航空
hángkōng · HSK 4
air travel
✈️ 航 = navigate + 空 = sky/empty; navigating the sky = aviation
航 originally meant boats crossing water (舟 boat radical), later extended to any navigation. 空 is sky or empty space. Together: navigating through the air — aviation, aerospace, air travel.
空 — cave radical 穴 + 工 (phonetic)
Originally a phono-semantic compound: 穴 (cave, hole) gives the meaning of emptiness or hollow space, while 工 supplies the sound. The earliest sense was 'hole' or 'cavity,' which extended to 'empty,' 'sky,' and 'in vain.' The reading kòng (4th tone) specifically means 'free time' or 'vacancy.'