Two combination instruments in the "standard six":
1. Turn indicator
Function: shows rate of turn in degrees per second.
- Classic turn-and-slip — horizontal gyro axis; shows only the yaw component.
- Turn coordinator (TC) — slightly canted gyro axis; shows roll AND yaw, more responsive at the onset of a turn.
Orientation of gyro axis
The orientation of the turn indicator's gyro axis is parallel to the aircraft's transversal axis — i.e. from wingtip to wingtip. This arrangement:
- Measures yaw rate about the vertical axis via precession.
- In the classic turn-and-slip the axis is exactly horizontal/transversal.
- In the turn coordinator (TC) the axis is tilted 30° → also senses roll.
Standard rate (rate 1) and 2-minute turn
| Rate | Turn rate | Time for 360° | TC marking |
|---|---|---|---|
| Rate 1 | 3°/s | 2 min | both wing marks on indicator |
| Rate 2 | 6°/s | 1 min | (rare in PPL) |
Bank angle for standard-rate turn — depends on TAS
The bank angle for a 2-minute (standard rate) turn is not constant — it depends on True Airspeed (TAS). At higher TAS a higher bank is needed to maintain the same turn rate.
Formula (rule of thumb): Bank for standard-rate turn [°] ≈ (TAS [kt] / 10) + 7
Examples:
- TAS 100 kt: bank ≈ 100/10 + 7 = 17°.
- TAS 120 kt: bank ≈ 19°.
- TAS 150 kt: bank ≈ 22°.
- TAS 400 kt (airliner): bank ≈ 47° — hence airliners fly half-standard-rate (1.5°/s) instead of full rate.
What the turn indicator does NOT show
The turn indicator gives no climb or descent indications. It only measures yaw/roll rate about the vertical axis — not vertical motion.
→ Climb/descent are read from VSI and altimeter.
2. Slip / skid ball (inclinometer)
Function: the slip indicator shows the apparent vertical of the aircraft — the resultant of gravity and centrifugal force (in a turn). From this you read:
- The rate of turn — implicit via bank-to-turn-rate ratio.
- The coordination of the turn — slip or skid.
A liquid-filled glass tube with a ball:
| Ball position | Meaning |
|---|---|
| Centred | Coordinated — bank and turn rate match |
| Inside the turn (ball toward inner wing) | Slip — too much bank for the turn rate |
| Outside the turn | Skid — too little bank for the turn rate |
"Kick the ball" — correction mnemonic
Classic flight-training mnemonic (FAA Airplane Flying Handbook): "Step on the ball" — press the rudder on the side the ball is on.
Practical application when the ball is well off centre: the pilot can return the ball to the middle by reducing bank and increasing rate of turn — an alternative to plain rudder input, producing a coordinated turn.
- Ball right → right rudder OR reduce bank + increase rate
- Ball left → left rudder OR reduce bank + increase rate
- Ball centred → coordinated
Practical use
- In climb with high power → ball usually left → right rudder to coordinate (compensates prop torque).
- In descent with low power → no strong prop yaw.
- In a turn → coordinate with rudder.