Weight and balance mystery

[Bruce]

Because I am experimental and the aircraft manufacturer I set the datum just like I set my gross of 1320. My aircraft is even approved for aerobatics.

[Clayton Harper]

Copy, Out.

[Jeff J]

The only mystery is why someone at Piper 70 years ago thought it would be a good idea to show a picture on a W&B sheet with a nose datum then, on the same sheet, use the WLE for the equipment list and the loading graph. The original flight manual, original owners manual, current type certificate (I don't have an original) and 2/3 of the original W&B sheet for a 1950 PA-20 all use the WLE as the datum. Even so, someone did that PA-20 the same way yours was done. As an IA I have to say the only thing that matters is the type certificate that Clayton was kind enough to post a section of. Having a negative number in W&B computation should not be a big deal. That is math we all had to do when we computed the removal of equipment, fuel, passengers or baggage from an airplane to become pilots and/or mechanics. Now that most of us (not me) carry computers on our person it is a lot easier to compute today than it was 20 years ago.

I have read several accident reports on Pipers where W&B sheets were incorrect because one mechanic used the WLE and another used the nose. These were listed as items of interest only and not "factors". I think I found those on the Cub forum in a thread about the same datum confusion a few years back. Since W&B sheets rarely get reviewed on these aircraft, it would be quite an event for one to crash because of an error on the sheet.

[Steve Pierce]

I agree, lot more confusing. Worked on a 2002 data plate Cub Crafters Super Cub where they used the wing leading edge for the rear weighing point datum and 60" forward of the wing leading edge for the main wheel datum. Took me a long time to straighten that one out. Should have just reweighed the airplane because of all the weight and balance revisions that had been done.

[zalto]

Something I tinkered with several years ago.
Thought some of you might want to play with it.
** Not for actual use! May be (probably is) flawed in one (or many) ways.

Dusty

[Jeff J]

I did something similar except I pretty much duplicated the original PA-20 W&B sheet and gave the user a choice which datum they wanted to use. It is nice to be able to punch numbers into a computer and have it instantly spit out the correct answer.

[Clayton Harper]

Has anyone gone though H8083-1A to see if is OK to change the datum. I haven't yet.

[Steve Pierce]

I just use the wing leading edge since that is what everything is referenced to on the TCDS.

[Clayton Harper]

I just use the wing leading edge since that is what everything is referenced to on the TCDS.

I can't see why anyone would do anything else on certificated aircraft. I'm just wondering what the FFA says about it in the Aircraft Weight and Balance Handbook FAA-H-8083-1A.

[Jeff J]

I can't see why anyone would do anything else on certificated aircraft. I'm just wondering what the FFA says about it in the Aircraft Weight and Balance Handbook FAA-H-8083-1A.

From page 2-1:
"The datum may be located anywhere the manufacturer chooses; it is often the leading edge of the wing or some specific distance from an easily identified location."

and from page 3-6:
"Location of Datum
It makes no difference where the datum is located as long as all measurements are made from the same location."

and AC 43-13-1B on page 10-1:
"The datum is an imaginary vertical plane from which all horizontal measurements are taken for balance purposes with the aircraft in level flight attitude. The datum is indicated in most Aircraft Specifications or TCDS. On some of the older aircraft, when the datum is not indicated, any convenient datum may be selected. Once the datum is selected, all moment arms and the location of the permissible c.g. range must be taken with reference to it.