The International Cessna 170 Association

The problem could be the weight and balance is being done from a incorrect datum point . It was stated the firewall was used ,I am under the impression the center of the axles should be used as the measurement datum point .

The datum for all models according to the TCDS is the front face of the firewall.

Cessna has you measure the distance between wheel weigh points, then convert to datum later in the calculation. as long as you keep all the numbers straight during the weigh, get a total, then convert the distances, you are OK. the original weight & balance form that has the weighing procedure takes it step by step, but assumes no fuel and no oil.... weigh the aircraft, then after you calculate the basic w&b, you convert to datum. then you add unuseable fuel and full oil for the "licensed empty weight" using arms from datum.

if you weigh with full fuel.... remove all fuel mathematically after the weigh point numbers are converted.... use the TCDS/Spec for the arm of the fuel. then the unuseable fuel is added back in. it's only unuseable in flight, in some attitude (it is the fuel left in any tank when the engine sputters during any maneuver the aircraft is allowed to do in the category it's certified in). it's not as accurate to do it this way... only more convenient

I usually weigh with full oil and make note in the weighing record. realistically, it's no different than any piece of fixed equipment, and all the adverse load calculations, etc. require full oil anyway

you can also use the weigh points as measured from datum and skip the formula.... same results and is easier to follow. btw, the metal Pipers are the same, the rag ones are worse, uses the same formula, but have 2 datums

Just thinking here and sometimes that can be dangerous but wouldn't you want the CG as far forward as possible when it's empty? Since the datum is the firewall, everything you load in the plane will move the CG aft. The plane is never going to go flying by itself so why worry that it is not in range when empty?

That just gives more latitude in loading and the pilot still makes the decision what to carry and where to load it based on whatever the final LOADED CG is going to be. I have a fairly stock plane (weight-wise anyway) and its empty CG is right at the front at 49.9 Index Units, right where I want it to get full usage of the CG range when loaded.

Am I missing something here?

Thanks Arash!!! That is what I was missing!

I am under the impression the center of the axles should be used as the measurement datum point .

Likely that was a senior moment and highly unlikely the reason that you were unable to mount a C/S .....but mounting a C/S was not an issue for me and I believe has not been a unsolvable issue for a few others.

I think only items loaded aft of the CG point will move the CG aft, right? The datum is an arbitrary point that just gives you a reference.

And the reason (in an effort to remove confusion) that modern aircraft have their datum situated in " space" forward of the nose so that all figures are positive.

I plan to reweigh my own some day as it was calculated too far aft. In any event, ....if you need to shift operational C.G. aft.... I've always preferrred to use a case of bottled WATER for ballast. It can be adjusted by removing individual bottles...and it can be VERY USEFUL in the event of survival needs.

A case of Vodka would be better it won't freeze until -16.51 degrees F.

You da REAL man,John!

Wait.... Will that affect my balance...???

In some parts of the world, Vodka, has been used as a Deicing Fluid.

What was the Vodka deicing? The airplane or the pilot?

In the case I am familiar with, the airplane was a 727. Vodka was used as a replacement for the empty tanks of deicing fluid. Since is was over 90 proof and 40 percent alcohol, it worked very well, and it was cheaper than the usual glycol..
Everyone was happy, almost.

One of the reported problems experienced by the defunct Soviet Air Force during the Cold War period was the impossibility of keeping the aircraft de-icing systems serviced with alcohol due to theft. The fluid was denatured by the addition of formaldehyde to no avail, and resulted in numerous hospitalizations and injuries.

(BTW, 80 proof vodka will freeze at approx. 16 degrees F so it might not be the best anti-icing fluid in Siberia)