Needle, Ball, and Airspeed

U. S. Federal Aviation Regulations forbid flight under Instrument Flight Rules (IFR), or blind flight, without a gyroscopic pitch and bank indicator, or artificial horizon. This instrument was not invented until 1929, yet skillful pilots were flying blind before that. They used the gyroscopic rate-of-turn indicator, invented about 1920 by Elmer Sperry, Jr. That instrument senses yaw rate and displays it to the pilot by a needle that deflects in the turning direction. The instrument, called a turn and slip indicator, incorporates a separate ball in a curved glass tube that acts as a lateral accelerometer or sideslip indicator. When the turn and slip indicator is combined with training to use elevator control and an airspeed meter to damp the phugoid mode, the blind-flying technique is called needle, ball, and airspeed. Charles Lindbergh retained control through clouds on his transatlantic flight by that technique.

Some modern light planes use a tilted-axis form of the turn and slip indicator. When the gyro’s gimbal axis, ordinarily aligned with the airplane’s longitudinal axis, is tilted nose upward about 30 degrees, the instrument measures rolling velocity as well as yawing velocity. Rolling velocity leads yawing velocity when starting a turn. The tilt provides anticipation of the turn. The tilted-axis turn and slip instrument is called a turn coordinator.

U. S. Federal Aviation Regulations for private-pilot certificates (Part 61, Subpart E) re­quires pilots to demonstrate the ability to control airplanes solely by reference to the usual blind-flying instruments, which include the artificial horizon and directional gyro. Yet, many of those pilots are still taught needle, ball, and airspeed blind-flying procedures. The limited blind-flying capability required for private pilots is not required for the FAA recreational pilot certificate (Part 61, Subpart D).

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