The turn coordinator is an essential instrument in an aircraft’s cockpit that assists in executing a coordinated turn. It provides visual feedback to pilots on the aircraft’s rate of turn and, to some extent, the bank state, which are crucial for executing flight maneuvers. Turn coordinator becomes a redundant instrument in the cockpit of a modern aircraft, and autopilot can now take care of coordinated flight. Here, a comprehensive overview of the instrument, its usage, and the functionality of the flight augmentation system is provided.
Turn Coordinator
The turn coordinator instrument bears some similarity with the turn slip indicator. Both have an inclinometer and turn rate indicator. The inclinometer part of the instrument has a ball suspended in a curved shape filled with a low-viscosity alcoholic fluid. This ball responds to side acceleration and indicates the amount of coordination. A perfect coordinated turn will have the ball in center.
The turn rate indicator on these instruments consists of a mechanical gyro that responds to the rate of turn, or more precisely to heading rate of change. For a standard 2 minute turn (2 min for 360° or 3°/sec), reference markers are placed in both the turn coordinator and the turn and slip indicator.
What is exactly the difference between both instruments? The turn coordinator has a mechanical gyro that is mounted at a 30°-45° angle to the longitudinal axis of the aircraft. This allows the instrument to simultaneously respond to the changes in roll and yaw. At the start of a turn, the heading rate is zero while the aircraft is banking; this state is detected by the turn coordinator and is depicted by an aircraft symbol. Do the instrument reports the correct bank angle? The answer is no; it only give an indication of bank.
Compared to the aircraft attitude indicator, the turn coordinator is more useful in executing a standard turn because it provides a direct indication of the turn rate. The amount of bank angle required will vary with the true airspeed of the aircraft and is given by the following relationship:
What is a coordinated turn?
A perfect coordinated turn is the one where the turn coordinator indicates the ball in center. It means the aircraft has no net side force/acceleration during this turn. The side force on an aircraft is directly proportional to the angle of side slip, which is the angle that air makes with the aircraft longitudinal axis. Both side slip and acceleration are undesirable in flight and need to be corrected by the use of a rudder.
In a perfect coordinated turn, the lift force is greater than the aircraft weight, but due to the perfect bank angle, the centrifugal force to maintain the turn rate is balanced by the planner component of the lift in the direction of the turn. The vertical component of the lift is equal to the weight of aircraft, and aircraft execute a level turn.
Slipping and Skidding Turn
An uncoordinated turn has lateral movement of the aircraft out of the turning plane. For a given true airspeed and turn rate/heading rate, if the bank angle is shallow, the centrifugal force will dominate and force the airplane out of turn; this is called a skidding turn. Similarly, if an airplane overbanks for a given true airspeed and turn rate, the lift vector will dominate and make a slipping turn. If the rudder input by the pilot is not perfect and is under or over compensating, the situation can be aggravated. A detailed explanation of executing a coordinated turn is given in Airplane Flying Handbook (FAA-H-8083-3C)
The vertical stabilizer provides the directional stability and dampens out the unnecessary side force; however, transition maneuvers can disturb the turn coordination. For high wing span airplanes, the adverse yaw produced by the outboard aileron contributes to uncoordinated turning. For high-performance fighters, a high roll rate can generate an adverse yawing moment from the side force generated by the vertical tail.
Importance of Coordinated turns
The uncoordinated turns can lead to airspeed variation and excessive banking that can lead to dangerous situations such as spin or a stall. A well-coordinated turn keeps aircraft in level flight with less variation in altitude, resulting in better flight management.
Effect of side Acceleration
- Spatial Disorientation: The pilots may experience spatial disorientation during prolonged uncoordinated turns, especially in low visibility conditions. This may cause incorrect inputs from the pilot that may further degrade the flight path.
- Passenger Comfort: The side acceleration caused by the uncoordinated turning flight can cause significant discomfort to the passengers as they feel pushed sideways in their seats. The situation will be even worse for flight attendants, thus making a bad flight experience.
- Workload and fatigue: In high air traffic conditions, added workload due to uncoordinated flight conditions and mental toll of flight forces can impair decision-making and reaction time during critical flight maneuvers such as the final turn for landing.
Effect of side Slip
The aircraft turn execution is based on the “Bank-to-turn” principle, which means turn is executed by banking, which provides a tilt in the lift vector. The side slip generated in both level and uncoordinated turning flight is not desirable due to the following reasons:
- Wing Lift: The aircraft wing is designed for a straight longitudinal airflow; the side slip can cause an uneven lift distribution between the left and right wing, generating an undesirable rolling moment. The fuselage wake during side slip further contributes to this effect. (See article on Aircraft Wing Design)
- Increased Drag: The side slip and resultant wake formation from the fuselage increase the overall drag that may lead to the drop in airspeed during entering into a turn.
Turn Coordinator in Modern Aircraft
Modern aircraft and airliners do not require separate turn coordinator instrument as the coordination is performed by the autopilot itself. Here is a brief overview of the techniques used for this purpose.
Yaw Damper
The yaw damper is part of modern aircraft flight control system as the third axis autopilot; this kind of system only respond to the yaw rate of the aircraft, and any other usage such as rudder trim is manually provided. For example, the Flight Augmentation Computer (FAC) of Airbus A-320 has following architecture to integrate the rudder command input and control the mechanical actuator.
Yaw damper serves two purposes: providing adequate stability and damping to dutch roll mode. The dutch roll is the oscillatory motion of the aircraft, where aircraft heading and roll angle change alternatively. The dutch roll is an important consideration in modern aircraft, as their swept wings are more susceptible to this phenomenon.
The control system of the yaw damper is very simple and effective in maintaining a coordinated flight. The aircraft yaw rate feedback from the Attitude and Heading Reference System (AHRS) is provided to the rudder servo control system after applying adequate filtering and gain.
The yaw axis rate gyros of the AHRS system is sensitive to both yaw rate and hearing rate of change (gyro is an inertial sensor and detects changes with respect to earth reference). In order to filter out the effect of heading rate from the sensor measurement, a high-pass washout filter is applied. In this way, the sensor responds to only yaw rate.
Aileron Rudder Interconnect (ARI)
ARI is a feed-forward controller or mechanical mixture that generates a rudder deflection command proportional to the aileron deflection generated by either autopilot or pilot input. The purpose of ARI is to mitigate the adverse yaw effect of ailerons.
Fully Automated Flight Control
In three axis autopilot, the rudder control is fully managed by the autopilot including the trim, yaw damper, and lateral acceleration regulator similar to a turn coordinator. The rudder control is integrated with other flight systems, such as autopilot and stability augmentation systems, allowing for coordinated responses to various flight scenarios. This system allows for more precise control of the rudder through electronic signals sent from the pilot’s inputs to the aircraft’s flight control computers.
