A ground level enhancement event occurred on 10–11 September 2017, associated with an X8.2 solar flare on the western limb of the Sun. We report the results of our manually conducted nowcast using WArning System for AVIation Exposure to Solar energetic particles. The maximum radiation dose rate at a flight altitude of 12 km was estimated to be approximately 3 μSv/h, which is less than half of the dose rate due to galactic cosmic rays. We also discuss a possible quasi‐parallel shock‐acceleration mechanism that may have led to the exceptionally soft proton energy spectrum as ground level enhancement events.