Weight and balance is important in any aircraft, not just heavies.
If your friend invites you and 3 other people for a golf outing in his Cessna 172, be very wary. If he fills the gas tanks all the way - don't get on the plane! The plane may be out of balance and it is certainly overloaded. Very few light airplanes can carry a full load of passengers and full gas tanks.
We have a bit more leeway in a heavy jet, but it's still critical. At its most basic, we care about two things: how much we weigh and where that weight is distributed.
Calculating the weight is a matter of simple addition. We start with an empty airplane and then add up the weight of everything we put in it. That weight is constantly changing - as we burn off fuel we get lighter. In a 757, every hour in the air we burn off roughly 7000 pounds of fuel. That's not a trivial amount. This is why our fuel gauges read in pounds and our fuel flow indicators read pounds per hour.
Some of these numbers are estimates. We use a standard planning factor for the weight of the crew and the passenger haulers do the same for their passengers. It assumes that the average person weighs X number of pounds and carries Y pounds of stuff (purses, laptops etc) with them.
The FAA recently made that number bigger to reflect that Americans are getting, well, bigger. I always seem to end up in the middle seat between two of these bigger Americans. "So which NFL team did you say you played for sir?"
So let's look a few of the numbers we care about:
EMPTY WEIGHT = Weight of the airplane empty (simple enough)
Add the (estimated) weight of the crew and their stuff and we get:
BASIC OPERATING WEIGHT - We're not going to operate without a crew so this makes sense.
Add the cargo or passengers and we get:
ZERO FUEL WEIGHT - This is an important number because by adding our fuel at any given moment to this we'll always know how much we weigh.
Add the fuel and we get:
RAMP WEIGHT - this is as heavy as we're going to be. Somewhat important. There is a limit to how heavy the plane can be even just sitting on the ramp.
Burn off a little bit of gas starting the engines and taxiing to the runway and we're at:
TAKEOFF WEIGHT - also a very important number. This number, along with the runway and weather (winds, temperature, pressure) will be used to calculate our takeoff data.
LANDING WEIGHT - after we burn off gas getting to our destination this is what we'll weigh. Of course when we get there we'll check what our weight actually is and use that number in our calculations. This is just to make sure that we're legal to go. The plane is only certified to land below a certain weight.
So that's the "weight" portion of weight and balance. Pretty simple really.
Don't try to put more weight in the plane than it can hold.
Don't try to takeoff heavier than the plane can fly.
Don't try to land heavier than the plane can handle or you'll put too much stress on it.
The "balance" part is a little more complicated but at its most basic we don't want the plane to be too nose heavy or too tail heavy or we may not be able to control it.
Note that I haven't had to manually calculate a weight and balance in a very long time. That's handled by the people who load our aircraft and a computer program spits out a weight and balance sheet. All I do is check it to make sure it's within acceptable limits.
The calculations use a fixed point somewhere in the aircraft called a reference datum. This is determined by the aircraft manufacturer and is normally towards the front of the plane. It's an imaginary point that we only care about for purpose of calculations. I've never seen a big X that says "Reference Datum Here" on an airplane.
When we add weight to the plane, like a big crate of chickens, we note how far it is from the datum. This distance is called the arm. Think of it as a lever. The longer the lever, the more the force is multiplied.
Weight x Arm = Moment
So one chicken sitting 10 feet back from the datum is equal to 10 chickens only one foot from the datum.
Weight x Arm = Moment
Now we just calculate the Moments for everything we put on the plane and add them all together. This gives us the Total Moment. Divide Total Moment by Total Weight and we get the Total Arm. This tells us where our Center of Gravity is.
As long as that Center of Gravity is somewhere in the happy zone, we're good to go.
Now, if we try to stick all the chickens up front, even if they'd fit we'd be too nose heavy. We might not have enough control force in the elevator to bring the nose up for takeoff. Not good.
Or, let's suppose all the chickens ran to the back of the plane (that's why we keep them in cages). We'd be too tail heavy. We might not have enough control force to keep the nose down. Even if we did, the plane might become unstable because as we remember from a few chapters back, Center of Gravity must be forward of Center of Lift for the plane to be stable.
Who says chickens can't fly.
So what about the fuel? The fuel is carried mostly in the wings and where the wings meet the fuselage. That happens to be fairly close to the Center of Gravity so as the fuel burns off the CG doesn't change all that much.
Some planes, however, have extra fuel tanks in the belly for long range flying. As long as you burn the forward and aft belly tanks fairly equally it's not a problem. You don't want too much gas in the front or too much in the back. I'll talk more about the fuel system in a later chapter. Today we're just worried about the weight of the fuel.
I haven't talked about lateral (side to side) balance, just fore and aft. It's not a big deal with the passengers or cargo. The plane is much longer than it is wide, so there's not a lot of side to side moment. If the all chickens move to one side of the plane we're not going to do a barrel roll.
The side to side fuel balance is quite important, because the wings are very long. We keep an eye on the fuel burn and make sure each side burns at about the same rate. A few hundred pounds heavier in one wing is no big deal. In the 757 we can have up to a 1950 pound imbalance (one of those numbers I had to remember).
Not much else to it. I can distill this down to three simple rules:
Don't try to carry too much stuff.
Make sure you put everything where it needs to go.
Make sure it stays where you put it.
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