June 1, 2012
June 2012 Turbine Pilot Contents Quick Look Dassault Falcon 20. System Synopsis Power control units. What's Your Type? Type rating basics. VNAV 101 Vertical navigation in light jets.
We’ve all seen old movies showing pilots straining like mad to turn, push, or pull a control wheel in a desperate effort to make the airplane do as commanded. Some of that was dramatic license, some of it wasn’t. It can take a lot of force to move an aileron that’s located, say, 40 feet away from the yoke; subjected to the aerodynamic loads involved in flying at high airspeeds; and controlled mechanically by cable runs, pulleys, and bellcranks.
That’s why FAR Part 25 transport category airplanes use control surfaces that are moved with the assistance of control units that, in effect, boost the pilot’s muscle power. These units, unsurprisingly, are frequently called power control units (PCUs) and use hydraulically powered servos. Mechanical inputs from those old-fashioned cables and pulleys are used to open and close servo valves that direct hydraulic pressure one way or the other, according to the pilot’s—or autopilot’s—commands. Now it takes less force to deflect the control surfaces, sparing the pilot or autopilot wear and tear.
There can be a down side to having “boosted” controls. It can be so easy to move the controls that there’s a risk of losing the kind of tactile feedback that pilots rely upon to sense changes in roll response, for example. To take care of that, artificial control feel devices sometimes are added to give pilots a more natural control feel. These could be as simple as center-weighted springs.
Another advantage of having PCUs is that they can serve a damping function. Hydraulic pressure stored in the PCUs can prevent control surfaces from being damaged by gusts when parked on the ramp.
For redundancy, there are two PCUs per control surface. If one PCU fails, the other can do the job. In the unlikely event of a total loss of hydraulic pressure, the crew would still be able to move the control surfaces. But because most POHs prohibit the use of the autopilot should a dual hydraulic system failure occur, it’s back to sheer muscle power—and the chance to fly like that old-fashioned pilot in the movies.
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