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Some aircraft use control surfaces that are driven indirectly through a flying tab system -- this is used to aerodynamically boost control forces, and works well enough in the normal case. However, it is possible for such a system to mechanically fail in a way such that the flight control and the tab are both capable of free movement throughout the control range, but the control surface does not respond to the tab's movement correctly, causing a reduction or loss of control authority.

How can a pilot catch this type of failure in a preflight or during a flight control check? "Full and free" clearly isn't enough to catch such a failure, since the tab will move freely throughout the entire control range even if the actual surface is frozen in place. Is it the case that the only way to catch this sort of failure is to disengage the gust locks and then try to move the control surfaces by hand through their full movement range? Should the control surface position be fed back to cockpit instruments in these aircraft, or is that already a requirement/feature on such types? Is it common practice or required in a "full and free" check to verify surface followup on such cockpit instruments when they are fitted?

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    $\begingroup$ At least for GA aircraft, I always move the control surfaces by hand and verify that 1) the opposite aileron moves in the opposite direction, and 2) the control column reflects proper movement. This also helps to remind me to remove the control lock, and I verify again during the "box check". $\endgroup$ – Ron Beyer Mar 23 '17 at 3:51
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The system you describe was used on large aircraft before hydraulic actuation became standard. The proper name is servo or Flettner tab.

The preflight check would ideally have one mechanic move the cockpit controls and a second mechanic walk around the aircraft and command control movements. While the actual control surface would not move, the servo tab would, and the second mechanic would hold on to the tab to check if it does so against moderate resistance. Then he would manually move the control surface to check if it moves freely through the full deflection range. This might require climbing on a ladder for the second mechanic, but would be the only way to ensure proper function of the control system.

One pilot by himself would not be able to properly check the airplane.

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    $\begingroup$ Or borrowing the de-ice truck whistles innocently $\endgroup$ – UnrecognizedFallingObject Mar 24 '17 at 22:57

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