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The Five-hundred-meter Aperture Spherical radio Telescope (FAST) completedits commissioning and began the Commensal Radio Astronomy FasT Survey (CRAFTS),a multi-year survey to cover 60% of the sky, in 2020. We present predictionsfor the number of radio-flaring ultracool dwarfs (UCDs) that are likely to bedetected by CRAFTS. Based on the observed flaring UCDs from a number ofunbiased, targeted radio surveys in the literature, we derive a detection rateof $\ge$3%. Assuming a flat radio spectrum $\nu L _{\nu}\propto \nu^{\beta+1} $with $\beta$ = -1.0 for UCD flares, we construct a flare luminosity function $dN/d L \propto L^{-1.96 \pm 0.45}$ (here $L=\nu L_\nu$). CRAFTS is found to besensitive enough for flares from UCDs up to $\sim$180 pc. Considering theGalactic thin disk, we carry out a 3D Monte Carlo simulation of the UCDpopulation, which is then fed to mock CRAFTS observations. We estimate that$\sim$170 flaring UCDs would be detected through transient searches in circularpolarization. Though only marginally sensitive to the scale height of UCDs, theresults are very sensitive to the assumed spectral index $\beta$. For $\beta$from 0 to -2.5, the number of expected detections increases dramatically from$\sim$20 to $\sim$3460. We also contemplate the strategies for following upcandidates of flaring UCDs, and discuss the implications of survey results forimproving our knowledge of UCD behavior at $L$ band and dynamos.

The Potential of Detecting Radio-flaring Ultracool Dwarfs at L band in the FAST Drift-scan Survey