Turn needle, ball & coordinator limits
Turn needle and ball: two separate instruments sharing one dial (turn-and-bank indicator or turn coordinator).
| Needle type | Responds to |
|---|---|
| Turn-and-bank | Yaw only |
| Turn coordinator | Yaw and roll: needle not a pure yaw read (TP 1102) |
Needle:
- Rapid yaw: large deflection. Slow yaw: small deflection.
- Adverse yaw moves needle. Rudder returns it to desired position.
- Ball centred: needle shows both direction of bank and direction of turn.
- Turbulence: needle oscillates. Smooth control inputs limit it. If it settles further to one side, average fluctuations and read that side as turn direction.
Ball:
- Affected by centrifugal force and gravity, not yaw.
- Centred: forces balance, manoeuvre co-ordinated.
- Off-centre: slip or skid toward side ball has rolled to.
| Needle | Ball | Condition |
|---|---|---|
| Centred | Off-centre | Slip (wings level, no turn) |
| Off-centre | Same side as needle | Slipping turn |
| Off-centre | Opposite side to needle | Skidding turn (turn direction is needle's side) |
Correcting:
| Flight | Correction | Ref |
|---|---|---|
| VFR | Step on the ball. Press rudder pedal on ball's side. | PHAK ch.8 |
| Instrument | Centre ball with rudder and aileron together | TP 1102 |
Needle errors (TP 1102 Straight Flight):
- Ball off-centre: needle deflection may not show direction of bank.
Turn coordinator limits (IFH ch.5):
- Senses roll and yaw, not pitch. Some dials marked "NO PITCH INFORMATION".
- Bank indication indirect (TP 1102 Instrument Flying: Straight Flight), via roll rate (IFH ch.7).
- Attitude indicator fails: backup bank source (TP 1102 Partial Panel).
- Ball shows only relationship between bank angle and rate of yaw, not amount of bank.
| Phase | Shows |
|---|---|
| Turn entry | Rate of banking |
| Stabilized turn | Rate of turn |
Trap: instrument flight, centre ball with aileron AND rudder together, never aileron alone (adds bank or turn) or rudder alone (adds unwanted yaw). Coordinator: not a pitch instrument; roll-in rate of banking is not rate of turn.
References
- TP 1102 Instrument Flying: Straight Flight, Ex.24 Straight Flight
- FAA-H-8083-25 PHAK ch.8, Inclinometer
- FAA-H-8083-15 IFH ch.5, Turn Indicators (turn coordinator limits)
- TP 1102 Instrument Flying: Partial Panel