Module 9 - Compass
Advanced Scenario Check: Compass
What you will learn
Manage an unexplained compass conflict during a night pilotage approach by protecting sea room, isolating likely interference and rebuilding a traceable direction picture.
This fictional scenario rehearses diagnosis and command discipline. It does not prescribe a course, speed or manoeuvre for a live vessel and cannot demonstrate practical Yachtmaster competence.
A yacht is approaching a fictional headland at night in cross-tide. After an alteration, the steering compass reads 112°C while the fluxgate shows 120° magnetic and GNSS COG varies around 126°T. The yacht is heeled. A portable loudspeaker and charging cable were placed beside the binnacle during the watch change. The plotter heading overlay and autopilot repeat the same fluxgate sensor.
The numbers do not form a majority. COG is ground movement, the two electronic displays share one magnetic sensor, and the steering compass may be affected by the new portable equipment or heel. The current card and variation predict 112°30′T from 112°C, but that is a hypothesis from recorded corrections, not the true answer to select.
State the first command before investigating detail: preserve the briefed safe water and reduce commitment as the vessel and traffic situation allow. Move portable magnetic and electrical items clear, maintain an outside lookout and assign one navigator to record each source, reference, time, heading, heel and configuration. Do not let troubleshooting displace vessel control.
Seek evidence that does not reuse the same failure path: a correctly identified charted transit or known bearing, depth trend, radar range, visual clearing limit and independently supported position as appropriate. If a reliable picture cannot be rebuilt before the last safe decision point, hold, divert or abandon the approach. Written revision can rehearse this judgement, but practical instruction and assessment aboard are required to demonstrate Yachtmaster command and instrument use.
Worked example
Give the first five-part command response to the 112°C / 120°M / 126°T conflict before the yacht reaches the commitment point.
- 1Decision: close the direction-integrity gate and preserve the briefed safe water; do not steer an averaged heading.
- 2Control: keep one person conning, one on helm and one maintaining the outside lookout while the navigator records the evidence.
- 3Isolation: remove the portable loudspeaker, cable and other suspect items without treating their removal as proof that the error is gone.
- 4Corroboration: obtain a known physical direction and independent navigation evidence, then repeat observations with time, heel and heading recorded.
- 5Trigger: if the conflict remains unexplained at the last safe point, hold, divert or abandon the approach and arrange competent compass or system investigation.
Sense check: The response first creates time, room and role clarity. Troubleshooting while continuing towards danger would spend the very margin needed to diagnose the problem.
| Observation | Question to ask | Command response |
|---|---|---|
| Portable speaker beside binnacle | Did the magnetic environment change? | Remove it, repeat a known-reference check and do not edit the card from one observation |
| Fluxgate, plotter and autopilot agree | Do they share one sensor or data path? | Treat shared outputs as one source and seek independent corroboration |
| COG differs in cross-tide | Is ground track being mistaken for heading? | Quantify tide and leeway or stop using COG as the heading comparison |
| Difference changes with heel or heading | Is the residual deviation pattern configuration-dependent? | Record conditions, restrict reliance and arrange competent investigation |
Brief evidence, not preferred numbers
The crew ask which heading display to follow while the conflict is being investigated.
1. Helm
Maintain the safe command given by the skipper and report the exact named source being followed; do not change source silently.
2. Lookout
Report physical cues, traffic and the clearing limit independently of the displays.
3. Navigator
Record source dependencies and convert only labelled compass or magnetic values with the current documented corrections.
4. Skipper
Challenge any unexplained agreement, protect the last safe decision point and state what evidence would reopen the approach gate.
Sense check: A clear source name and decision limit let the team detect a silent display change or a shared-data failure.
Common mistake or limitation
- Removing a nearby object is a useful isolation step, not proof that the previous compass error has disappeared on every heading.
- A plotter and autopilot repeating one fluxgate are not two confirmations of that sensor.
- Selecting the source closest to the planned course turns the desired answer into evidence.
- An improvised correction from one observation is not a substitute for a current deviation record and competent compass assessment.
Recap
- Protect the safe option before troubleshooting.
- Remove likely interference and record the exact configuration.
- Use known directions and genuinely independent navigation evidence.
- Hold, divert or abandon the approach if a material conflict remains unexplained.
Optional quick check
Section 9 of 9
In cross-tide, a compass and shared fluxgate displays disagree while a portable speaker sits beside the binnacle. What is the best first response?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
Continue studying Compass
This public lesson is part of Revision Module 9. Full revision access adds the guided Learn, Practise and Test flow, flashcards and revision progress.