Parallel Array Multiple Search - Mandatory And Optional

I have 4 arrays that all contain data linked by the index. I have 3 drop down boxes on a web page that give the user 3 ways to search for data. The top one is mandatory the other two are optional. I've managed to create the code to search using the mandatory box but cant figure out how to expand the search.

Here is the function that deals with the search data:
function findFlights(){
var yourAirline = readTheAirline();
var result = 'Here are your flights <BR>';
var choose = 'Choose your destination';

for (var index = 0; index < flightTimes.length; index++){
var flight = flightDestinations[index];
var airline = flightOperators[index];
if (yourDestination == flight){
var b = flightTimes[index];
var c = flightDestinations[index];
var d = flightOperators[index];
var e = flightFares[index];
var message = b + ' to ' + c + ' operated by ' + d + '. £' + e + '.';

result = result + message + '<BR>';
}if (yourDestination == choose){
result = 'Please choose a destination city from the destination menu and then click Find flights again.';
}}displayMessage(result);}