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I have my own theories, but I have never experienced a turn coordinator or turn and slip indicator's gyro failing first-hand.

If the gyro isn't spinning, would the instrument behave like a weak accelerometer but on a different plane of movement, or would it just do nothing at all?

For example, would the weight of the motionless gyro of a turn and slip indicator resist movement laterally when the plane is rolled and cause a momentary turn indication from roll or are the centering-springs strong enough to overcome virtually any inertial resistance offered by the weight of the gyro?

Another way of asking this is if you held a turn and slip indicator unit in your hand with the display window facing your palm and you rotated (rolled) the unit side-to-side rapidly in sort of a door knob turning motion, would the turn rate indicator needle tilt at from the center position on the display at all?

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It depends on how it fails, there are lots of possibilities on what caused it to stop spinning and they can all lead to different outcomes. In this instance it just kind of bobbed around as if it were free floating. But if something is jamming the mechanics it may simply stop moving altogether so it really just depends on how it fails.

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Turn coordinators have a number of failure modes because they are normally electrically powered and depend on very precise gyro RPM, so the gyro motor runs on fixed frequency 115v ac and there is a teeny tiny static inverter in the unit to convert the airplane's 12 or 24 vdc power to fixed frequency a/c for the motor. If the motor's not running so the gyro is stationary, there will be indications from gyro's mass inertia as the housing moves, but it will be quite random and the needle will randomly tilt and move around.

Because they are sensitive to bank, Turn coordinators also use an internal damper in the gyro mechanism to filter out small movements. The electric gyro can keep running, but if the damper fails, and it is bumpy at all, the airplane symbol will dance all over the place and you really can't use it (I had one go bad like this, where the gyro still worked but the indicator became hyper-sensitive and moved constantly with the slightest motions of the plane, making it pretty much as unusable as one with the gyro failed).

So for an electric turn coordinator, the main failure modes will be an electrical failure in the inverter or a/c motor driving the gyro, failure of the internal damping system, or failure of the bearings.

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Dave is right in that there are different failure modes that could cause your turn coordinator to behave in different ways. However your ball-in-a-tube slip/skid indicator will not be affected by the failed gyro, and will continue to work.

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  • $\begingroup$ "However your ball-in-a-tube slip/skid indicator will not be affected by the failed gyro, and will continue to work."-- !! Unless gravity fails. $\endgroup$ – quiet flyer Jun 16 at 18:21

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