Load intensities which exceed the UD load intensities, “concentrated loads”… — Page 63, Lesson 81

Let’s pick this up right where the load limits left off. We’ve been talking about the uniform distributed load — the UD load — that a cargo floor is certified to carry. Now I want to walk you through what happens when you want to carry something that exceeds that UD load intensity. That’s what we call a concentrated load. A concentrated load is simply a load intensity that exceeds the UD load intensities. The key point is this: you can carry it, but only on one condition — you must use approved load spreaders. The spreaders distribute the load over a sufficient area so that the loading limits are not exceeded. In other words, the spreader takes the concentrated force and spreads it out across more floor, bringing the actual intensity back down under the limit. Now, why do we need floor protection at all? Because when you carry concentrated loads — or items that have hard or sharp areas — they can damage the floor. The normal practice is to use standard load spreaders of 2 inch thick timber. That’s the standard tool: 2-inch-thick timber boards placed under the load. Let me give you a worked example so you can see exactly how this calculation runs. This is the kind of thing you’ll be doing in the exam, so follow the logic carefully. We have an item of cargo with dimensions 3 ft × 4 ft × 12 ft, and it weighs 900 kg. The compartment has two limits we care about: the maximum running load is 20 kg/in, and the maximum load intensity is 70 kg/ft². We need to find the running load values and the floor intensity values, and decide whether there are any limitations on how we may carry it. And note the conversion: 1 ft = 12 inches. Let’s start with the running load — that’s the underfloor protection. The running load is the load per inch of length along the aircraft’s longitudinal axis. The rule is: you divide the load by a length, and you get three values depending on which length you use. The max running load uses the shortest length. The mid running load uses the mid length. The min running load uses the longest length. So, max running load = 900 kg divided by (3 ft × 12 inches/ft). That’s 900 divided by 36, which gives 25 kg/in. And the limit is 20 kg/in — so this exceeds the limit. That’s a problem. Mid running load = 900 kg divided by (4 ft × 12 inches/ft). That’s 900 divided by 48, giving 18.75 kg/in. That’s below the limit — OK. Min running load = 900 kg divided by (12 ft × 12 inches/ft). That’s 900 divided by 144, giving 6.25 kg/in. Also below the limit — OK. So for running load, the only orientation that breaks the limit is when the 3-foot dimension is along the aircraft. We’ll come back to that. Now the distribution load intensity — that’s the floor protection. Here we divide the load by an area, not a length. Same three-tier logic: max distribution load uses the smallest area, mid uses the medium area, min uses the largest area. Max dl = 900 kg divided by (3 ft × 4 ft). That’s 900 divided by 12 ft², giving 75 kg/ft². The limit is 70 kg/ft² — so this exceeds the limit. Mid dl = 900 kg divided by (3 ft × 12 ft). That’s 900 divided by 36 ft², giving 25 kg/ft². Below limit — OK. Min dl = 900 kg divided by (4 ft × 12 ft). That’s 900 divided by 48 ft², giving 18.75 kg/ft². Below limit — OK. So the conclusion is this: the cargo can be carried, but only under two conditions. First, it must be placed on the cargo bay floor on either its medium area or its largest area — that prevents floor damage. Second, either the 4-foot length or the 12-foot length must be placed parallel to the longitudinal axis of the aeroplane — that prevents underfloor damage, because it spreads the load across the underfloor frames. So the takeaway: you’re not just checking one number. You check the running load along the aircraft axis, and you check the distribution intensity on the floor. If either one exceeds its limit in a given orientation, you rotate the cargo to a different face — and the spreaders handle the rest. That’s the full picture of concentrated loads and load spreaders.