Why most navigation accidents begin quietly, long before the grounding or collision
Contents
Use the links below to jump to any section:
- Introduction – Navigation Systems Do Not “Break”, They Drift
- The Myth of the Single Failure
- Dependency Chains – How One Sensor Feeds Everything
- The Difference Between Failure and Degradation
- Why Early Warnings Are Missed
- Alarm Systems and False Reassurance
- When Information Conflicts on the Bridge
- Why the First Error Is Rarely the Fatal One
- Accident Pattern – How Investigations Reconstruct Failure
- Professional Navigation in Imperfect Conditions
- Officer and Master Responsibilities During Degraded Navigation
- Closing Perspective
- Knowledge Check – Navigation System Failures
- Knowledge Check – Model Answers
1. Introduction – Navigation Systems Do Not “Break”, They Drift
Navigation systems rarely fail in a dramatic way.
There is almost never a moment where an officer looks at the bridge and says, “The navigation system has failed.”
What happens instead is far more dangerous.
Systems drift.
Inputs degrade.
Data becomes slightly wrong, then more wrong, then trusted because it still looks familiar.
Most navigation accidents occur with screens still lit, positions still plotted, and alarms still silent.
The danger is not loss of information.
The danger is believing information that is no longer reliable.
2. The Myth of the Single Failure
Accident reports are full of phrases like:
- “The gyro was inaccurate”
- “The GPS position was wrong”
- “The radar target was misinterpreted”
These statements describe the final visible problem, not the real failure.
Navigation systems are integrated.
They depend on shared inputs.
A single sensor error can quietly corrupt multiple systems at the same time.
By the time the ship grounds or collides, the original fault may be hours old.
3. Dependency Chains – How One Sensor Feeds Everything
Modern bridges are not collections of independent instruments.
They are information networks.
A single faulty input can affect:
- ECDIS position and vectors
- Radar overlay and ARPA calculations
- AIS CPA/TCPA
- Track control and alarms
- UKC assessment
For example, a gyro error does not announce itself as “gyro failure”.
It appears as:
- Slight radar bearing offset
- AIS targets that “don’t quite look right”
- ECDIS vectors that slowly diverge
- A ship that seems to drift off track for no clear reason
Nothing screams failure.
Everything whispers uncertainty.
4. The Difference Between Failure and Degradation
True failures are obvious.
Degradation is not.
A frozen ECDIS display is a failure.
A GPS antenna intermittently losing satellites is degradation.
Degraded systems still provide data.
They just provide increasingly unreliable data.
This is why officers often continue navigating confidently right up to the point of grounding.
The system never stopped working.
It simply stopped being truthful.
5. Why Early Warnings Are Missed
Early warning signs are subtle and easy to rationalise.
Common examples include:
- Small discrepancies between systems
- Repeated alarm acknowledgements with no obvious consequence
- Minor track deviations explained away by wind or current
- Targets behaving “oddly” but not dangerously
Human beings are excellent at normalising small errors, especially under routine conditions.
By the time doubt becomes obvious, options are already reduced.
6. Alarm Systems and False Reassurance
Alarms are often treated as protection.
In reality, they are confirmation tools, not safety nets.
Most alarms activate:
- After limits are exceeded
- After geometry has already deteriorated
- After time margins have been consumed
An alarm that never sounds does not mean the system is safe.
It may simply mean the alarm thresholds are wrong, disabled, misunderstood, or dependent on faulty inputs.
Many groundings occur with no alarms active at all.
7. When Information Conflicts on the Bridge
One of the most dangerous moments on a bridge is when systems disagree.
ECDIS says one thing.
Radar suggests another.
Visual cues feel different again.
This is where experience matters.
Junior officers often search for which system is correct.
Senior officers ask a better question:
“Which system could be lying right now?”
Safe navigation under failure conditions depends on cross-checking physics, not screens.
8. Why the First Error Is Rarely the Fatal One
The first error usually:
- Does not violate COLREGs
- Does not trigger alarms
- Does not look unsafe
It merely reduces margin.
The second error reduces options.
The third error removes recovery time.
The fourth error becomes the accident.
Investigations often find that if action had been taken at the first sign of inconsistency, the outcome would have been trivial.
9. Accident Pattern – How Investigations Reconstruct Failure
Investigators rarely find a single cause.
They find sequences:
- Initial sensor degradation
- Over-reliance on integrated systems
- Failure to cross-check
- Delay in reducing speed or calling the Master
- Continued execution of a plan that no longer matched reality
The final grounding or collision is often the least interesting part of the report.
The real lessons lie in what was ignored hours earlier.
10. Professional Navigation in Imperfect Conditions
Professional navigation assumes:
- Systems will degrade
- Data will conflict
- Alarms will lag reality
The goal is not perfect information.
The goal is maintaining time, distance, and options.
Reducing speed, increasing margins, and simplifying the situation are not admissions of failure.
They are signs of control.
11. Officer and Master Responsibilities During Degraded Navigation
Officers are expected to:
- Recognise uncertainty early
- Communicate doubt clearly
- Reduce risk before limits are reached
Masters are expected to:
- Support early intervention
- Avoid hindsight-driven criticism
- Encourage conservative decision-making
Most accidents are not caused by lack of skill.
They are caused by delayed authority and delayed doubt.
12. Closing Perspective
Navigation systems rarely fail suddenly.
They fail quietly, gradually, and convincingly.
The most dangerous bridge is not one with dark screens.
It is one where everything appears to be working — but no one is checking why.
Safe navigation begins with recognising that trust must be constantly earned by the data, not assumed because the system is modern.
13. Knowledge Check – Navigation System Failures
- Why do most navigation systems fail gradually rather than suddenly?
- Why is a single sensor failure rarely isolated?
- How can gyro errors affect multiple systems at once?
- What is the difference between failure and degradation?
- Why are early warning signs often ignored?
- Why do alarms not guarantee safety?
- What makes conflicting information particularly dangerous?
- Why is the first error rarely the accident trigger?
- What typically reduces recovery options during failure sequences?
- Why do investigations focus on early decisions rather than final events?
- How should officers respond to uncertainty before alarms activate?
- Why is reducing speed often the most effective response?
- What role does human behaviour play in system failure?
- Why is “everything looks normal” a dangerous assumption?
- What separates professional navigation from system dependence?
14. Knowledge Check – Model Answers
- Because inputs degrade while systems continue displaying data.
- Because modern bridges share common sensor inputs.
- By corrupting bearings, overlays, vectors, and calculations.
- Failure stops data; degradation corrupts it.
- Because small errors are easily normalised.
- Because alarms activate after margins are already reduced.
- Because decision-making becomes delayed and confused.
- Because it only reduces margin, not control.
- Delay in recognising and responding to uncertainty.
- Because early decisions determine whether recovery remains possible.
- By cross-checking, slowing down, and increasing margins.
- Because it restores time and manoeuvrability.
- Through over-trust, fatigue, and normalised deviation.
- Because normal-looking data can still be wrong.
- The ability to navigate safely with imperfect information.