When Magnetometers (And Compasses) Go Bad

The latest-technology glass panels do many more things than the old, tried-and-true “steam gauges” with which many of us grew up. Through the “magic” of a software-driven display supported by various sensors, modern flight instrumentation can provide easy-to-read attitude information and a wealth of other data that simply wasn’t available before. But there’s no free lunch. Along with their additional capabilities and accuracy, glass panels also bring different failure modes to the cockpit. One of these new-tech failure modes involves the way in which they determine the aircraft’s heading, along with other information, most of which firmly belongs in the nice-to-have-but-not-critical category. In many situations, losing heading information isn’t the end of the world—especially if GPS navigation remains available—but it can have a ripple effect on the panel’s various other systems and capabilities. I recently learned the hard way how failure of the heading sensor(s), usually a magnetometer, may not be a failure at all, depending on how the equipment is designed and installed.

Key Takeaways:

• Modern glass panel systems, while advanced, introduce new failure modes, exemplified by erroneous heading information from magnetometers.
• The article describes an incident where dual electronic flight system (EFIS) magnetometers and the magnetic compass provided incorrect heading data due to natural magnetic anomalies in the Earth's crust, often associated with mineral deposits.
• These geological magnetic disturbances can lead to inaccurate heading and wind calculations, yet may not trigger warnings on all EFIS units and are generally not covered in standard pilot training.
• Pilots should be aware of these natural phenomena and consult detailed charts for reported magnetic disturbances when planning flights, especially in geologically active regions.

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