#### Answer

$16\frac{1}{4}$ hours

#### Work Step by Step

Each aircraft searched for $3\frac{1}{4}$ hours.
Thus, the total number of hours the 5 search planes searched for the missing aircraft is:
$\\=5 \times 3\frac{1}{4}
\\=5 \times \frac{4(3)+1}{4}
\\=5 \times \frac{13}{4}
\\=\frac{5(13)}{4}
\\=\frac{65}{4}
\\=16\frac{1}{4}$