Flight Controls — Page 180, Lesson 240

I want to walk you through the trimming systems on a large aircraft, because this is where the pilot actually fine-tunes the aircraft's balance in flight. We're going to look at three distinct devices: the trimming tab, the fixed tab, and the variable incidence tailplane. Let's start with the trimming tab. This is a small adjustable surface set into the trailing edge of a main control surface. So imagine the aileron or elevator as the big surface, and the trimming tab is a smaller hinged flap built into its back edge. Its deflection is controlled by a trim wheel or a switch in the cockpit, and it's usually arranged to operate in an instinctive sense — meaning if you want the nose to go up, you move the trim in the direction that feels natural for that. Here's the key operating principle. To maintain the primary control surface in its required position, the tab is moved in the opposite direction to the control surface. So if the elevator is deflected up, the tab goes down. Why? Because the tab's hinge moment then balances the control surface's hinge moment. The airflow hitting the tab creates a force that helps hold the big surface in place, so the pilot doesn't have to keep holding pressure on the control column. Now, the fixed tab. Some trimming tabs are not adjustable in flight at all. They can only be adjusted on the ground, and their purpose is to correct a permanent out-of-trim condition — a built-in asymmetry in the aircraft. They're usually found on ailerons. They operate in exactly the same manner as the adjustable tabs, but you set them once on the ground and they stay put. Finally, the variable incidence tailplane. This is a more powerful trimming system, and it may be used on both manually operated and power operated tailplane-elevator controls. Let me explain the manual case first, because it's subtle. In a manual system, the load on the elevator is felt on the control column — you can feel the aerodynamic force through your hands. But the load on the tailplane itself is not felt. So to trim, you adjust the tailplane incidence with the trim wheel until the total tailplane and elevator load, with the elevator free, is equal to the balancing load required. In plain terms, you're changing the angle of the whole tailplane until the aircraft stays in balance even when you let go of the controls. Now, as an alternative to the trim wheel, the variable incidence tailplane may be operated by trim switches that operate in pairs. These are usually on the control wheel, and there may also be a pair of levers mounted on the centre console. Here's the critical design point: one switch or lever controls the power, and the other controls the direction of movement of the trimming device. Both must be moved simultaneously in order to trim the aircraft. This deliberate two-handed design is to prevent inadvertent operation of the longitudinal trim system — and that failure mode has a specific name: 'Trim-runaway'. If one switch were enough, a single accidental touch could drive the tailplane to a dangerous position. Requiring both inputs together is the safeguard. So to tie it together: the trimming tab is your in-flight fine adjustment on a control surface, the fixed tab is a ground-set correction, and the variable incidence tailplane is the powerful longitudinal trimmer that moves the whole tailplane, protected against accidental activation by its paired-switch design.