What to check on a Heading Control System.
Within autopilot functions, heading control is the simplest mode: the system takes a heading set by the officer of the watch and drives the steering gear to hold it, correcting for yaw from wind, sea and current using feedback from the gyrocompass. It does not know or care where the ship's track lies on the chart; a track control system, by contrast, drives the ship along a planned route from the ECDIS, automatically adjusting heading at waypoints and correcting for cross-track error. Heading control is what is actually in use…
What sets a heading control system apart
Within autopilot functions, heading control is the simplest mode: the system takes a heading set by the officer of the watch and drives the steering gear to hold it, correcting for yaw from wind, sea and current using feedback from the gyrocompass. It does not know or care where the ship's track lies on the chart; a track control system, by contrast, drives the ship along a planned route from the ECDIS, automatically adjusting heading at waypoints and correcting for cross-track error. Heading control is what is actually in use for most of an ocean passage even on ships fitted with track control, because it is simpler to monitor and sufficient once the ship is well clear of other traffic and hazards.
Main components
Control unit
Takes the set heading from the operator, compares it continuously to the actual heading from the gyrocompass, and generates a rudder demand signal to correct any error.
Adaptive control parameters
Rudder gain, how much rudder is applied per degree of heading error, and counter-rudder, how early the system starts opposing the turn as the ship approaches the set heading, are tuned to the vessel's loaded condition and sea state; wrongly set, the ship either wanders off heading or oversteers into a hunting oscillation.
Interface to steering gear
Sends the rudder demand to the steering gear's control system, which drives the actual hydraulic rudder actuator; the autopilot itself does not move the rudder, it commands the steering gear to do so.
Off-heading alarm
Monitors deviation from the set heading and alarms if it exceeds a set limit, a required safety function since an unmonitored autopilot can carry a ship well off course before anyone notices.
Selection and sizing
- Compatibility with the vessel's existing gyrocompass and steering gear interface, since heading control systems are rarely a clean standalone fit on a retrofit.
- Adaptive or self-tuning capability, which reduces the manual retuning burden as loading and sea state change through a voyage.
- Redundancy and changeover arrangements to manual or emergency steering, tested as part of the vessel's steering gear tests.
- Integration requirement: whether the system needs to interoperate with a track control system already fitted or planned.
Regulations and class
SOLAS Ch. V requires steering gear performance standards that heading control systems must not compromise, including the ability to revert quickly to manual steering. IMO performance standards for heading and track control systems set minimum requirements for accuracy, alarm functions and changeover behaviour. Class societies require periodic testing of the steering control system, including changeover between control modes, typically verified during the mandatory steering gear test carried out before departure and at other required intervals.
Typical faults
| Fault | Cause | Consequence |
|---|---|---|
| Hunting, repeated over-correction | Rudder gain and counter-rudder not retuned for a change in loading or sea state | Excess rudder activity, wasted fuel, faster steering gear wear |
| Slow response in following seas | Parameters tuned for head seas applied unchanged in a following sea | Larger heading excursions before correction takes effect |
| Undetected drift off heading | Off-heading alarm limit set too wide, or muted without being reset | Ship well off intended heading before the watch notices |
| Loss of control on changeover | Poorly tested or unclear procedure for reverting from autopilot to manual steering | Delay regaining rudder control at a critical moment, for example entering a traffic scheme |
What to look for in a supplier
- Type approval to the applicable IMO performance standard for heading, and if fitted track, control.
- Clear, tested interface specification against the vessel's actual gyrocompass and steering gear model.
- Documented adaptive tuning behaviour, and manual override of gain and counter-rudder for the watch officer when needed.
- Established service support for software updates and periodic performance verification.
Retune rudder gain and counter-rudder after any significant change in loading condition or when heading into markedly different sea conditions; a setting that was smooth in ballast can hunt badly when the ship is loaded deep.