Weight and Balance Question

bcso48

bcso48

Filing Flight Plan
Joined
Mar 23, 2010
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18
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Paradise CA.
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Display name:
George Mahon
OK I am going to start off with throwing myself under the bus and asking a very stupid question. I believe I know the answer but was curious if there is more to this then what I am seeing.
When I first got instruction on weight and balance I was taught the old school method of weight times arm of course equals moment.
My instructor would have me write it out long hand on a sheet of paper total everything and then see if we were in the CG envelope.
My question is with Cessna I've noticed on the 172, 182, and even the 210 they never provide an arm for the fuel. They give an arm for every station besides the fuel. I understand this arm would change because the fuel is the one item where weight changes in flight.
I remember training questions for weight and balance where there was a fixed arm for fuel on board. Isn't there other aircraft like from piper that provide a fixed station arm for fuel ? Thanks.....George
 
They do indeed provide an arm for the fuel... look again in the AFM/POH/PIM.

Hint - if you don't see it in a table - see if there's a graph that shows gallons and moment, and then work it out.

I can tell you authoritatively that the 172R and S model information manuals do show an arm for fuel....172R and S and 182S - all on page 6-9.
 
Hey Tim,
I do look it up on the graph and work backwards to get the Arm. I'm looking now at the POH for a 1977 182Q Cessna. There are published ARMS for the Pilot passengers and baggage but not for the fuel. To get it is easy enough you simply go to the loading graph get the moment and work backwards.
I will look up those 172 models you mentioned. I have just noticed on some Pilots Operating Handbooks you have to do some simple math to get the Arm and others they publish the number and its posted by the loading graph.
 
Last edited:
I have a POH for a Cessna 172S, and they have a chart on page 6-14 (probably doesn't help you) that has "Weight (lbs)" on the y axis, and "Load Moment/1000 (pounds-inches)" and various lines for different stations in the aircraft.

Such lines might be "Pilot and Front Pax", "Fuel (6 lbs/gal)", "Rear Passengers", "Baggage Area". The slope of these lines relate to the moment arm.

So, You don't necessarily need to know the "arm" for fuel, when you can use plots like that to detemine how much fuel weight produces this moment. All you need to calculate CG, is total weight and total moment, which can be determined from such charts.

http://www.langleyflyingschool.com/...e Graph (Cessna 2), Langley Flying School.gif

That link shows a chart similar to what I am talking about (don't use THAT particular chart for anything, as a I don't know what it is for).

With all that said, why do you need to know the fuel arm? Are you able to get the moment without knowing the fuel arm?
 
bcso48 said:
Hey Tim,
I do look it up on the graph and work backwards to get the Arm. I'm looking now at the POH for a 1977 182Q Cessna. There are published ARMS for the Pilot passengers and baggage but not for the fuel. To get it is easy enough you simply go to the loading graph get the moment and work backwards.
I will look up those 172 models you mentioned. I have just noticed on some Pilots Operating Handbooks you have to do some simple math to get the Arm and others they publish the number and its posted by the loading graph.
Click to expand...
If the AFM is set up so that there's a CG section (section 6)... read it carefully - Cessna mentions the usable fuel arm in the starting of the section, NOT with the table or the section where they talk about the pilot seat.
 
Hey Tim,
Thanks for your time on this. I looked at my POH and there it was. Don't know how I missed that before. George
 
bcso48 said:
I understand this arm would change because the fuel is the one item where weight changes in flight.
Click to expand...

The arm for fuel in a Cessna single effectively does not change as fuel is burned. The CG solution for the sum of everything on board changes as the fuel weight changes just as it would change if you threw a passenger out the door in flight.
 
Generic 172 and models A-H, TCDS 3A12.pdf

"Fuel Capacity 42 gal. total, 37 gal. usable (two 21 gal. tanks in wings at +48)"

There's your arm. Get the TCDS for the aircraft you're interested in have fun.

[soapbox on]

In addition to explaining the AFM/POH, the CFI should also be explaining where to find the TCDS and how to read it for airplane being used.

[soapbox off]
 
murphey said:
Generic 172 and models A-H, TCDS 3A12.pdf

"Fuel Capacity 42 gal. total, 37 gal. usable (two 21 gal. tanks in wings at +48)"

There's your arm. Get the TCDS for the aircraft you're interested in have fun.

