What is the best method to do Weight & Balance for a Cessna 172? (If it's possible with a small tutorial!)
Thanks
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2 Answers
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The Calculation section of “Center of gravity of an aircraft” on Wikipedia gives an overview of the steps.
- Determine the weights and arms of all mass within the aircraft.
- Multiply weights by arms for all mass to calculate moments.
- Add the moments of all mass together.
- Divide the total moment by the total weight of the aircraft to give an overall arm.
The Basic Empty Weight (BEW) of the aircraft will come from the Weight & Balance information that is unique to each aircraft. The computed arm for the BEW may have some rounding error, so treat it as informational. Use the precise figures for Weight and Moment from your aircraft’s W&B.
The arms, distances from some reference datum, will also come from your aircraft’s W&B info: fuel, front seats, rear seats, and baggage areas on a Cessna 172. Copy those into your table as well.
Avgas (100LL) weighs 6 pounds per gallon, so the weight is 6 times the number of gallons.
I like to work top-to-bottom and left to right. For the grand finale, divide the sum of the Moment column by the sum of the Weight column to get the overall center of gravity.
The example below is from my PP-ASEL checkride taken in a Cessna 172. The items in boldface are computed values. The digits after the decimal are there for typographical purposes and do not necessarily indicate significant figures.
$$\begin{array}{c|r|r|r|} & \textit{Weight} & \textit{Arm} & \textit{Moment} \\ \hline \text{Airplane} & 1{,}665.6 & 39.2 & 65{,}928.93 \\ & & & \\ \text{Fuel (50 gal)} & \bf{300.0} & 48.0 & \bf{14{,}400.00} \\ & & & \\ \text{Bacon} & 180.0 & 37.0 & \bf{6{,}660.00} \\ \text{Shelton} & 172.0 & 37.0 & \bf{6{,}364.00} \\ & & & \\ \text{Duffel} & 10.0 & 73.0 & \bf{730.00} \\ \text{Shelton bag} & 10.0 & 73.0 & \bf{730.00} \\ & & & \\ \text{Backpack} & 15.0 & 93.3 & \bf{1{,}399.50} \\ \hline & \bf{2{,}352.6}& \bf{40.9} & \bf{96{,}212.43} \end{array}$$
In this particular case—given for information only—the center of gravity (40.9 ≈ 96,212.43 / 2,352.6) at the total weight is within limits for the Normal category as given in the aircraft’s Pilot Operating Handbook (POH). Your case may vary. The Pilot-in-Command is responsible for computing Weight & Balance for each flight.
As a point of interest, my DPE was Clyde Shelton who recently flew his ten thousandth checkride.
answered Oct 29, 2015 at 17:51
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$\begingroup$ Shouldn't you also add the weight of the pilot and passengers? $\endgroup$ Apr 16, 2019 at 9:33
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$\begingroup$ @ROIMaison Yes, the pilot in this case is the row labeled Bacon, and Shelton was the one passenger. $\endgroup$ Apr 17, 2019 at 10:25
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$\begingroup$ Ah right, I was looking for something along the lines of passenger, and completely missed the names! $\endgroup$ Apr 18, 2019 at 8:07
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Aircraft weight and balance is done the same for all aircraft (at lest all GA aircraft as far as I know). This PDF from the FAA provides a nice overview of it. You will need a few of the constants out of the POH for your plane such as dry weight and moments for various things. As well as specifics from your airplane is other avionics have been installed. Other than that this walkthrough covers it pretty well.
