Did that flight arrive or depart yet? You called the airline... you checked on-line, perhaps even your cell phone received a flight status message... so why are they all different? Ok, this situation does happen, but it certainly isn't the norm. First, lets talk about the typical reasons that cause flights to depart or arrive late.
- Weather This is probably the most obvious, and most common cause for delayed flights. There are three areas where weather affects flight schedules: at the origin airport, in-flight, and at the destination airport.
At the origin airport, weather can affect the scheduled flight arrival in many ways, such as a passing thunderstorm or snow squal, or the need to de-ice the aircraft. These normally temporary issues have lingering affects on flight arrival & departure times. When aircraft cannot depart on-time, later scheduled flights have to wait due to the backlog, as the spacing interval between takeoffs is fixed and cannot be compressed (imagine cars in traffic that have a required two car lengths between them). The net affect is that all of the flights scheduled to arrive from the airport with the weather problems will arrive late, as will many flights hours after the weather passed.
- Traffic Many factors can cause air traffic, and all impact both flight arrival and departure times. Congestion isn't only a problem when you're driving to the airport. It affects aircraft as well and ultimately scheduled flight arrivals. Weather delays cause disruptions in normal air traffic patterns. Flights normally scheduled to arrive on time might have to 'cirlce' around the airport area as congestion requires air traffic controllers to hold some flights in a prescibed pattern before they are cleared to land.
So if you check on the flight status of a particular flight within 30 minutes of the scheduled arrival time, you may find a discrepancy, as aircraft placed in holding patterns around the airport may be reported as having arrived, even though the flight is still in the air.
- Go-Arounds A go-around occurs when an aircraft is just about to land at anairport and the pilot decides it is not safe to land. So you will see (or feel) the plane suddenly increasing altitude, as if it were taking off again. Of course this affects the arrival time for that flight, as it could take up to 30 mins for the aircraft to return and land again.
- Mechanical Problems The bane of travelers and airlines alike, mechanical problems affect arrival schedules too. If a scheduled flight has a mechanical problem while at the origin airport, the airlines and FAA will update the flight status so travelers know when a particular flight should land at an airport. This is by far the greatest majority of cases - on-ground mechanical problems. However, if the flight experiences a significant mechanical problem while in the air, the flight might be re-routed to another airport. The FAA might not send updates fast enough to inform travelers and service providers (such as this site) of the change, and consequently the flight arrival status could indicate all is well while the flight actually landed in another airport.