We know that 1 knot is equal to 6080 feet per hour. So, the airspeed in feet per second is given by:
\[ u = 400 \, \text{knots} \times \frac{6080 \, \text{ft}}{\text{hour}} \times \frac{1 \, \text{hour}}{3600 \, \text{seconds}} = 675.56 \, \text{ft/sec} \]
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