Definitions and Calculations — Page 63, Lesson 79

Let's start with the power drive units, because that's how cargo actually moves inside the aircraft. The PDU, as we call them, are mounted in the floor. They're controlled by a control joystick, and their job is to move containers in two directions. Laterally, that's side to side, and longitudinally, that's along the length of the aircraft. They do this in the doorway area, and then just longitudinally in the compartment forward and aft of the doorway. So in the doorway you get full movement, but once the container is past the door, it only travels along the length of the hold. Now, the control panel for this is what you see in Figure 2.17, the cargo bay control panel. And there's a specific switch on it called the Eight Direction Selector Switch. That's exactly what it sounds like — it lets the operator select one of eight directions for the PDU to push the container. So you're not just limited to forward and back; you have a full set of directional choices. Once the containers are in position, the aircraft has guides, rollers, stops, locks, and tie-down fittings placed in the appropriate spots. Their purpose is to provide adequate 'G' restraint. That means they hold the containers in place against the forces of acceleration and deceleration in flight, so nothing shifts around. Now let's move to the big topic: floor loading. The floors of the passenger cabin and the freight areas are limited in the load they can carry. And I want you to understand why this matters so much. If you place excessive loads on the structure, you might see visible panel creasing and local indentations. But the more serious problem is that it significantly accelerates structural fatigue. Fatigue is cumulative — it builds up over time, and it can lead to major structural collapse with little or no warning. So this isn't just about cosmetics; it's a safety-critical limit. The floor loading is defined in two ways: by linear loading and by area loading intensity. Let's take them one at a time. First, linear loading, which is also called running loading. The limitation is expressed in pounds per linear foot, or kilograms per linear inch. Its purpose is to protect the aircraft underfloor frames from excessive loads. Here's the key definition: depending on the units used, it is the total load permitted in any one inch or one foot length of the aircraft, irrespective of load width. So the width of the load doesn't matter for this calculation — only how much weight sits on a given length of the floor. Let me walk you through the calculation, because Figure 2.18 shows it clearly. In example (a), you have a load of 500 kilograms spread over a 10-inch length. So the linear load is 500 divided by 10, which gives you 50 kilograms per inch. Now, the limit here is 45 kilograms per inch. So 50 exceeds 45 — that's unacceptable. But here's the clever part: the way you locate an item in the aircraft can turn an unacceptable situation into an acceptable one. In example (b), you simply rotate the same load through 90 degrees. Now that 500 kilograms is spread over 20 inches instead of 10. So the linear load becomes 500 divided by 20, which is 25 kilograms per inch. That's well within the 45 kg per inch limit. So the rule of thumb is this: the least linear loading occurs when the longest length of the load is placed at right angles to the floor beams. That's the practical takeaway — orient your load so its longest dimension runs across the beams, not along them. Now for the second type: area load limitations. These are expressed in kilograms per square metre, or pounds per square foot. Their purpose is to protect the aircraft floor panels, not the frames. And there are two permissible intensities of pressure: "Uniformly Distributed" loads, which we call UD loads, and "Concentrated" loads. UD loads are the general floor loading limitations for cargo. They're given as allowable pounds per square foot. And here's the condition: providing 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 other checks. First, you must not exceed the linear load limitations. Second, you must respect the individual and accumulated compartment load limitations. So the UD limit isn't the whole story — you still have to check the linear load and the total weight per compartment. Finally, concentrated loads. These are load intensities that exceed the UD load intensities. So if you have a heavy item on a small footprint, it's a concentrated load. You can still carry it, but only if you use approved load spreaders. Those spreaders distribute the load over a larger area so the floor panels aren't overstressed. So to tie it all together: linear loading protects the frames underneath, area loading protects the panels on top, and when you have a point load that's too intense, you spread it out with an approved device. That's the complete picture of floor loading.