Flight Controls — Page 186, Lesson 243

Let’s start with the Mach trim system, because it’s the heart of this excerpt. As an aircraft accelerates toward high subsonic speed, something important happens to the wing. The centre of pressure — that’s the point on the wing where the total aerodynamic lift force effectively acts — moves rearward. When that point shifts back, it creates a large nose-down pitching moment. In other words, the aircraft wants to pitch its nose down. We call this tendency “tuck under.” It’s a serious condition, so the aircraft must be fitted with an automatic system to correct this change in attitude. That system is called “mach trim.” Now, the key design feature here: mach trim is designed to operate whether or not the autopilot or some other method of automatic flight control is engaged. So it’s independent — it doesn’t rely on the autopilot being on. The system senses speed increases above a datum mach number. A datum mach number is simply a reference mach number — a set threshold. When the aircraft’s speed rises above that datum, the system detects it. Then, through a servo system — a servo being a powered actuator that converts a small control signal into a larger mechanical movement — it produces the appropriate movement of the horizontal stabilizer, or alternatively a centre of gravity shift, to maintain the trimmed flight position. So there are two ways mach trim can correct the tuck under: by moving the horizontal stabilizer, or by shifting the centre of gravity. Let me show you the centre of gravity shift method, because that’s what Figure 8.18 illustrates — mach trim by fuel transfer. Look at the figure. You’ll see the centre of pressure and the centre of gravity marked. The centre of gravity is the point where the aircraft’s weight effectively acts. There are two trim tanks — a front trim tank and a rear trim tank. The idea is fuel transfer to balance. When the centre of pressure moves rearward and tuck under occurs, the system transfers fuel between the front and rear trim tanks. By moving fuel, it shifts the centre of gravity — moving it forward or rearward as needed — to counteract the pitching moment and maintain trimmed flight. That’s the fuel-transfer method of mach trim. Now let’s move to the cockpit controls. The trim, flap, and speed brake selectors are located on the centre pedestal — that’s the central console between the pilots. For longitudinal trim — that’s pitch trim, controlling the nose up and down — there’s usually a large wheel. For lateral and directional trim — that’s roll and yaw — there are smaller wheels or switches. So you have a big wheel for pitch, and smaller controls for roll and yaw. The flap and speed brake selectors are also on the centre pedestal. The flap lever — the control for the flaps — usually has a detent or gate between each flap position. A detent is a notch or a mechanical stop that prevents inadvertent operation — it stops the lever from being moved accidentally. The flap lever typically has between three and five positions, depending on the aircraft type. The speed brake selector is shown in Chapter 7, so we won’t go into its detail here. Let me summarise what we’ve covered. Mach trim is an automatic system that corrects tuck under — the nose-down pitching moment caused by the centre of pressure moving rearward at high subsonic speed. It operates independently of the autopilot, senses speed above a datum mach number, and uses a servo system to move the horizontal stabilizer or shift the centre of gravity via fuel transfer between front and rear trim tanks. Then, on the centre pedestal, we have the trim controls — a large wheel for longitudinal trim, smaller wheels or switches for lateral and directional trim — plus the flap lever with detents between positions, and the speed brake selector. That’s the core of this section