Ahead of the summer travel season , airlines in the United States usually compete to sell tickets and fill seats. But the airlines that operate the grounded Boeing 737 Max planes have a new problem : there are not enough seats to meet the demand .

The revenue(收益)is right in front of them . They can see it , but they can’t meet it , said Mike Trevino. He is an airline industry expert for Southwest Airlines Pilots Association .

The grounding of Boeing’s 737 Max came after two deadly crashes in five months . Southwest Airlines is the world’s largest 737 Max operator. The company has 34 of the planes .

American Airlines operates the second-most, with 24. These planes have been removed from use until at least August, The grounding of the planes has led Southwest to cancel 160 of its 4,200 daily flights between June 8 and August 5.

American Airlines will cancel 115 daily flights, or 1.5 percent of its total summer flights . Southwest only flies Boeing 737s. It had estimated $ 150 million in lost revenue between February 20 and March 31 --- mostly because of MAX cancellations .

The 737 Max was grounded worldwide in March following a deadly Ethiopian Airlines crash . It came five months after a Lion Air crash in Indonesia. All on board both planes died .

Boeing is under pressure to provide additional software. Experts are examining the original software as a possible reason for the crashes .

Boeing must prove to worldwide regulators that the plane is safe to fly. That process may take more than 90 days. Planes in the United States are usually mostly full during the months of June , July and August .

Sleeper trains occupy a romantic corner of any traveler’s soul. One of Hercule Poirot’s most fascinating adventures takes place on the Simplon Orient Express, which used to run from Paris to Istanbul. A famous scene in Alfred Hitchcock’s “North by Northwest” features a night train entering a tunnel. James Bond, meanwhile, detects a spy on a sleeper train after noticing him behave suspiciously in the dining car.

In some parts of the world, the nostalgia(怀旧)lives on. The Caledonian Sleeper, complete with smartly dressed waiters, neeps and tatties(白萝卜泥和土豆泥)and a selection of whiskies, is the best way to travel between London and Scotland. Elsewhere, however, sleepers are on their last legs. Flights across Europe have become so cheap that fewer and fewer travelers bother with sleeper trains. Sensing that the end is approaching. Andrew Martin, a British writer, has written a book about the sleeper.

“Night Trains” is a brief history of the mode, combined with accounts of journeys Mr. Martin has taken on sleeper routes across Europe. The reader joins him on a train Munich, where he eats a tuna sandwich on board. Travelling from Paris to Venice, he thinks he has been robbed of $105. The service to Nice is cancelled, yet such is his love for sleeping aboard that he spends the night on the train as it sits on the platform.

These stories make clear that the golden age of the sleeper train is long past. How different things were in the 19th century, when a passenger on the Orient Express could dine on delicacies and good wines. The only modern-day sleeper train which comes up to the Mr. Martin’s exacting standards is the Nordland, which travels towards northern Norway.

Those who have no experience of the sleeper trains often ask sleeper enthusiasts: “Do you sleep?” After a read of Mr. Martin’s book, the answer would seem to be a definite “no”: the noise of the train wake him up time and again. Still, it is hard not to be won over by his enthusiasm. Catch the sleeper train, before it’s too late.

Getting the Cheapest Flights

After interviewing 10 travel agents in the area, we’ve compiled (汇编) the very best tips for finding the cheapest flights.

Book your flight 21 days before you want to fly. Booking a flight is like playing a game of chicken. Airlines want to get the most money possible for their tickets and have difficult computer programs that adjust their prices automatically. Booking too early, you might miss out on some major deals.【小题1】The best advice is to book your flight 21 days before your planned departure date to get the best deals.

【小题2】Most airlines update their reservation systems on Tuesday at 7 p.m. GMT. People tend to buy their tickets on weekends, so airlines make changes on one of the slowest days of the week---Tuesday.

Fly on Tuesday or Wednesday. Tuesday and Wednesday are the least busy days at airports, and are also the cheapest days to fly. 【小题3】Friday and Sunday are the peak flying days of the week, and are the worst, and most expensive days to fly.

Book early for international flights. 【小题4】You can also try flying into smaller airports, which sometimes have better deals.

Eat your cookies（网络标记)． Delete your Internet browser cookies if you’ve visited an airline site within the last 30 days. 【小题5】

Compare prices. Use travel search engines, in addition to airline websites, to make sure you're getting the best deal possible.