Quadrantal error is caused by an NDBNDB —Non-Directional Beacon signal bending towards the magnetic axis of the aircraft, causing the signal to appear to come from an incorrect direction. It is at a minimum when the signal comes from directly in front of (relative bearing 000) or behind the aircraft (relative bearing 180) or directly from either side (relative bearing 090 or 270). It is at a maximum when coming from halfway between any of these points – so relative bearing 045, 135, 225 or 315.

Note that all of these are relative bearings – not QDMQDM —Magnetic bearing from an aircraft to a beacon or stations, QDRQDR —Magnetic bearing from a beacon or station to an aircrafts or any other absolute bearings.

To calculate relative bearing, note that:

$\text{Magnetic Heading} + \text{Relative Bearing} = \text{QDM}$

Therefore:

$\text{QDM} - \text{Magnetic Heading} = \text{Relative Bearing}$

An important point to note is that quadrantal error is never zero (minimum does not mean the same thing as zero!) This is often one of the wrong answers.