Definition
A condition in high-latitude regions where the Earth's magnetic field lines converge so steeply toward the magnetic poles that magnetic compasses and heading systems referenced to magnetic north become inaccurate or unusable. In areas designated as regions of magnetic unreliability, navigation must be conducted using true north references rather than magnetic north, and charts in these areas show courses and bearings in degrees true.
Plain English
Near the magnetic poles, a magnetic compass stops giving trustworthy headings because the magnetic field is pulling almost straight down instead of pointing sideways. In those areas, pilots use true north for navigation instead of magnetic north.
Context Anchor
Seen in instrument procedure and off-airway route discussions, especially for high-latitude or polar operations where courses may be given in true degrees instead of magnetic degrees.
Derivation
"Magnetic" refers to the Earth's magnetic field, the reference normally used for compass headings. "Unreliability" simply means "cannot be relied on." Together the phrase names the specific zones where that normal magnetic reference can no longer be trusted.
Why Pilots Care
Pilots must use true headings or non-magnetic references such as GPS to avoid large navigation errors.
Grounding Statement
Near the magnetic poles, a compass may not have a strong, steady pull toward magnetic north, so small position changes can create large direction errors.
Intuition Check
Do not read magnetic unreliability as a minor compass inconvenience. It means magnetic direction information may be unsuitable as the main navigation reference in that area.
Example Sentence 1
When planning a flight into northern Canada, the crew noted the area of magnetic unreliability on the chart and set up the flight plan using true headings.
Example Sentence 2
Instead of magnetic headings, the crew flew true courses through the area of magnetic unreliability.