Compass swing after avionics upgrade
A twin turboprop has just had a glass cockpit and a new flux-valve remote-reading compass installed. On the compass swing at the run-up pad (magnetic heading 090°, all electrics on), the engineer reads a deviation of +6°. At 180° the deviation is −2°, and at 270° it is +4°. The aircraft's magnetic variation at the airport is 3°W. The crew then notices that switching on the windshield heat produces an additional 5° swing on the standby direct-reading compass, while the remote-reading display does not move. The MEL allows dispatch with a maximum acceptable deviation of 5° on any heading.
1. Compute the magnetic and true headings the crew should expect, and decide whether the aircraft may be dispatched as swung.
Read the worked answer
Deviation is the angle between magnetic north and the compass needle, east-positive. So:
- Compass 090° + dev +6° = magnetic 096°
- Compass 180° − 2° = magnetic 178°
- Compass 270° + 4° = magnetic 274°
True heading = magnetic + easterly variation (or − westerly). With 3°W variation:
- True = 096° − 3° = 093°
- True = 178° − 3° = 175°
- True = 274° − 3° = 271°
All deviations are within the 5° MEL limit, so dispatch is permissible. The key judgement is that the ±6° figure is a deviation, not an error to be added to variation blindly; the sign convention must be applied consistently. Note also that the 5° extra swing from windshield heat affects only the standby direct-reading compass, which is a known installation effect and not part of the swung remote-reading system.
2. The crew wants to know which compass to trust in flight if the two disagree by 8°. Explain the reasoning.
Read the worked answer
The remote-reading (flux-valve) system senses the Earth's field remotely and is unaffected by the windshield-heat current, so it is the primary source. The direct-reading standby compass is subject to that additional 5° field distortion plus its own residual deviation. An 8° disagreement is therefore consistent with the known electrical interference on the standby unit, not evidence of a failed flux valve. The crew should cross-check against the standby only with windshield heat off, and treat any residual difference as a compass-error problem to be resolved by the engineer. The judgement tested is separating installation-induced error from instrument failure before declaring a compass unserviceable.