What does the turn coordinator show?
Two things. The miniature airplane, seen from behind, tilts with the rate of roll and the rate of turn. The ball in the curved glass tube below it shows whether the aircraft is turning in balance, slipping or skidding.
The little airplane tilts like a banking aircraft, but it is showing the rate of turn, not the bank angle. For the bank angle, look at the attitude indicator.
How do you read the markings?
| Marking | What it means |
|---|---|
| Wing marks at level | The wings sit here when the aircraft is not turning. |
| L and R marks below them | A standard-rate turn: 3° a second to the left or right. Put the wing tip on the mark and the aircraft turns a full circle in 2 minutes. |
| 2 MIN. | Reminds you that the marks mean a 360° turn in 2 minutes. |
| The ball | Centred: the turn is in balance (coordinated). Off to one side: slipping or skidding. |
| NO PITCH INFORMATION | Printed on the dial. This instrument says nothing about the nose being up or down. |
| D.C. ELEC. | The gyro runs on the aircraft’s electrical power. |
What do slip and skid mean?
The ball is an inclinometer: a ball in a curved tube of liquid. In a balanced turn, the pull of gravity and the push outwards in the turn even out, and the ball stays in the middle.
- Slip: the ball moves to the inside of the turn. The aircraft is banked too much for its rate of turn, or there is too little rudder.
- Skid: the ball moves to the outside of the turn. There is too much rudder for the bank.
Step on the ball. Press the rudder on the side the ball has moved to, and it comes back to the middle.
How does it work inside?
Inside is a small gyroscope spun by an electric motor. Because it is electric, it keeps working if the vacuum system fails and the attitude and heading indicators stop. That makes it an important backup.
The gyro is mounted at a tilt, so it feels the aircraft starting to roll as well as turning. That is why the little airplane moves as soon as the aircraft starts to roll into a turn.
An older instrument, the turn-and-slip indicator, has a needle in place of the little airplane. Its gyro is not tilted, so it shows only the rate of turn.
What can go wrong?
- Reading it as a bank indicator. The tilt is the rate of turn. At a different speed, the same rate needs a different bank.
- Looking for pitch. It shows none, as the dial says. Use the attitude indicator, the altimeter and the vertical speed indicator.
- Ignoring the ball. Keep it in the middle with the rudder, so every turn is balanced.
Questions
What is a standard-rate turn?
A turn of 3 degrees a second, so a full 360 degree turn takes 2 minutes. On the turn coordinator, the wing tip sits on the L or R mark.
Does the turn coordinator show bank angle?
No. Its little airplane shows the rate of turn. Read the bank angle on the attitude indicator.
What does "step on the ball" mean?
If the ball is off to one side, press the rudder pedal on that side to bring it back to the middle.
Why does it say "no pitch information"?
Because the turn coordinator cannot tell you whether the nose is up or down, only how fast you are turning and whether the turn is balanced.
What powers the turn coordinator?
In most light aircraft, electricity. The attitude and heading indicators usually run on the vacuum system, so the turn coordinator is a backup if the vacuum fails.
Source
- Federal Aviation Administration, Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), Chapter 8, Flight Instruments.