So the MAC itself is 3.5 m long — Page 58, Lesson 77

I want to walk you through the cargo handling section now. This is the part of mass and balance where we move from theory — calculating CG positions and fuel loads — into the physical reality of how we actually get baggage and freight into the aeroplane, and how that loading affects the balance we've been calculating. Let me first finish the worked problem that's hanging over from the previous page, because it's a classic exam-style question. We have an aeroplane with a zero fuel mass of 47,800 kg and a performance limited take-off mass of 62,600 kg. The leading edge of the mean aerodynamic chord, the MAC, is 16 m from the datum, and the trailing edge is 19.5 m from the datum. So the MAC itself is 3.5 m long. We need to move the CG from 30% MAC to 23% MAC — that's a 7% shift forward along the chord. The tank arm is 16 m aft of the datum, and the fuel specific gravity is 0.72. The question asks how much fuel, in imperial gallons, must be taken up to achieve that CG movement. I'll leave the arithmetic for you to work through — the answer is on page 92 — but the principle is exactly what we've been doing: adding mass at a known arm changes the total moment, and we solve for the mass that produces the required CG shift. The specific gravity of 0.72 is there to convert the mass of fuel into a volume in imperial gallons, because fuel is measured by volume on the aircraft. Now, the graphical presentation. In practice, we rarely do these calculations longhand on the flight deck. We use graphs. There are two things common to any mass and CG envelope graph. First, the CG must be within the envelope or on the line of the envelope — that's the safe operating region. Second, the mass of the aeroplane is always shown on the vertical scale. The horizontal scale can vary: it may be the CG position in inches, metres or centimetres; it may be the moment of the CG in kg inches, kg metres or kg centimetres; or it may be the percentage of the CG along the mean aerodynamic chord. The example for the SEP1, the single-engine piston aeroplane in CAP 696, is unusual because it uses both mass and moments on the vertical scale. The MEP1, the multi-engine piston aeroplane, uses CG position on the horizontal scale, and the MRJT, the medium-range jet transport, uses the MAC percentage. So the same idea — plot mass against CG position — but the units and the horizontal scale differ between aircraft types. Now let's move into cargo handling itself. The cargo compartments are in the lower deck, and they accommodate baggage and cargo. These compartments feature fire resistant sidewalls, ceilings and walkways. They're usually pressurized and heated, and they typically have fire detection and protection equipment. Two load limits matter here: a maximum floor loading, measured in kg per square metre, and a maximum running load value, measured in kg per metre. The floor loading tells you how much weight per unit area the floor can bear; the running load tells you how much weight per unit length along the compartment it can bear. These are structural limits we must respect when we distribute cargo. There are three ways cargo is carried. Containerized cargo: baggage and cargo loaded into standard size containers designed to fit and lock into the cargo compartment. Each container has an individual maximum mass limit and an individual floor loading limit — again, mass per unit area. Palletized cargo: cargo loaded onto standard size pallets and restrained with cargo nets or strops. Typically the forward area of the forward cargo compartment is configured to take palletized freight. Bulk cargo: cargo loosely loaded in the area at the aft of the rear cargo compartment, separated from the containers by a restraining net attached to the floor, ceiling and sidewalls. So we have containers locking into place, pallets netted down, and bulk cargo held back by a net. Finally, the cargo handling systems. The forward and aft cargo compartments typically have separate cargo power drive systems to move containers and cargo pallets. The power drive system is operated by a control panel at the door area of each cargo compartment, and it's capable of loading and unloading fully loaded containers or pallets in wet or dry conditions. That's an important operational point — the system must work in rain as well as in dry weather. A typical panel is shown in Figure 2.17. So the key thread here: every piece of cargo, whether in a container, on a pallet, or loose, has a position and a mass, and that position and mass feed directly into the CG calculation we did at the start. The structural limits — floor loading and running load — protect the airframe, and the envelope graph tells us whether the resulting CG is within limits. That's the whole loop.