Weight and balance problem

[dreamed2fly]

How to find arm or moment when it is not stated in the poh?
Ive looked everywhere in the poh for 172. It only states EW but no moment or arm

[Ixixly]

Go look at the Aircrafts W & B where they actually weigh the Aircraft.

[john_tullamarine]

Perhaps you can provide a scan of the relevant POH data .. the story sounds a tad strange .. with a picture we can offer competent comment.

The POH provides generic (probably - depends on how old the book is) GAMA format loading data.

The weigh data - usually residing in the POH - provides the actual aircraft's starting data for doing some sums.

If you have a problem posting scans, send them to me via email and I'll run up some comments for you.

[AbsoluteFokker]

Each aircraft is different and therefore requires a properly calibrated measurement of its exact empty weight, fuel capacity and appropriate arms and weights.

Do not expect this from a generic POH for a given aircraft type.

Can't find it? You can't go flying.

[poonpossum]

You should see a printed page or some kind of obvious addition to you manual that lists the empty weight for the specific aircraft along with the arm.

If you can't find it you're technically buggered, practically, call up whoever owns the maintenance logs for the thing and ask them ;)

[uncle8]

What you need is in the Aircraft Flight Manual.

[training wheels]

How to find arm or moment when it is not stated in the poh?
Ive looked everywhere in the poh for 172. It only states EW but no moment or arm

For the C172, you use the loading graph in figure 6.6 of the POH to get the moment for each station.

http://i.imgur.com/AOGEjKW.jpg

Refer to the example given in figure 6.5 of the POH. Once you have added all the weights and moments for each of the different stations, you then plot the total weight and moment on the CoG Moment Envelope graph (figure 6.7).

http://i.imgur.com/cWG1Zno.jpg

[skkm]

Yes, but that is all completely useless if one doesn't have a moment/arm for the BEW.

[john_tullamarine]

Some comments -

completely useless if one doesn't have a moment/arm for the BEW

Indeed.

a properly calibrated measurement of its exact empty weight

Let's not get carried away here.

Empty weight (and CG) are NEVER "exact" .. they always carry a measure of error associated with the scales and weighing techniques adopted.

A very well run weigh on a 172 with appropriate platform scales and hangar may provide an empty weight accurate in the vicinity of 5kg and empty CG accurate in the vicinity of 5mm. Many weighs are not all that well done and errors can be considerably more. Use jackpad cells and the problems escalate.

Do not expect this from a generic POH for a given aircraft type

For civil light aircraft, the general rule is that the POH is generic and, for anything recent, will be to the GAMA format. Generally, this is not the case for military aviation where the preference is for each flight manual to be tailored to the aircraft.

For each aircraft, the weighing data provides the input to operational calculations.

For the C172, you use the loading graph in figure 6.6 of the POH to get the moment for each station

The graphs shown are typical GAMA format. One notes that all the calculations bypass the need to know the specific CG using, rather, the moment.

The present problems are

- the 172P may not be relevant to the OP's question

- the OP doesn't make all that much sense until we can see the data with which he/she is playing. This is so in respect of "It only states EW but no moment or arm"

[Sunfish]

You should have a W & B certificate specific for each aircraft that states the BEW and arm. If you don't have that you can't fly.

[9 lives]

Dream,

The arm is found with a measuring tape, if not from the flight manual. The moment is the arm multiplied by the weight. It's a big number, but don't worry, it works itself out again.

The flight manual, and/or the W&B documents with the aircraft will specify the datum. For a 172 (or any other single Cessna) it will be the base of the firewall. But be careful, for other aircraft types it will not be, and indeed, might not even be along the airframe at all, 'could be out in front.

If you are to load something in the aircraft, determine it's C of G (a point you can measure to), and measure that from the datum. That's the arm of the item.

I was told by a very informed source that only one in ten aircraft were weight as they went out the door new at a well known manufacturer. The other nine were a guess, I suppose. Then, people start installing and removing things, and do the math. At best, the aircraft is reweighed correctly.

Sometimes, things happen which change the weight or balance, and they are not recorded. Removing wheel fairings seasonally is an example. You must have a valid W&B, and you have to believe it. Then you're responsible for the loading relative to that. But don't get carried away. If you are correct, and within limits to within a few pounds, and an inch, the plane will be safe to fly. Within one pound and 1/2 inch is very optimistic.

[training wheels]

Yes, but that is all completely useless if one doesn't have a moment/arm for the BEW.

Ah yes, agreed. I thought he was asking how to do a weight and balance for the C172. If this aircraft doesn't have a moment stated for the BEW in the POH, then it makes you wonder what pilots previously have been using to calculate their weight and balance before flying this aircraft?

[john_tullamarine]

The flight manual, and/or the W&B documents with the aircraft will specify the datum.

One important caveat.

If your aircraft has a trimsheet loading system (and there are four seaters around with such) it is highly likely that the designer has used a non-OEM datum for the sheet to maximise accuracy. Otherwise the trimsheet will likely be a bit of a dog's breakfast.

The two datum references can't be mixed unless you know the formula. Easy enough to reverse engineer from the trimsheet if you know what you're about.

For those who may be interested, the general formula is

IU = constant1 + (empty weight x (FS-trimsheet datum))/IU constant2

and should apply across the board. Constant1 applies only to the IU entry argument at the top of the sheet.

For the larger aircraft (ie heavies) the OEM usually specifies a trim datum. There is no compulsion on the trimsheet designer in the field to use that OEM trim datum, though, so one needs to be a little careful at times as Captain Speaking. I've used a third datum for a number of heavy sheets where I didn't particularly like the OEM choice of trim datum.

[MakeItHappenCaptain]

Weight x Arm = Moment,
therefore, Arm = Moment/Weight qed

Just remember to return you moment to the original order of magnitude and stay in corrcet units, ie. Metric or Imperial.
(Imperial only used to stick a big finger to the British. With a Louisianan drawl, "Oh but Farenheight and inches are SO much easier than metric!":ugh:) Now THAT'S resistance to change.

[paramagarwal]

A request for a link or answer
If the load spreader is wider than stipulated, is it detrimental or useful for the loading process? Any calculation of limitations imposed due the min or max width of the load spreader? Can anybody give any links for the calculations used for determining the min height and min width of a load spreader? Any precautions/SOP/technical limitations stipulated when loading special load manually?
Thanks everybody.