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As airlines pack more passengers than ever onto each plane, a single storm can reverberate through the system—leaving flyers stranded as agents scramble to find ever-rarer empty seats on later flights. But you aren't entirely at the mercy of the airlines. Should your itinerary be delayed, cancelled, or overbooked, knowing your rights can be the difference between a $300 voucher and a long night at the airport.

"You have to be your own advocate," says cheapflights.com editor Melisse Hinkle. "If you don't know the rules, you may not get what you're entitled to."

The bad news first. Airlines aren't bound to their schedules. They promise to get you from point A to point B, but make no guarantees about timeline. You blew a lucrative business deal because of a cancelled flight? Good luck getting any airline to pay you damages. Airlines set their own policies about what they'll offer delayed passengers—there's no federal guideline dictating a consistent course of action.

In fact, the only time airlines are required to compensate inconvenienced customers is when they're bumped from an oversold flight. In that case, all the airlines should offer pretty much the same deal: They'll put you on their next available flight. If you reach your destination one to two hours later than originally scheduled, they'll pay you 200 percent of the one-way fare (up to $650). More than two hours later, and you're entitled to 400 percent (up to $1,300).

The problem? Most travelers don't receive the full amount they're entitled to, according to AirHelp, a new app that offers to collect unpaid compensation from the airlines. Last year, involuntarily bumped passengers received $391 to $439 each, but AirHelp estimates those payouts should've averaged $643.60. Check your airline's offer against the rules.

If you're bumped, some airlines (United is one) will book travelers on another carrier if you request it; others (such as Delta) will do it at their discretion, not yours. Still others (including Southwest and Spirit) don't have any agreements with competing airlines, so their planes are your only options.

To know who does what, you have to decipher the legalese laid out in each airline's contract of carriage. Though cumbersome, it's the only way to be certain of your rights, says Hinkle, because "you can't assume that the airline's agents know the rules."

Some airlines are more pro-passenger than others. JetBlue actually compensates delayed passengers on a sliding scale published in its Customer Bill of Rights. Weather slowdowns don't merit a payout, though: The delay has to be a Controllable Irregularity (in other words, it has to be JetBlue's fault).

That's true for all airline-issued compensation. Storms, riots, labor strikes, bombings—all are considered "acts of God" that the airlines can't control, and won't reimburse you for. Only when an airline's bungling causes you to be stranded overnight will it offer accommodation (but again, check your carrier's contract of carriage). United also kicks in meals, and Delta has been known to wheel snack and drink carts through gate areas during delays, but most other carriers make no promises regarding food.

Passengers get more perks when flying on an E.U. carrier out of the European Union, where regulations require airlines to reimburse passengers (from $170 to 800, depending on flight distance and length of delay) and cover costs for food, phone calls, and a hotel. But they may not pony up unless you press the issue. When his Croatia Airlines flight was cancelled (resulting in a 24-hour delay), journalist Ramsey Qubein had to remind agents of E.U. regulations—and only then did they produce the $300 he was owed.

But when flying from other international locations and within the U.S., you're better served by focusing on rescheduling, not remuneration. If you have access to an airline's lounge, head there for fastest service, suggests Qubein, who logs 350,000 air miles per year. No club membership? Dial the airline's customer service number while you wait to speak with an airport agent. "They may not always be able to help you over the phone, since sometimes your reservation can be under 'airport control,' but they can help give you an idea on alternate flight options to request," says Qubein.

The best rebooking resource—albeit a spendy one—is ExpertFlyer's pro subscription. The $100 annual fee lets you search alternate flights to see which ones have open seats in your class of service (you can actually see the ticketing codes). "Knowledge is power," says Chris Lopinto, ExpertFlyer's CEO and co-founder. "You're better off suggesting possible alternatives to the agent, rather than letting them suggest their most convenient option."

Just remember the Golden Rule, says Qubein. "Gate agents have heard it all, so being forceful or demanding will not get you far." If you simply must vent, post your complaint on Twitter or Facebook. "That's the airline's public profile, so it'll typically be speedy about addressing customer care issues posted there," says Hinkle.

And when lengthy delays make you want to throw in the towel? You can. Slowdowns of 90 minutes or more give you the right to request a refund instead of a rebooking. If your scheduled trip was short—or if a staycation suddenly seems preferable to the airport fracas—you can always take the money and run.

What Does Your Airline Say?