I'm not entirely sure what you're trying to achieve with $string, but providing it's correct: 
 
<?
       $flights = PIREPData::getLastReports($userinfo->pilotid, '10');                                                                      
       $string = "";
       if(!$flights){
               $string = 'No flights have been filed!';
       } else {
               foreach($flights as $flight)
               {       
                       $string = $string.$flight->depicao.'+-+'.$flight->arricao.',+';
               }
       }                                                                       
?>
 
Your string will end up looking like this, for example: "EGKK+-+KJFK,+KJFK+-+EGKK,+EGKK+-+KMIA,+KMIA+-+EGKK,+" and so on... 
If that's what you intended?