August 3, 2021 Summer School

The Liberty Gazette

August 3, 2021

Ely Air Lines

By Mike Ely and Linda Street-Ely

Linda: I’ve noticed a school bus passing by our house every weekday. I suppose summer school is in, and that made me think that all the adults sitting around, not in school, could use a lesson in something. I have just the thing: How to figure weight and balance of an airplane. 

Why would you want to know this? In addition to possibly learning something new, you will finish this article with a new appreciation (and forgiveness) for when the pilot of your next sightseeing flight asks how much you weigh and tells you where to sit.

Let’s think of a Cessna 172, the most popular training airplane. When aircraft are assembled, the weight of everything and its position are recorded. But there must be a reference point from which to measure. 

The way these small airplanes are built, there’s a firewall between the engine compartment and the rest of the airplane, e.g., where you sit. That firewall is, for weight and balance purposes, the reference (datum) point that Cessna uses. With the only variables being what you put into the plane, you can use this point to figure out how much weight you can put where. If you have too much weight too far forward, it will be hard for the nose to lift off the ground. Too far back and the airplane will nose up too fast. Think of it like the playground seesaw. But wait, we’re not ready for recess yet.

Here’s how you figure weight and balance: 

The pilot’s operating handbook will tell you the empty weight of the aircraft and its maximum gross weight. You figure out the weight of all the people who will climb into the airplane, plus any bags they’re carrying, and the weight of the fuel. Aviation fuel weighs six pounds per gallon. Let’s say your 172 has 30 gallons, and you’re a young couple with two children. (See fig.1)

Once you have all the weights jotted down, you multiply each by the arm. That is, the location where that “weight” will be in the airplane, which is measured by its distance from the datum. Do that little math problem and your answer is called the moment. 

Next, add the weights to get the total weight. Add the moments to find the total moment. Then divide the total moment by the total weight. This answer gives you the center of gravity.

Take the numbers that are total weight and center of gravity and find them on the chart supplied in the pilot’s operating handbook. (See fig. 2) As long as they fall within the limits shown on the chart, you’re good to go. Of course, you’ll burn fuel as you fly, so the airplane will be lighter when you land, so you should figure your anticipated landing weight and balance because that center of gravity will shift as fuel is used, but you get the idea.

Below is your homework. You may use a calculator. Turn it in to the Gazette office. Class dismissed!

ElyAirLines.blogspot.com

	Weight	Arm	Moment
Basic empty weight	1600	x 39	=
Pilot (wife)	130	x 37.0	=
Pilot (husband)	185	x 37.0	=
Rear passenger (child)	50	x 73.0	=
Rear passenger (child)	40	x 73.0	=
Baggage area	50	x 95.0	=
Usable fuel (30 gal x 6 lbs per gal)	180	x 47.9	=
TOTAL WEIGHT & MOMENT		---

____________    X   __________   =   ____________

Total Moment   X   Total Weight   =  C.G.

 Go to graph (Figure 2)

                            Figure 2