Can someone explain this

The laminated or taped reference card observed by a passenger aboard an Air Milford Cessna operating between Milford Sound and Queenstown is almost certainly a **compass deviation card** — a federally mandated airworthiness placard required in virtually every certificated aircraft carrying a magnetic standby compass, including under New Zealand Civil Aviation Rules (CAR Part 91) and the equivalent U.S. FAR 91.203(a)(2). The card documents the measured difference between what the aircraft's magnetic compass *reads* and the actual magnetic heading, recorded at multiple intervals (typically every 30°) around the compass rose. These deviations arise not from pilot error or instrument failure, but from the aircraft's own internal magnetic fields — generated by avionics, wiring harnesses, control cables, ferrous airframe components, and even cargo — which distort the local magnetic environment sensed by the compass needle. A qualified avionics technician performs a "compass swing" on the ground, precisely orienting the aircraft on known magnetic headings and recording the error at each point. That error table is the document the passenger photographed.

The presence of a Garmin glass cockpit — likely a G500, G600, or integrated Garmin avionics suite common to the Cessna 172 or 206 platforms Air Milford operates — does not eliminate the regulatory requirement or the practical utility of the deviation card. Glass cockpit systems derive their heading information from a remote flux gate magnetometer or AHRS unit mounted in a magnetically clean location (often a wingtip or tail cone), which produces its own calibrated magnetic heading largely free of cockpit-level interference. That heading is then corrected for programmed magnetic variation and displayed digitally. The standby magnetic compass, however, is an entirely separate, self-contained instrument with its own deviation characteristics, and it remains both required by regulation and essential as a backup. Should the primary avionics bus lose power, the pitot-static system fail, or a glass panel go dark over rugged terrain like the Fiordland corridor, the wet compass and its associated deviation card become the primary heading reference. The card is not redundant to the Garmin — it specifically supports the instrument the Garmin cannot replace in a complete electrical failure scenario.

For pilots operating scenic tour or air taxi flights in the Southern Alps environment — some of the most demanding terrain-flying in the Southern Hemisphere — this distinction carries real operational weight. The Milford Sound to Queenstown route traverses narrow valleys, high passes, and areas of significant magnetic anomaly where situational awareness is non-negotiable. New Zealand's CAA mandates compass serviceability and current deviation documentation as part of pre-flight airworthiness, and operators like Air Milford are subject to CAR Part 119 air operator certification standards that include maintenance and instrument currency requirements. A missing or out-of-date deviation card is a write-up item, not a cosmetic issue. The passenger's instinct — that the card might serve as a backup to the electronics — is substantively correct, even if the full regulatory and technical picture is more specific than that framing suggests.

More broadly, the compass deviation card represents one of the most enduring and underappreciated items in the certificated aircraft cockpit. As avionics have grown increasingly sophisticated, passengers and even some newer pilots can mistake legacy required instruments and their associated documentation for anachronisms or workarounds. In practice, regulatory frameworks in the U.S., New Zealand, EASA jurisdictions, and elsewhere have deliberately preserved the magnetic compass as a last-resort backup precisely because it requires no electrical power, no software, and no satellite signal to function. The deviation card is the calibration document that makes that backup instrument trustworthy. Its presence taped to a panel next to a modern Garmin suite is not a contradiction — it is the system working as designed.