Definition
False or unreliable magnetic compass indications caused by electrical disturbances acting on the compass, most commonly from precipitation static (P-static) building up on the airframe and discharging through or near the compass system. The compass card may swing, oscillate, or settle on a heading that does not reflect the aircraft's actual magnetic heading.
Plain English
The magnetic compass is showing the wrong heading because electrical noise on the airframe — usually from flying through precipitation — is upsetting it.
Context Anchor
Seen in weather and instrument flying discussions, especially when flying through precipitation or other conditions that can create static buildup on the aircraft.
Derivation
Erroneous comes from the Latin errare, meaning to wander or stray. A wandering compass needle is exactly the symptom — the reading drifts away from the true magnetic heading rather than pointing where it should.
Why Pilots Care
Relying on the compass during these conditions can produce large navigation errors and loss of situational awareness.
Grounding Statement
If static builds up on the airplane, the compass may show a heading change even though the airplane has not actually turned.
Intuition Check
Do not assume “erroneous” means the compass is permanently broken. Here it means the reading is wrong or unreliable in that moment, possibly because static electricity is interfering with it.
Example Sentence 1
While flying through heavy snow, the crew noticed erroneous magnetic compass readings and relied on the heading indicator until the precipitation static subsided.
Example Sentence 2
After landing the pilot noted that the erroneous magnetic compass readings disappeared once the aircraft was clear of the storm.