Airlines don't guarantee their schedules, and you should realize this when planning your trip. There are many things that can-and often do-make it impossible for flights to arrive on time. Some of these problems, like bad weather, air traffic delays, and mechanical repairs, are hard to predict and beyond the airlines' control.

If your flight is delayed, try to find out how late it will be. But keep in mind that it is sometimes difficult for airlines to estimate the total duration of a delay during its early stages. In so-called "creeping delays," developments occur which were not anticipated when the carrier made its initial estimate of the length of the delay.

Weather that had been forecast to improve can instead deteriorate, or a mechanical problem can turn out to be more complex than initially determined. If the problem is with local weather or air traffic control, all flights will probably be late and there's not much you or the airline can do to speed up your departure.

If there's a mechanical problem with the plane for your particular flight or if the crew is delayed on an incoming flight, you might be better off trying to arrange another flight, as long as you don't have to pay a cancellation penalty or higher fare for changing your reservations. (It is sometimes easier to make such arrangements from your cell phone than at a ticket counter.)

If you find a flight on another airline, ask the first airline to endorse your ticket to the new carrier; this could save you a fare increase. Remember, however, that there is no rule requiring them to do this. Persistence can pay big dividends and get you to your destination on time.

If your flight is canceled, most airlines will rebook you on the first flight of theirs to your destination on which space is available, at no additional charge. If this involves a significant delay find out if another carrier has space, and ask the first airline to endorse your ticket. Finding extra seats may be difficult, however, especially over holidays and other peak travel times.

Each airline has its own policies about what it will do for delayed passengers waiting at the airport; there are no federal requirements. If you are delayed, ask the airline staff if they will pay for meals or a phone call. Some airlines, often those charging very low fares, do not provide any amenities to stranded passengers. Others may not offer amenities if the delay is caused by bad weather or something else beyond the airline's control.

Contrary to popular belief, airlines are not required to compensate passengers whose flights are delayed or canceled. Compensation is required by law only when you are "bumped" from a flight that is oversold. Airlines almost always refuse to pay passengers for financial losses resulting from a delayed flight. If the purpose of your trip is to close a potentially lucrative business deal, to give a speech or lecture, to attend a family function, or to be present at any time-sensitive event, you might want to allow a little extra leeway and take an earlier flight.

In other words, airline delays and cancellations aren't unusual, and defensive counter- planning is a good idea when time is your most important consideration. When booking your flight remember that a departure early in the day is less likely to be delayed than a later flight, due to "ripple" effects throughout the day.

Also, if an early flight does get delayed or canceled, you have more rerouting options. If you book the last flight of the day and it is canceled, you could get stuck overnight. You may select a connection (change of planes) over a nonstop or direct flight because of the convenient departure time or lower fare.

However, a change of planes always involves the possibility of a misconnection. If you have a choice of connections and the fares and service are equivalent, choose the one with the least-congested connecting airport, so it will be easier to get to your second flight. You may wish to take into consideration the potential for adverse weather if you have a choice of connecting cities.

When making your reservation for a connection, always check the amount of time between flights. Ask yourself what will happen if the first flight is delayed; if you don't like the answer, pick another flight or ask the agent to "construct" a connection that allows more time.