A second technology outage within six months at Delta Air Lines left planes at a standstill Sunday and likely lost the company millions of dollars in revenue.

Travelers also lose time and money when airline systems go south. But what are passengers owed by a carrier when a flight is delayed or canceled?

Not much if anything, according to the U.S. Department of Transportation. There are no federal requirements, and each airline sets its own policies about how it treats passengers booked on delayed or canceled flights.

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Generally, a passenger whose flight is substantially delayed or canceled is entitled to a refund or to be rerouted at no additional cost. The rub: each airline may define “substantial” differently in the fine print known as the airline’s contract of carriage, or customer service plan.

The federal department says passengers need to realize that bad weather is beyond an airline’s control and mechanical and IT problems unpredictable.

The Transportation Department provides the following tips to avoid airline delays:

Depart earlyA departure early in the day is less likely to be delayed than a later flight, due in part to the "ripple" effects of delays throughout the day.

Book a nonstop flight

You are less likely to be delayed on nonstop flights. Changing planes involves the possibility of a misconnection.

Consider the season

If you have to make a stop or connection, consider the season while booking flights. For example, airports in the south might have fewer winter snowstorms but more spring and summer thunderstorms.

Allow time

When booking a connection, check the amount of time between flights and add a time cushion in case the first flight is delayed.

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Choose the best airport

Examine the flights that serve your destination city and check the on-time performance of each flight you are considering.

Check performance data

Monthly on-time performance data of the major U.S. airlines is available. DOT requires the major U.S. airlines to make this information available upon request if you make a reservation through the carrier. More detailed flight delay information is available on the Bureau of Transportation Statistics website. The FAA also provides notifications about current delays at specific airports.

Call ahead

If there’s a problem, try to rebook over the internet or phone. While airlines often try to notify passengers of schedule changes, it may not be possible due to a delay on short notice.

Check other flights

If a delay is weather-related, all flights will probably be late and there’s little you or the airline can do to speed up departure. If there is a mechanical problem or the crew is delayed on an incoming flight, arranging another flight could be a better option as long as the fare is near the same and there is no penalty changing the reservation.

Check other carriers

If a flight is canceled, most airlines will rebook passengers on the next flight to the destination at no extra charge provided space is available. If this involves a significant delay, passengers may check to see if the first airline will endorse your ticket to another carrier with an open seat to the same destination.

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Check ahead

Each airline has its own policies about what it will do for delayed passengers at the airport. There are no federal requirements regarding these amenities or services. Some do not provide any amenities to stranded passengers; some do. Ask. Airlines’ customer service plans are required to list amenities airlines will provide passengers in certain circumstances.

Know your rights

Contrary to what many believe, airlines are not required to compensate passengers when domestic flights are delayed or canceled. Compensation is required by law on domestic trips only when a booked passenger is “bumped” from an oversold flight. On international itineraries, passengers may be able to recover reimbursement under Article 19 of the Montreal Convention for expenses resulting from a delayed or canceled flight by filing a claim with the airline.

On international itineraries, passengers may be able to recover reimbursement under Article 19 of the Montreal Convention for expenses resulting from a delayed or canceled flight by filing a claim with the airline.

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