Gyro Error after a Blackout at High Latitude
At 0345, in latitude 58°N, a 180 m ro-ro is steering 310°T at 18 knots. A power interruption has reset the gyrocompass. Forty minutes after the alarm, the gyro repeater bearing of a lighthouse is 072.0°G while the charted true bearing is 069.0°T. The master proposes to steer the next leg, true course 124°T, on the gyro repeater and to log the gyro error as 3°E. The bridge team also reports that repeater No. 2 is reading 4° lower than No. 1, and that the magnetic compass is swinging through about 8° as the vessel yaws in a following sea. The gyro alarm is off. You must decide whether the gyro can be used, what course to steer by gyro, and what immediate checks are required.
1. Determine the gyro error and the gyro course for the next leg. Can the 3°E be treated as a fixed correction?
Read the worked answer
Gyro error = gyro bearing − true bearing = 072.0 − 069.0 = +3.0°, i.e. 3°E (gyro reads high). To make true course 124°T, the gyro repeater should read Cg = T + error = 124 + 3 = 127°G. The arithmetic is valid only if the gyro has settled and the repeater is synchronised. A 3°E error is plausible as a temporary settling error, but it is not a permanent correction: after a blackout the gyro needs time to settle, and the error will change as the course changes. Do not simply log 3°E and treat it as fixed.
2. The vessel is at 58°N making 18 knots on 310°T. Does speed error explain the 3°E? What checks must be made before relying on the repeater?
Read the worked answer
Approximate speed error magnitude is (V cos C tan L)/60 = (18 × cos 310° × tan 58°)/60 ≈ (18 × 0.643 × 1.600)/60 ≈ 0.31°. That is an order of magnitude smaller than 3°, so speed error alone does not explain the observed error. Also, repeater No. 2 is 4° away from No. 1; with repeaters disagreeing, at least one transmission or repeater is suspect. Before using the gyro: compare all repeaters at the same instant, synchronise and check failure flags; check the gyro’s latitude/speed correction setting; take a second independent bearing after the gyro has had longer to settle; and compare with the magnetic compass only as a rough check, since it is yawing 8°. If no independent check agrees, steer by an alternative reference and do not use the gyro for the pilotage leg.