Harmon Technologies, Inc.

Turn Coordinator

The turn coordinator is a gyro instrument that helps the pilot to fly the airplane when the horizon is not clear due to haze or clouds.  While the attitude indicator is powered by vacuum, the turn coordinator is powered by electricity.  In this way, some gyro information is still available to the pilot even if either the vacuum pump or electrical system fail.  This sort of redundancy is pervasive in aircraft design and helps to make flying safe.  This instrument looks a lot like the attitude indicator since it has a miniature airplane that can bank to the left or right but it does not provide any pitch (nose up or down) information.  It does provide information about the rate of turn.  If the pilot banks the airplane so that the miniature airplane's wings line up with the white lines on the instrument, the airplane will be turning at exactly 3 degrees per second.  This is called a "standard rate turn".  When in such a turn it will take one minute to turn around and two minutes to turn a complete circle.  Standard rate turns are used in holding patterns and when intercepting instrument approaches in order to provide a predictable and repeatable maneuver.  Air traffic controllers expect airplanes on instrument flight plans to make turns at standard rate unless otherwise instructed.

One interesting result of various laws of physics is worth mentioning here.  The bank angle needed to create a standard rate turn gets steeper at faster airspeeds.  N4906J will produce a standard rate turn at a gentle bank angle of 18 degrees at cruise airspeed.  Most airliners require much steeper bank angles because of their faster speed.  In order to not upset the passengers with steep banks, the rules provide a maximum angle of 30 degrees for standard rate turns. 

The turn coordinator also has an inclinometer (also called the "ball").  This is used to let the pilot confirm that bank and turn are in the proper proportion to each other.  If they are not, the pilot can correct be "stepping on the ball" (applying rudder to center the ball).  Keeping things coordinated keeps the passengers coffee from spilling!  In the old days, this instrument used a vertical needle instead of a miniture airplane as an indicator and was know as the "needle and ball" by pilots.

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