The turn & slip indicator and turn coordinator are often conflated with one another because they provide pilots with similar data. However, there are some important differences to consider. The major difference between these two gyroscopic instruments lies in the fact that the turn & slip indicator shows the rate of heading change, while the turn coordinator indicates the rate of heading change and rate of roll, or movement around the longitudinal axis.
To better understand these instruments, we must cover how gyroscopic instruments operate. First, it is important to note that gyroscopic instruments rely on a spinning gyroscope housed within the instrument. The gyroscope generally spins at 10,000 to 15,000 revolutions per minute, exhibiting immense rigidity in space. This means that the gyroscope remains stagnant as the aircraft moves around it.
The second force exerted on the gyroscope is gyroscopic precession, which is the movement of a gyroscope in response to forces acting on it. For instance, if force is applied to a spinning gyroscope, the resulting force will be 90 degrees in the direction of rotation. Fortunately, the turn & slip indicator’s gyroscope is mounted in a way that negates gyroscopic precession. By contrast, the turn and bank indicator takes advantage of gyroscopic precession to present the rate of bank data.
Turn & Slip Indicator
It is worth noting that the turn & slip indicator consists of two instruments in one. While a majority of the instrument is composed of a turn needle that is attached to the gyroscope inside the housing, the other part is made up of three indices. The index at the top is where the needle will point when there is no rate of turn being detected, and the indices on either side are distinguished with an “L” on the left and an “R” on the right. Furthermore, these two are often called a “doghouse” due to their shape.
In a majority of general aviation aircraft, when the turn needle is aligned with either doghouse, the standard rate of turn is 3 degrees per second. Meanwhile, in high-speed aircraft, there are indicators for half-standard rate of turns. At the bottom of the turn & slip indicator, there is a “ball,” or inclinometer. Keep in mind that the rate of turn for any given bank angle usually decreases as speed increases.
Within a turn coordinator, the gyroscope is mounted at a 30-45 degree angle with the front of the gyroscope being higher than the backend. In this configuration, gyroscopic precession can be utilized to displace the instrument indicator in a way that indicates the rate of roll. When the aircraft heading changes, the instrument indicates the rate of turn in a similar fashion to the turn & slip indicator. More than that, it indicates movement in the vertical axis and longitudinal axis and displays this information within the cockpit for pilots to interpret.
In a VFR flight, the turn coordinator is utilized to coordinate turns during the banking process. When an aircraft banks, there is a slight lag before the rate of turn is developed. It can also be used as a reference to measure the rate of bank, understand how much rudder is required to combat adverse yaw, and keep everything coordinated.
In a IFR flight, the real value of the turn coordinator comes to the surface, especially during partial-panel flight. Because the turn coordinator tells the pilot if the aircraft is moving about the longitudinal axis, during partial-panel operations, they are particularly critical. As a result of the turn coordinator being electrically powered, redundancy in the case of a vacuum failure is provided.
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