Aircraft Turn Indicator – The Importance of the Turn Coordinator in Aviation:

The turn coordinator (TC) variation and the turn and slip indication (T/S, also known as turn and bank indicator) are effectively two aircraft flying instruments in one. One part shows the speed at which the aircraft is turning or changing its heading, while the other part shows whether the aircraft is in coordinated flight by displaying the slip or skid of the turn. The slip indicator is essentially an inclinometer that, when the aircraft is at rest, shows the angle between the transverse axis and the horizontal, and when it is in motion, shows this angle as changed by the aircraft’s acceleration. [1] Degrees per second (deg/s) and minutes per rotation (min/tr) are the most often used quantities.

Name:

Although the gauge does not immediately respond to bank angle, the turn and slip indication can sometimes be referred to as the turn and bank indicator. The turn coordinator doesn’t either, but it does react to roll rate, allowing it to react to the beginning of a turn more rapidly.

Operation:

Graphic of a turn and slip indicator and a turn coordinator.

Turn indicator:

The turn indication is a gyroscopic device that operates on the precession principle. A gimbals contains the gyro. The rotational axis of the gyro is parallel to the aircraft’s lateral (pitch) axis, but the gimbals’s range of motion is constrained around the roll axis.

A torque force is delivered to the gyro as the aircraft yaws, and this torque force induces gyro precession around the roll axis. With relation to the direction of the applied yaw torque force, the gyro spins on an axis that is 90 degrees off-center. In opposition to a calibrated spring, the gyro and gimbals rotate with some restriction (around the roll axis).

The angle at which the gimbals and gyro become positioned, which is directly connected to the display needle and so indicates the rate of turn, is where the torque force against the spring reaches equilibrium. [3] The gyro in the turn coordinator is canted 30 degrees away from horizontal so that it can react to both roll and yaw.

Hash marks are present on the display for the pilot to use as a guide during turning. When the needle aligns with a hash mark, the aircraft is making a “standard rate turn,” which is three degrees per second and is referred to as “rate one” in some nations. This equals two minutes for every 360-degree turn (a complete circle). With “2 min turn” for indicators whose hash marks correspond to a standard rate or two-minute turn and “4 min turn” for those used in faster aircraft that show a half standard rate or four-minute turn, indicators are labeled according to their sensitivity[4]. Many military jets and the supersonic Concorde jet are two examples of aircraft that employ 4-minute turn indicators.

Inclinometer:

A technique known as the “ball in a tube” inclinometer is used to achieve coordinated flight indication. In an inclinometer, a ball is enclosed in a curved glass tube together with a liquid that serves as a damping medium. As the tube is curved in the opposite direction from the bubble and the bubble are replaced by a ball in the original shape of the indicator, it is essentially a spirit level. [5] The indicator in some early aircraft was just a pendulum with a dash pot for dampening. The ball indicates whether the airplane is gliding, sliding, or flying steadily. Gravity and the centripetal acceleration of the airplane are what move the ball.

A simple alternative to the balance indicator used on gliders is a yaw string, which allows the pilot to simply view the string’s movements as rudimentary indication of aircraft balance.

Turn coordinator:

Image showing the face of a turn coordinator during a standard rate coordinated right turn.

The display and the axis on which the gimbals is attached are the main differences between the turn coordinator (TC) and the turn and slip indicator (T/S). The exhibit is a perspective shot of a tiny airplane. This has an attitude indicator-like appearance. In order to minimize confusion regarding the aircraft’s pitch, which may be determined from the artificial horizon sensor, “NO PITCH INFORMATION” is typically put on the instrument.

The gimbals of the TC is pitched up 30 degrees from the transverse axis, in contrast to the T/S. This prompts the instrument to yaw and roll in response. Because the instrument will respond to the change in roll before the aircraft has even started to yaw, it will be able to show a change more immediately. Although responding to changes in the aircraft’s roll, this instrument does not show the roll attitude.

When the attitude indicator fails, the turn coordinator may be used as a performance tool. “Partial panel” operations are what this is known as. If the pilot is unaware that the indicator is showing roll rates in addition to turn rates, it may be excessively difficult or even impossible. If the internal dash pot is damaged, the utility is likewise reduced. The latter situation occurs when the instrument is under damped and exhibits enormous full-scale deflections to the left and right, all of which are really roll rate responses.

Practical implications:

Skidding. The corrective action is to press the “left” pedal adequately.

A sloppy turn is also referred to as slipping and skidding within a turn since it might make the pilot and passengers feel uncomfortable. Passengers feel gravity directly beneath their seat when the airplane is in a balanced turn (the ball is centered) (force perpendicular to seat). Unless they are looking at objects outside the aircraft, passengers may not even notice the aircraft is turning during a well-balanced turn.

Although skidding and slipping of the aircraft during a normal turn that maintains altitude are frequently undesirable, slipping of the aircraft can be employed for useful purposes. The terms “ahead slip” and “side slip” refer to intentionally placing an aircraft into a slip.

Current Legal Status:

Although it was believed that the Turn and Slip Indicator (and later the Turn Coordinator) was an essential and needed instrument for flight under Instrument Flight Regulations, the FAA has more recently determined that these instruments are no longer relevant in the current flying environment. According to Advisory Circular No. 91-75, which was released on June 25, 2003: [Paragraph 5 b] “…there is minimal requirement for precisely measured standard rate turns or timed turns based on standard rate in today’s air traffic control system.” The rate-of-turn indicator is no longer as useful as an instrument that provides both horizontal and vertical attitude information, according to the Advisory Circular and all other comments, it seems.

Consequently, it is now acceptable to substitute a second attitude indicator, ideally powered by a source other than the primary flying display, for a Turn-and-Slip or Turn Coordinator instrument. Hence, the second attitude indicator should be electric if the aircraft’s primary display is vacuum powered, and vice versa. This provides a redundancy of systems safety measure and additional flying information than the rate-of-turn indication. The “ball,” or slip indication, is still necessary. A slip indicator can now be found in various attitude indicators’ displays rather than having to be positioned separately in the panel.

Conclusion:

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