The International Cessna^® 170 Association

I also notice that the tail doesn't really want to come up when there's a load in the back. Part of that is that I have 180 gear legs,they're swept farther forward than stock 170 legs so there's more weight on the tail. I roll in more forward (nose-down) trim for take-off when I'm aft-loaded. If the trim is set right for flight I can just take off more from a 3 point attitude,the tail doesn't have to come up first. Just be careful to set the trim correctly to avoid a departure stall type situation.

Eric

Hi,

I had my plane weighed at my annual (6/18/03) at Twiss Air Service at Astoria, OR. The plane has a nav/com, Apollo GPS, Oil filter, Std rear seat (heavy), Xponder, two gyros, Wheel paints, NO gas, 5 qts of Oil. The weight was 1353 lbs. Left wheel 631 lbs. Right wheel 617 lbs. Tail Wheel 105 lbs. Arm is 39.61

I've been thinking about my weight and balance lately, and since I didn't have it memorized, I had to look up the arm for fuel. I pulled out the weight and balance sheet generated by a local FBO who annualed and weighed the aircraft last sept.

In the column marked arm(inches), their value was 10.7.

222 lbs of fuel = only 2375.4 moment? I don't think so. Frankly, I'd never understood fully the inches vs index units thing, and then I found this thread. I knew someone here would have discussed it.

The FBO used a computer program, which someone somewhere in softwareland must have entered data for the 170B from the POH and not understood the significance of index units, because it plugged in 10.7 for my fuel arm. I think it should be somewhere around 48 inches, right? I have to go back to them and have them crunch a new one and explain why I didn't catch this sooner.

I haven't done any serious loading of my aircraft other than 1 pax and my lunch, so I never noticed the numbers not crunching right until tonight when I went to enter the weights and arms into my 496 W&B calculator.

So can I derive my absolute fuel arm by this calc:

37 gal useable x 6lbs/gal = 222 lbs

10.7 index = (222*Arm)/1000

10700 = 222 * Arm

Arm = 48.19 inches

That should be the absolute arm for the fuel station on any 170B, using 37 gallons useable, correct?

Why doesn't the POH just list station arms in inches? I'm reading TCDS A-799 to get the station arms in inches...is that acceptable?

It does not matter what method you use to figure your weight and balance a long as it's correct. As you found out you can mix and index with an arm. You can figure an index with the arm or the arm with the index.

I don't use the index method because the Army who taught me W&B doesn't and I've never found it useful or necessary.

For a 170A or B model you should have 5 gallons or 30 lbs of unusable fuel at an arm of 46" per the TCDS. The moment would be 1380.

Any other fuel in the main tanks has an arm or 48" per the TCDS. So 37 gallons at 6lbs per gallon is 222lbs. 222 times the arm of 48" gives a moment of 10656.

I believe your calculation of the arm doesn't work out because the moment was rounded to 10700 rather than 10656.

The TCDS lists the Arm for fuel tanks for a B model at 48 inches, which may be used for calculating Wt and Bal for fuel.