Turn Indicators and the Attitude Indicator

The gyro principles now become instruments. This lesson covers the panel's rate family, the rate-of-turn indicator with its slip ball and its younger sibling the turn coordinator, and then the instrument a pilot in cloud trusts with everything: the attitude indicator, including the manoeuvres that fool it and the markings that make it readable at a glance.

The rate-of-turn and slip indicator

The instrument's purpose is a pair of answers on one face: how fast the aircraft is turning (the needle) and whether the turn is balanced (the ball). The needle's engine is a rate gyro: a single-gimbal gyro whose gimbal is held by a spring. When the aircraft yaws, the gyro precesses against the spring (the last lesson's ninety-degree rule put to work) and the gimbal settles where spring force balances precession, an angle proportional to the rate of turn. The needle simply displays that angle. The benchmark rate is the rate-1 turn: three degrees per second, a full circle in two minutes, marked on the dial by the rate marks (the doghouses) the needle sits on in a standard turn.

Bank, rate and TAS

The rate the gyro measures and the bank the pilot flies are joined through speed, a relationship Principles of Flight derived in full: for a given rate of turn, the faster the TAS, the more bank is needed. No numbers are required here, only the consequence: at high speed the bank needed for a rate-1 turn grows uncomfortably large, so bank angle becomes the limiting factor. Jet transports turn at a capped bank (typically 25 to 30 degrees) and accept less than rate 1, rather than roll steeply for the standard rate.

The ball

The balance (slip) indicator is mechanically the simplest thing on the panel: a ball in a curved, liquid-damped tube, a pendulum in disguise. It sits wherever the apparent gravity, the combined pull of gravity and the turn, points: centred when the two are aligned with the aircraft's vertical, displaced when they are not. Reading it with the needle in a turn:

Balanced turn — needle deflected, ball centred: the bank exactly matches the rate for this speed.
Slip — too much bank for the rate: the aircraft slides toward the inside, and the ball falls to the inside of the turn, the same side the needle points.
Skid — too much rate for the bank: the aircraft is flung outward, and the ball rolls to the outside of the turn, opposite the needle.

The turn coordinator

The turn coordinator repackages the same rate gyro with one deliberate tilt: its gimbal axis is inclined about thirty degrees, so the gyro responds to roll rate as well as yaw rate. The display is a miniature aircraft that banks: the instant the aircraft begins to roll into a turn the symbol banks with it, before any heading has changed, and once the turn is established the symbol's angle shows the rate, its wingtip on the mark for rate 1. Two cautions belong to it: the symbol resembles an attitude indicator but shows no pitch information whatever, and its bank-like display is rate dressed up as attitude, not attitude itself.

The attitude indicator

The attitude indicator exists to answer the one question instrument flight cannot survive without: which way is up. It gives pitch and bank at a glance when the real horizon is gone. Its heart is an earth gyro: a two-degrees-of-freedom gyro with its spin axis held vertical, the case and display moving around that fixed vertical as the aircraft pitches and rolls. Two types share the panel: the plain attitude indicator (AI), and the attitude and director indicator (ADI), the same attitude sphere with the flight-director command bars drawn over it, telling the pilot not only where the aircraft points but where the computed guidance wants it pointed.

When the attitude indicator is fooled

The gyro's vertical is maintained by an erection system that gently precesses the axis toward what it senses as "down", and that sensing is a pendulous device, fooled exactly as the compass's pendulum was:

Prolonged acceleration (a take-off roll, a go-around) — the apparent down tilts aft, the erection system dutifully follows it, and the instrument shows a false climb (the instrument twin of the somatogravic illusion, the body's own false pitch-up sense under acceleration), inviting a push-over the aircraft does not need.

Sustained turn — the apparent down leans toward the outside of the turn, the erected vertical follows it, leaving small pitch and bank errors during and just after the turn.

Modern instruments blunt both with erection cut-outs that pause the chasing when acceleration or bank is sensed; the residual errors are small, transient, and worth knowing exactly because they appear when the pilot leans hardest on the instrument.

Reading the face

The display is a picture of the world:

Sky-and-ground sphere — light above, dark below, split by the horizon line.

Pitch ladder — degree marks above and below the horizon line.

Bank scale — around the top rim, with marks at 10, 20, 30, 60 and 90 degrees (some faces add 45) converging on the sky pointer.

Fixed miniature aircraft — in the centre.

The logic every pilot internalises is that the symbol stays, the horizon moves: pitch up and the horizon sinks below the symbol, bank left and the horizon tilts right behind it, exactly as the real world would.