), not in the three point?

skywagon8a said:

It would be a good idea to actually measure the distance from the datum to the tail wheel as the TC sometimes is in error. I weighed an airplane once and used the TC numbers. They were wrong. The tail wheel was a few inches different than it was supposed to be.

Click to expand...

ya the 200" is for the location of the cg of the tail wheel assembly itself(say for computing going from a small tail wheel to a bush wheel one), NOT the location of the rear weighing point... think its more 203? ish..(tail wheel axel center line) and each cub is a different length, think there are 3 lengths to a cub depending on which jig it came out of????

benflyn said:

When you weight the plane it is level(as in set for rigging), not in the three point?

Click to expand...

Yes, level is correct.

John Scott

Another tip: The rear point for the CG envelope is 20" aft of the leading edge. That does not change with weight or CG. But the front of the envelope does move depending on the loaded weight. To easily compute that number divide the loaded weight by 125 and the resulting number is the forward limit in inches from the LE. For example if the loaded weight is 1550 pounds the forward point is 12.4".

Good tip Dan. Thanks

Bill

Interesting short cut. Only works between 1312.5 and 1750lbs though.

John Scott

T.J. said:

Another tip.
Most mechs weight the tail with the scale under the wheel, not the tail post. So, instead of (Weight at tailpost X 186), use Weight under tailwheel X 200. The 200" is from Piper Report 834, page 7, Tailwheels. All tailwheels are listed as +200".
Hope I didn't add to any confusion!:roll:

Click to expand...

I'm a proud new cub owner and updating the flight manual.

I want to confirm it is okay to use 200 for the formula instead of 186, as IA W+B took tailwheel measurement with scale under tailwheel and not at tailpost as mentioned. He put 200 as arm for tailwheel.

Do I need to update the flight manual to show I used 200 instead of 186? Are you allowed to do that?

As long as you make sure the C/L of the tail wheel axle is 14” behind the tail post.

Use the measurements of actual locations. No guesswork. With the plane in level attitude, of course.

This is opinion:
I believe the AFM should be "as delivered" with supplements as things change. To that end, what Mark probably needs is a copy of an original Piper Report # 765, with original wt/bal and equipment lists, and original signature.
Then, he can update that info on a separate sheet of paper with the new scale numbers and different tailwheel arm.
I don't have a clean copy of that document, since mostly I see 150 Super Cub paperwork.
If anybody has a clean copy with original factory weights, I think that would be useful to a lot of folks.

When I rebuilt my -12, completed a little over a decade ago, the supervising IA had me create a completely new Weight and Balance report, which listed all installed "stuff", with weight and arm, together with as-weighed empty weight data for the plane. I think his perspective was that a factory W&B was pretty much irrelevant at that point in time. I don't recall if I even had a factory W&B report.

The AFM is like a data tag - irrelevant unless someone wants to make a big deal out of it. The Super Cub 150 AFM is so generic all you need to do is make sure your N-number is on it - apparently they all weighed 1077 when they left the factory.
I would not worry about it, except, unlike the data tag, it is so dirt easy to make sure a "legal" document is on board. I am not sure, but I believe the original is required, and any changes come as addendums. Mark needs the numbers from an original 135 AFM, and page 3 with signatures.

Again, not sure, but I believe that is true of the weight and balance and equipment lists as well, although at this point they have all been superseded.

The TCDS will list "Required Equipment" for each model, and Item 401 (some letter) will be the required Flight Manual Report which is only a couple pages. All the flight manuals for a given model are the same, just make a copy and put your serial number and registration on the lines provided. Note 1 of the TCDS says "Current weight and balance report including list of equipment included in certificated weight empty, and loading instructions when necessary, must be in each aircraft at the time of original certification and at all times thereafter." The Weight and balance document from Piper is not part of the Flight Manual, it has a different Report Number. The original one from Piper is not required, just a Weight and Balance and Equipment List that meets the requirements of Note 1. Don't forget that many STCs require a Flight Manual Supplement to also be installed, so verify that all required supplements are also there.

I don't understand why people make this so complicated!

It is not complicated. But I beg to differ - all I have is a clean copy for the 150, and "Report 834" appears on all pages, including weight and balance and all equipment pages.

My opinion is that a PA-18 driver would do well to carry this unmodified document on board, along with any supplements, revisions, etc., that are necessary.
My opinion is that once the AFM is signed, we do not have any authority to change it.

To that end, Mark needs a clean copy of Report 765, so he can do as dga, Steve, and I suggest. Then if any subsequent alterations require an AFM Supplement, he needs to staple that on the back.

Finally, a current weight and balance and equipment list needs to be at least available - probably on board.

Guess Piper did it different on the -18 than other models. My -16 doesn't have a report number on the W&B section, and neither did my PA-22. The PA-16 actually tells you to make pen and ink changes when a metal prop is installed or when you install skis.

I'm sure we can all agree that the original Piper W&B is no longer applicable (except as a starting point to do W&B calculations instead of weighing), and the Equipment List is also likely no longer applicable. Most airplanes today show up with a stack of W&B calculations showing equipment removed and installed, but very seldom have I seen anyone actually go through the trouble of actually making ONE list of the equipment currently installed.

I do it. I also list all ADs in one place in the log, with date and page number of compliance - and alterations are also listed in one spot. In the logbook. Easy for Cubs. Tough for Lycomings.