By Jim Davis    2025-11-01   (Last Updated November 1, 2025)

Turn and Slip Indications

Helicopter pilots use turn and slip indications to maintain coordinated flight and measure the rate of a turn. They are key indications monitored by pilots and incorporated into the primary flight display (PFD) of modern helicopters.

What are turn and slip?

Turn and slip indicators measure two distinct things. The “turn” in “turn and slip” refers to yaw rate. Yaw rate is how fast the heading of the helicopter changes and is typically measured in degrees per second.

The “slip” in “turn and slip” refers to perceived lateral acceleration. This includes both gravity (when the helicopter is at a nonzero roll angle) and acceleration of the helicopter in the lateral direction (e.g. centripetal acceleration in a turn).

Below is an image of a legacy turn and slip indicator including some explanation of the readings.

Turn indication usage

In straight flight, the turn indicator should read 0. In this condition, a pilot may not pay attention to this indicator, as she/he will monitor the heading indication to ensure it maintains a desired value. An exception is if the heading indication fails.

The turn indicator is more useful for maneuvers. When turning, for example, pilots may aim for a certain yaw rate. A “standard rate turn” of 3 degrees per second is common, and also referred to as a 2 minute turn since it takes two minutes to turn 360 degrees. Some turn indicators, including almost all old indicators, had a bar to mark this rate.

Slip indication usage

The slip indicator, showing perceived lateral acceleration, should be centered with an exception discussed shortly. In other words, the pilot should not perceive any lateral acceleration, even when turning. They should only feel a downward push into their seat, including both gravity and aircraft acceleration.

Even in a high speed, high rate, turn pilots “coordinate the turn” to give the perception of no lateral acceleration. When turning left, for example, the pilot feels a centrifugal force pulling right. By rolling the helicopter, left side down, a component of gravity pulls the pilot left. The pilot rolls the helicopter (or adjusts the turn rate) until these two forces are equal and opposite. This is achieved when the slip indicator is centered.

If the slip indicator is left of center there’s a force pulling the pilot left. This can be countered by either (1) adding left pedal to increase the rate of the left turn or (2) moving the cyclic right to reduce the bank angle. The former adds rightward centrifugal force while the latter reduces the leftward force due to gravity. The phrase “step on the ball” helps beginner pilots remember which pedal (left or right) to depress when the ball is not centered.

There’s an exception here, unique to helicopters. In hover, a helicopter’s tail rotor produces substantial thrust, creating a lateral force. The main rotor must be tilted through a combination of roll and flapping to counter this, preventing the tail rotor from accelerating the helicopter laterally.

When balanced in hover, a helicopter may require 3 degrees or more of roll. This means the slip indicator will not be centered in many hover conditions.

Classical Turn and Slip Indicators

Turn Indicator

The original indicators were self-contained packages, like the one shown above. On the front, a needle and gauge for turn indication are visible to the pilot. A physical slip ball is also present, facing the pilot. Behind this, hidden to the pilot, are the gyroscope and other hardware to measure yaw rate.

The turn indicator needle is linked to a rotating disk (gyroscope). Inertia typically keeps this disk in a fixed (almost vertical) plane, but when the aircraft yaws the disk moves out of this plane by an amount closely related to the yaw rate. This out-of-plane movement drives the needle to point to the associated yaw rate.

Slip Indicator

The slip indicator (inclinometer) is a ball sealed in a curved glass tube. The tube contains a liquid to effectively dampen the motion of the ball (without the liquid, the ball would jump around too erratically).

At rest, with only gravity acting on the ball, it moves to the center of the tube. When the helicopter accelerates left, the inertia of the ball effectively moves it right in the tube. If the helicopter is rolled left side down, but not accelerated, gravity pulls the ball to the left side of the tube (the ball is denser than the liquid surrounding it).

Modern turn and slip indication

The use of mechanical, gyro-based turn indicators and physical slip balls is declining in modern helicopters with "glass cockpits" and advanced avionics. They have been largely replaced by more reliable, sophisticated, and integrated electronic systems known as Attitude and Heading Reference Systems (AHRS), explained below.

Modern glass cockpit displays (like computer monitors) still show values like the old turn and slip indications, only in digital form. An example is shown below. These values are often synthesized from a few sensors, making them robust to the failure of any one sensor.

The old mechanical instruments are still present, but out of the pilot’s primary view, to serve as back-up if the modern systems fail. This can be seen in the image below.

AHRS: The source of modern turn and slip values

So-called attitude and heading reference systems (AHRS) provide the turn and slip data shown in modern glass cockpits. These systems include solid-state sensors and computer processors to measure yaw rate, replacing the spinning mechanical gyro. They also include linear accelerometers, including one or more in all three directions: vertical, lateral, and longitudinal. The digital slip ball is primarily (or exclusively) governed by the lateral accelerometer.

AHRS uses a computer and algorithms to integrate the data from all sensors. This provides an accurate and stable representation of the helicopter's orientation and motion. The calculated information is fed to an Electronic Flight Instrument System (EFIS) for display on a Primary Flight Display (PFD). The yaw rate is shown on a digital version of the turn coordinator, often as a tape or moving indicator on the instrument display.

Advantages of modern systems

AHRS provides several benefits for helicopter pilots. Some of these are listed below.

• Increased reliability: Unlike mechanical gyros, modern systems have no moving parts to wear out or fail, which increases their longevity and reliability.
• Improved accuracy: The sensor fusion improves the accuracy of attitude and heading data.
• Reduced weight and size: Solid-state AHRS units are smaller and lighter than their mechanical predecessors.
• Fast startup: Modern AHRS units initialize much faster than traditional gyro-based systems, reducing pre-flight checks.
• Integrated information: A single AHRS provides a wealth of information to the flight displays and other systems, including the autopilot and flight data recorders.