You cannot have multiple id tags, which is probably why it's not working. Try changing it to a class instead. See below for example in jquery.
HTML:
<input type="radio" class="ac" value="<?php echo $aircraft->name ;?>"><?php echo $aircraft->name ;?><br />
JAVASCRIPT:
function radio_is_on() {
if ($('.ac:checked').val()) {
return true;
}
else {
alert('Please select the aircraft first!');
return false;
}
}