Definitions and Calculations — Page 68, Lesson 87

Let’s start with the stabilizer trim units, because that’s the heart of this page. The stabilizer is the horizontal tail surface that the pilot can move to trim the aircraft — to balance the pitching moment so the control column forces are neutral. The trim setting is expressed in units, and it’s read off a graph as a function of two things: the centre of gravity position, given as a percentage of MAC — that’s Mean Aerodynamic Chord — and the flap setting in degrees. So look at Example 1. The question: what is the stabilizer trim setting if the CG is 15% MAC and the flaps are moved from the 5° to the 15° position? From the graph, with CG at 15% MAC and flaps at 5°, the stabilizer trim setting is 4.25 units nose-up. Now move the flaps to 15° — same CG, 15% MAC — and the setting becomes 3.5 units nose-up. So you can see the flap deflection changes the required trim. Example 2 introduces a CG movement. Suppose the traffic and fuel load shift the CG from 15% MAC to 24% MAC. What’s the change in stabilizer trim? With 5° take-off flap, the trim goes from 4.25 units at 15% MAC down to 3 units at 24% MAC. So as the CG moves aft — from 15 to 24% MAC — the required nose-up trim decreases. Now, a practical note: to let the pilot set the stabilizer trim correctly for take-off, there is a trim indicator on the instrument panel, or it may be integrated into an electronic display unit. That’s how you know what the setting actually is in the cockpit. Then the page moves to a different topic: cargo compartment limitations, referencing MRJT1 Figure 4.9. These tables detail the cargo compartment limitations that must be considered when items of cargo are loaded, to ensure the limitations are not exceeded. The key concept here is running load. The running load is the fore/aft linear load — that is, the weight distributed along the length of the compartment. Let me walk you through the example. Take a box with dimensions 3 feet by 3 feet by 3 feet — a cube — weighing 100 kg. Its running load is 100 divided by 3, which gives 33.3 kg per foot. And if you want it per inch, divide by 12: 2.78 kg per inch. The warning is important: be very careful to use the correct units — feet versus inches — because the running load limit is specified in a particular unit, and mixing them up would give you a wrong answer. So, to summarise the two ideas on this page: stabilizer trim is set from a graph using CG as % MAC and flap angle, and the trim indicator in the cockpit shows the setting. And for cargo, running load is the linear fore/aft load in weight per unit length, and you must respect the compartment limits and use the right units.