Definitions and Calculations — Page 68, Lesson 87

Right, let's pick this up with the stabilizer trim units. This is a practical, graph-reading skill, so I want you to think of it as a direct link between the aircraft's centre of gravity and the control surface you set before take-off. First, the term itself: stabilizer trim units. The horizontal stabilizer is that smaller wing-like surface at the tail. On a large transport aircraft, the whole stabilizer can be pivoted to trim the aircraft — to balance the aerodynamic forces so the pilot doesn't have to hold constant back or forward pressure on the control column. The setting is expressed in units, and the graph in Figure 2.19 plots those units against the centre of gravity position, for different flap settings. Let me walk you through Example 1. The question is: what is the stabilizer trim setting if the CG is 15% MAC and the flaps are moved from the 5° to the 15° position? MAC is the Mean Aerodynamic Chord — the average chord of the wing, and the CG position is expressed as a percentage of that chord. So 15% MAC is a specific CG location. Reading the graph: with the CG at 15% MAC and the flaps at 5°, the stabilizer trim setting is 4.25 units nose up. Now, keep the CG the same at 15% MAC, but move the flaps to 15°. The graph gives 3.5 units nose up. So you can see the flap setting changes the trim required, even with the same CG. Now Example 2 — this is the one that tests your understanding of the relationship. Suppose the traffic and fuel load cause the CG to move from 15% MAC to 24% MAC. What is the change in stabilizer trim? We stay with the 5° take-off flap setting. At 15% MAC, the trim is 4.25 units. At 24% MAC, the graph shows 3 trim units. So the change is from 4.25 down to 3 units — a decrease of 1.25 units as the CG moves aft. There's an important note here: to enable the pilot to correctly set the stabilizer trim for take-off, there will be a trim indicator on the instrument panel, or it may be an integrated part of an electronic display unit. So you're not guessing — you set the value and confirm it on the indicator. Now let's move to a different kind of limitation: the cargo compartment limitations. These tables detail the limitations which must be considered when items of cargo are loaded, to ensure the limitations are not exceeded. The key concept here is the running load. The running load is the fore/aft linear load — that is, the weight distributed along the length of the compartment. Let me give you the worked example. Take a box that is 3 feet by 3 feet by 3 feet — so it's a cube, 3 feet long — and it weighs 100 kg. The running load is the weight divided by the length: 100 ÷ 3 = 33.3 kg per foot. And if you want it per inch, you divide by 12, because there are 12 inches in a foot: 33.3 ÷ 12 = 2.78 kg per inch. The critical warning here is to be very careful to use the correct units. The running load can be expressed in kg per foot or kg per inch, and you must know which one the limitation table is using before you compare your cargo against it. Get the units wrong and you could exceed a structural limit without realising it. So, to summarise what we've covered: stabilizer trim units are read from a graph as a function of CG position and flap setting, with the trim indicator confirming the setting; and cargo running load is the fore/aft linear load, weight divided by length, expressed per foot or per inch — and you must match the units to the limitation table.