Clearing Bearings in the MCA Oral Exam: Applying Gyro Error Correctly

The question

"You are passing a dangerous shoal marked by a lighthouse. The passage plan says the lighthouse must bear not more than (NMT) 045°T to remain clear. The lighthouse now bears 048° by gyro, and gyro error is 1°E. What do you do?"

The answer

Easterly gyro error means the gyro reads low, so add it: 048° + 1° = 049°T. The limit is NMT 045°T, so you are on the dangerous side of the clearing line.

Act at once: alter course away from the danger to bring the bearing back inside the limit, call the Master, and confirm your position by an independent method such as a radar range or GNSS. Consider reducing speed while you regain safe water.

Applying gyro error

"Error East, compass least; error West, compass best." With an easterly error the gyro reads less than true, so you add the error. With a westerly error, you subtract it.

• Gyro 048°, error 1°E → 049°T
• Gyro 048°, error 1°W → 047°T

Getting it the wrong way round turns a 1° error into a 2° mistake, which matters when you are close to a limit.

NMT and NLT

A clearing bearing means nothing until you know which side is safe, so it is always stated with a limit:

• NMT (not more than): the bearing must stay at or below the value.
• NLT (not less than): the bearing must stay at or above it.

Hatch the danger side on the chart and brief it to the watch. If an examiner gives you a clearing bearing without NMT or NLT, ask which it is. That alone shows you understand the tool.

What the examiner wants to hear

• The arithmetic, said out loud.
• Which side of the line you are on.
• Immediate, practical action, not just "keep monitoring".
• Calling the Master and cross-checking the position.
• Prevention: clearing bearings planned in advance, marked on the chart and ECDIS, and checked at set intervals.

Exam tips

• Sketch it on your plotting sheet. A quick drawing of the line and the danger prevents sign errors.
• Say how you found the gyro error that watch, for example by transit, azimuth or amplitude.
• Mention backing up bearings with clearing ranges or parallel indexing.