Definitions and Calculations — Page 50, Lesson 65

Let's pick this up right where the calculation left off. We've just worked out the Take-off Mass and its moment, and now we're finishing the flight in the log. First, in the 'MASS' column, we subtract the fuel used during the flight from the Take-off Mass to get the Landing Mass. In our example, that's 3642 minus 240, which gives us 3402 pounds. So the Landing Mass is 3402 lb. Next, we check the Operating Manual to make sure the Regulated Landing Mass hasn't been exceeded. That's the certified maximum weight we're allowed to land at, and we must confirm our calculated landing mass is at or below it. Now, in the 'MOMENTS' column, we determine the sign of the fuel moment. Fuel burned is a removal, so its moment is negative. We add or subtract it as appropriate to the Take-off Moment to get the landing moment. Here, the Take-off Moment is 310,286, and the fuel moment is 240 pounds times 75 inches, which is 18,000. Subtracting that gives us a Landing Moment of 292,286 pound-inches. In the abbreviated form, that's 2922.9 pound-inches. Here's an important point about large versus light aircraft. For large aircraft, if the CG is within limits at the Zero Fuel Mass, it's not normally necessary to calculate the landing CG position. Why? Because, as we stated earlier, the CG will stay within limits throughout the flight. But for light aircraft, it's usual to determine the landing CG position. And that's simply done by dividing the landing moment by the landing mass. So 292,286 divided by 3402 gives us 85.92 inches aft of the datum. It has a positive sign, which tells us it's aft of the datum. Now, there's another way to find the CG — graphically. We can use the Centre of Gravity Envelope for the SEP1, which is shown in Figure 2.10. This is a graphical representation of the mass and centre of gravity limits. The vertical axis is the mass in pounds. The horizontal axis is the CG position in inches aft of the datum. And the slanted lines represent the moment divided by 100. So you can plot your mass and moment on this envelope and see at a glance whether you're within the certified limits. That's the full picture — we've taken the Take-off Mass, subtracted fuel to get Landing Mass, checked the regulated limit, adjusted the moment, and then found the landing CG either by division or by reading the envelope.