A turn indicator can be one of two things, relating to either ground vehicles or aircraft. In ground vehicles, a turn indicator is normally a system of blinking lights linked to controls within the driver's compartment. The driver activates the turn indicator to notify other drivers that he is planning to make a turn. In an aircraft, a turn indicator is an instrument that allows the pilot to see if the aircraft is turning, something that can be difficult under low-visibility conditions. It helps the pilot maintain course for landings and other maneuvers.
On ground vehicles, a turn indicator is more often called a turn signal, or blinker. The turn signals are commonly mounted at each corner of the vehicle, sometimes in pairs, so that at least one of them at each corner is visible from any angle. They are often red or amber colored and are activated manually by the driver. They flash on and off and deactivate automatically after the turn is made and the vehicle returns to straight-line travel. In the past, turns were often indicated by hand signals, and while these are no longer legally recognized as acceptable turn signals for most motor vehicles, they are still used in some areas, particularly by those riding two-wheeled vehicles.
The term turn indicator more commonly refers to an aviation instrument that employs a gyroscopic mechanism to sense changes in the attitude of an aircraft. This instrument is used by pilots when flying by instrument and allows the pilot to see the direction and rate of a turn. In most aircraft, these instruments are of the type called a turn and bank indicator, which also shows the degree of bank in the aircraft's attitude during the turn. They are often marked with graduated lines, allowing the pilot to make standard turns of varying degrees by aligning a needle to a specific point for a certain time.
Most turn indicators have been replaced, in many modern aircraft, by an improved instrument called a turn coordinator. A turn indicator, while a useful instrument, does not provide the pilot with information relating to roll, the rotation of the aircraft around the longitudinal axis. A design improvement allows the turn coordinator to relay information regarding the roll of the aircraft in addition to turning data. Aircraft that are used for acrobatics, however, often employ the earlier versions of turn indicator as some of the more extreme maneuvers can cause the gyroscope in a turn coordinator to flip and become unreliable for short periods of time.