[soapbox on]

In addition to explaining the AFM/POH, the CFI should also be explaining where to find the TCDS and how to read it for airplane being used.

[soapbox off]
Click to expand...

Good point and I'll admit I don't know where to find it.... where are they available - faa.gov?
 
They are interesting yes, but I don't think you need the TCDS to get the info necessary to fly an airplane. The POH/Owner's manual, W&B calcs and any supplements should have all the info you need. TCDS have only been readily available for a few years. I don't think any instructor has ever discussed with me. My A&P, yes. When I rented, I don't remember ever seeing one for the planes I flew or needed it for that matter.
 
WOW!- Thanks!!! Now I can answer a lot of my hypothetical questions.

Is this data "better" than the POH?
 
Q: Why does the TCDS for the 172P say Oil Capacity is 2 Gal, 3.5 Gal usable???
 
Jaybird180 said:
WOW!- Thanks!!! Now I can answer a lot of my hypothetical questions.

Is this data "better" than the POH?
Click to expand...

I think the POH is supposed to reflect what is on the TCDS. IOW, the TCDS is gospel and the POH info comes from that source. If/when the TCDS changes, a revision to the POH should be issued.
 
ejensen said:
They are interesting yes, but I don't think you need the TCDS to get the info necessary to fly an airplane. The POH/Owner's manual, W&B calcs and any supplements should have all the info you need. TCDS have only been readily available for a few years. I don't think any instructor has ever discussed with me. My A&P, yes. When I rented, I don't remember ever seeing one for the planes I flew or needed it for that matter.
Click to expand...

But as the OP pointed out, the documentation he has doesn't have it easily available. I've never liked reading printed graphs for something like this because of the print poor quality and lack of accuracy.

Besides, there's all sorts of interesting tidbits in the TCDS. For example, a child's optional jump seat is permitted under certain circumstances, what props are legal (the 172A & B have 2 different props allowed), and where the datum is located. It even lists, based on serial number, how much fuel is considered unusable. We discussed placards in another thread. And at the bottom, specs for ferry flights.
 
Greg Bockelman said:
Misprint. I think they mixed units. Capacity is 2 Gallons (8 Quarts) and I think that should read 3.5 QUARTS usable.
Click to expand...

Pg 14, rev 80. And this is the May 2010 update to the TCDS. Someone forgot to proofread!

Wonder who to send a note to about this?
 
murphey said:
But as the OP pointed out, the documentation he has doesn't have it easily available. I've never liked reading printed graphs for something like this because of the print poor quality and lack of accuracy.

Besides, there's all sorts of interesting tidbits in the TCDS. For example, a child's optional jump seat is permitted under certain circumstances, what props are legal (the 172A & B have 2 different props allowed), and where the datum is located. It even lists, based on serial number, how much fuel is considered unusable. We discussed placards in another thread. And at the bottom, specs for ferry flights.
Click to expand...

It should be on the W&B report (as required by the TCDS :wink2:). The generic 78 POH I have does show a sample but only gives the moment so you have to calculate the arm (+48). The actual POH should be customized for the aircraft. It looks like the 88 gal tanks change the arm to 46.5.

And I'm not arguing the usefulness of the TCDS (and any STCs). I even plan to read the ASTM for the S-LSAs since there is no TC.
 
murphey said:
Generic 172 and models A-H, TCDS 3A12.pdf

"Fuel Capacity 42 gal. total, 37 gal. usable (two 21 gal. tanks in wings at +48)"

There's your arm. Get the TCDS for the aircraft you're interested in have fun.

[soapbox on]

In addition to explaining the AFM/POH, the CFI should also be explaining where to find the TCDS and how to read it for airplane being used.

[soapbox off]
Click to expand...

A-freakin'-men!!!

I have a whole folder full of TCDSs on my laptop, and it's the first place I go for a lot of information like that.

According to the TCDS, all 172 fuel tanks are at +48. 182 tanks are also at +48, except for the models with 92 gallon tanks (87/88 usable) which are at +46.5. I don't have the 210 TCDS, but you can find any TCDS you want here:

http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/MainFrame?OpenFrameSet
 
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