Powered Flying Controls — Page 196, Lesson 251

Let’s pick up right where the feel computer leaves off, because that’s the bridge into the trim system. The feel computer also incorporates a load relieving trim system. That system connects the horizontal stabilizer to the relief valve, and it does so through two components: the stabilizer position cam and the bellows. Here’s the problem it solves. When you operate the elevators, that places a stick force on the pilot’s controls. Once you’ve completed the control movement, you don’t want to keep holding that force. So to remove it, the pilot trims the variable incidence stabilizer. You trim it until the stick force is cancelled and the elevator returns to the neutral position. That’s the whole point of the load relieving trim system — it relieves the load you’d otherwise have to hold. Now let’s look at the feel trim system itself. Figure 9.9 shows a basic sketch of a hydraulically operated artificial feel unit with feel trim included. Normal operation of the controls creates a stick force which requires trimming out. You achieve that by operating the trim wheel. The trim wheel relieves the downward pressure on the metering valve by allowing the bellows to expand downwards. At the same time, it trims the tailplane or elevator to fly ‘Hands Off’. So the trim wheel does two things at once: it relieves pressure on the metering valve, and it trims the surface so you can fly hands off. Then Figure 9.10 shows a simplified schematic sketch of a powered flying controls system as found on a modern civil aircraft. That brings us to the big topic: Fly by Wire systems, or FBW. A fly by wire system is a powered flying control system that uses electronic inputs to a solenoid operated servo valve, rather than the mechanical inputs you’d find on conventional power controls. Let me unpack that. The pilot operates the flight deck controls. Those may be a side stick, as with Airbus aircraft, or a conventional control column and rudder pedals. That mechanical input goes into transducers. The transducers convert the mechanical input into an electrical output. That electrical output is amplified, then processed by computers. The processed command signal then provides the input to the servo valve, which controls the movement of a hydraulic actuator. So the chain is: pilot input → transducer → electrical signal → amplified → computer processing → servo valve → hydraulic actuator. The A320 is a typical example of an aircraft with an FBW system. In the A320, all surfaces are actuated hydraulically and are electrically or mechanically controlled. Let me walk you through the main control architecture. Pitch control: the elevator is electrical. The stabilizer is electrical for normal or alternate control, and mechanical for manual trim control. Roll control: the ailerons are electrical, and the spoilers are electrical. Yaw control: the rudder is mechanical, with electrical for yaw damping, turn co-ordination, and trim. Slats and flaps are electrical. Speed brakes are electrical. The flight deck controls consist of two side sticks, conventional rudder pedals, and pedestal mounted controls and indicators. Now, electrical control is by three types of computer. First, ELAC — that’s the Elevator Aileron Computer. There are two of these, and they control the ailerons, elevators, and stabilizer. Second, SEC — the Spoilers Elevator Computer. There are three of these, and they control the upper wing surfaces and the standby elevator and stabilizer. Third, FAC — the Flight Augmentation Computer. There are two of these computers for electrical rudder control. So to tie it together: the feel computer gives you the artificial feel and the load relieving trim, the trim wheel lets you trim out stick force hands off, and then the whole system becomes fly by wire where your mechanical inputs are converted to electrical signals, processed by ELAC, SEC, and FAC computers, and turned into hydraulic actuator movement. That’s the architecture of a modern FBW system.