/// <summary>
        /// Used to close expired counters
        /// </summary>
        internal void CloseExpiredCounters()
        {
            // Closes counter if checkin period is done
            foreach (Counter counter in Counters)
            {
                Flight flight = counter.Flight;

                // When flight is no longer ready for check in
                if (!flight.IsReadyForCheckIn())
                {
                    counter.Close();
                }
                // When check in amount is the same as the reservation amount
                if (flight.GetCheckinAmount() == flight.Reservations.Count)
                {
                    counter.Close();
                }
            }
        }