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Tom Ferraro, A&P/IA, performed a Weight and Balance on our 1969 Cessna 172K. He goes into complete detail on the process.
There are a few things people think about right away. As we age, we all get heavier, so do airplanes. Don't ask why, but we are putting in fancy interiors, new paint jobs, a lot of avionics, changing batteries and other modifications, which is part of the reason.
Basically, to get a good weight and balance is more that just putting on the scales. There are some steps you'll need do to start the process.
You can get some of the information you'll need from your aircraft records.
You can go to the FAA website under aircraft registration to get the n number, make and model number (like ours, which is a Cessna 172K). The specifics for your airplane are unique to the particular model.
On the airplane itself, look for the serial number, manufacturer, tachometer and hobbs reading. Having the weight and balance information that came from the manufacturer is also beneficial. That will give you the arms and locations in the equipment list.
One last thing that people often forget, is whether there are any STC's on the airplane, like extra fuel tanks or anything that change the gross weight of the aircraft.
Another important step, is to go to the type certificate data sheet. This is the certified document that came from the manufacturer with the airplane that tells you specifically all the details. Go to the FAA.gov website to get the certificate data sheet and the specific model number, gross weight and usable fuel. Our Cessna 172K hold 42 gallons of fuel of which 38 is considered usable fuel. When the aircraft is weighed with full fuel tanks, the 38 gallons of usable fuel is deducted. The 4 gallons difference stays with the weight of the airplane. If the aircraft has no fuel, 4 gallons are added the aircraft. Our aircraft was filled to its capacity and the usable fuel was deducted. You'll also need he datum location, where you measure all the arms to the waypoints of the aircraft.
To determine the leveling means, the aircraft must be level. The type certificate data sheet will indicate where the leveling means is. In our aircraft, it's the top door sill, where we used a regular bubble leveler. Most times the tires pressures will need to be adjusted to make the aircraft sit level.
All this information was written down on a worksheet for later use.
The aircraft was then pulled up onto the scales which are load cell and aircraft specific. The scales are calibrated and kept up-to-date. These scales allow for the position of the aircraft on the scales, whether is might offset a little.
Using a bubble style level, the aircraft is checked to make sure it's sitting level.
The actual weights are then read. Left (red line, Center (yellow line) and Right (Green line); the 2 mains and the nose.
The actual weight of our aircraft is shown as 629, 628 and 435.
All the above information is added to a spreadsheet for a quick calculation.
At this point, the data from the worksheet is entered into a simple spreadsheet on a laptop computer. This spread sheet is used repeated and contains all the calculation and formulas necessary.
All the information is added to the aircraft log book and permanent aircraft records.
The left main was 629 pounds. The arm is 58 inches according to the aircraft records, which is the distance between the datum reference point and the main wheels. The nose wheel was 435 pounds and the arm is minus 7 which is critical because the datum is the firewall, so the nose wheel is in front of the firewall which makes it a minus number. The main gear makes the arm a positive number. We subtract 228 pounds for the 38 gallons of usable fuel. The final weight of the aircraft is now shown as 1,464 pounds and the center of gravity is 40.24 inches. The useful load of the aircraft is 836 pounds.
The form was printed out and given the aircraft owner (me) for my records.
Watch the video below to see the actual weight and balance process: