during flight, when are the indications of a magnetic compass accurate?

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A magnetic compass gives accurate readings mainly while the aircraft is flying straight and level. Sharp turns, climbs, descents, or accelerations can result in compass errors due to magnetic dip and other factors.

There are multiple factors that cause errors. These include acceleration errors, turning errors and deviation errors, largely due to the Earth's magnetic field not being uniform, local magnetic fields in the airplane, and the compass's geographical position.