To convert the airplane's speed from kilometers per hour to miles per hour, you can multiply the speed in kilometers by the conversion ratio of 0.62 miles per kilometer.
So, the calculation would be:
\[ 900 \text{ kilometers/hour} \times 0.62 \text{ miles/kilometer} = 558 \text{ miles/hour} \]
Therefore, the airplane's speed in miles per hour is 558.