A defective crystal in the AFC section of a RADAR unit can cause what visual defect?

A defective crystal in the Automatic Frequency Control (AFC) section of a RADAR unit can indeed cause bright flashing pie sections on the Plan Position Indicator (PPI). This phenomenon occurs because the AFC is responsible for keeping the radar frequency stable and properly aligned. If the crystal, which regulates the frequency oscillation, is malfunctioning, it can lead to fluctuations in the radar signals.

These frequency instabilities cause the radar returns to be displayed erratically, leading to distinct areas or "pie sections" on the PPI screen that appear brighter or flash due to those shifts in frequency. This visual artifact is a direct result of the incorrect processing of signals caused by the defective crystal. Consequently, the radar unit displays these intermittent bright sections as it struggles to maintain a coherent and stable representation of the environment being scanned.