Definitions and Calculations — Page 63, Lesson 79

Right, let's get into the heart of the load control side of mass and balance. We've already talked about the big picture—total mass, centre of gravity, the envelope. Now we're going down to the floor itself, because the aircraft structure has its own limits that we have to respect. First, let's deal with the machinery that moves the cargo. In a modern freighter, we have Power Drive Units, or PDUs. These are mounted in the floor of the aircraft. They're controlled by a control joystick, and their job is to move the containers around. Specifically, they move containers laterally and longitudinally in the doorway area, and longitudinally in the compartment forward and aft of the doorway. So, right at the door, you can push them sideways and along the aircraft; once you're past the door, it's just along the length of the fuselage. On the control panel you'll see an Eight Direction Selector Switch—that's the switch that lets the operator choose which direction the PDU system drives the containers. Now, once those containers are in position, we need to hold them there. That's where the guides, rollers, stops, locks and tie down fittings come in. These are placed in the appropriate locations in the floor to provide adequate 'G' restraint for the containers in flight. That means they hold the cargo against the forces of acceleration, deceleration, and manoeuvring—the 'G' forces—so nothing shifts around. Now, the critical part: Floor Loading. The floors of the passenger cabin and the freight areas are limited in the load they can carry. This is a structural limit, and it's serious. If you place excessive loads on the structure, you might see visible panel creasing and local indentations. But that's just the cosmetic warning. The real danger is that it will significantly accelerate structural fatigue. Fatigue is cumulative—every overstress adds up—and it can lead to major structural collapse with little or no warning. So we don't guess at this; we calculate it. The floor loading is defined in two distinct ways: by linear loading and by area loading intensity. Let's take them one at a time. Linear / Running Loads. The linear, or running, loading limitation is expressed in lb per linear foot or kg per linear inch. Its purpose is to protect the aircraft underfloor frames—the structural members that run across the aircraft. It's the total load permitted in any one inch or one foot length of the aircraft, irrespective of the load's width. So it doesn't matter how wide the item is; what matters is how much weight is concentrated over that one-inch or one-foot length of frame. Let me show you how this works with the example in Figure 2.18. In example (a), we have a 500 kg load sitting on a 10 inch length of floor. The linear load is 500 kg divided by 10 inches, which is 50 kg per inch. Now, the limit here is 45 kg per inch. So 50 exceeds 45—that's unacceptable. But here's the clever part: the way you orient the load can turn an unacceptable situation into an acceptable one. In example (b), we simply rotate that same load through 90 degrees. Now the 500 kg is spread over 20 inches. That's 500 divided by 20, which is 25 kg per inch—well within the 45 kg per inch limit. The key lesson here: the least linear loading occurs when the longest length of the item is placed at right angles to the floor beams. You're spreading the weight across more frames. Now, the second limitation: Area Load Limitations. These are expressed in kg per square metre or lb per square foot, and they protect the floor panels themselves—the panels that sit between the frames. There are two permissible intensities of pressure: Uniformly Distributed loads, or UD loads, and Concentrated loads. The general floor loading limitations for cargo are referred to as UD loads, given as allowable lb per square foot. If a load, or a series of loads, is within the allowable UD limitations for the floor area on which it rests, then the loading is subject only to two further checks: first, that you don't exceed the linear load limitations we just discussed; and second, that you don't exceed the individual and accumulated compartment load limitations. So UD loads are the baseline—if you're within the UD limit, you just have to check those two other things. But what if your load intensity exceeds the UD limit? Then you have a concentrated load. These can still be carried, but only on one condition: that approved load spreaders are used. A load spreader is a device that distributes the concentrated weight over a larger floor area, bringing the effective pressure back down to an acceptable level. So, to tie it all together: linear loading protects the frames, area loading protects the panels. You check both. And if you have a heavy, concentrated item, you spread it out with an approved spreader to keep the panels safe. That's the structural side of the floor—now you know why we're so careful about how we position every single piece of cargo.