To convert kilometers per hour to miles per hour, you can use the conversion factor where 1 kilometer is approximately equal to 0.621371 miles.
Given that the airplane travels at 900 kilometers per hour, you can use the following formula:
\[ \text{Speed in miles per hour} = \text{Speed in kilometers per hour} \times 0.621371 \]
Calculating it:
\[ \text{Speed in miles per hour} = 900 \times 0.621371 \approx 558.36 \]
Thus, the airplane's speed is approximately 558.36 miles per hour.