function agregarPista(url){
  var i = parseInt($('aerodromo_npistas').value);
  new Ajax.Request(url,
  {
    method:'post',
    parameters: {'i':i},
    onSuccess: function(transport){
      $('pistas').insert(transport.responseText) ;
      $('aerodromo_npistas').value = i+1;
    },
    onFailure: function(){ 
	//alert('Something went wrong...') 
	$('pistas').insert(transport.responseText);
    }
  });

}