Powered Flying Controls — Page 196, Lesson 255

Let's pick up with the protection against jamming of controls. Redundancy alone isn't enough — the aircraft must also survive a jammed control surface or a jammed actuator. So I want to walk you through the three main ways manufacturers build that protection in. First, some aircraft give the pilots disconnect levers. These are usually brightly painted on the centre pedestal, and they operate pitch and roll disconnects. Pulling or turning one handle disconnects the elevator, leaving each pilot in control of his own side. The other handle is the roll disconnect, and this usually gives the captain control of the roll spoilers and the co-pilot control of the ailerons. Now, an important point — there are many different disconnect systems out there, and you should note that there may be a reduction in roll rate when you use them. So the protection works, but you lose some performance. Second, other aircraft types use cross-linked control systems. These employ a number of override or breakout connectors to protect against single point jams. The idea is that if one actuator jams, the connector breaks out or overrides, so the jam doesn't lock up the whole system. Third, there are aircraft that use a series of shear rivets. In these systems, the yokes are usually tied with shear rivets in the aileron, elevator, and rudder systems. Here's how it works: should the ailerons become jammed, the pilots turn the yokes towards each other to shear the rivet. That allows the remaining aileron to be used. Similar procedures are used for the elevators and the rudder. One thing to keep in mind — rudder centring may be lost when this type of protection is used. So you trade centring for the ability to keep controlling the aircraft. So the key takeaway: redundancy gives you multiple paths, but jamming protection gives you a way to break free of a jammed component. Each method has its trade-off — reduced roll rate with disconnects, or loss of rudder centring with shear rivets.