Sometimes it's not stormy weather, security holdups or late cabin crews that delay flights.
Instead, it could be trouble with the galley coffeepots.
If one of the $US7,000 to $US20,000 coffee makers on the plane isn't working, the ground crew needs to make sure there's not an electrical problem that could cause a fire or other hazard. Once that is ruled out, it's a matter of the airline's choosing the lesser of two evils.
"If it fixes the coffee maker, there's a delay; but if the flight leaves without a coffee maker, the passengers will complain there's no coffee," said Ronald Carr, an assistant professor at Embry-Riddle Aeronautical University.
Carr is a former pilot for American Airlines, which is particularly intent on improving its departure-time ratings.
In a recent podcast for the airline's employees, American's chief of operations, Robert Isom, said that an "inordinate amount of coffee maker problems" were causing short delays.
Isom came to American as part of its merger in 2013 with US Airways, where he had helped achieve one of the industry's best on-time records. But American's on-time performance - measured in part by the percentage of flights departing within 15 minutes of schedule - is only middling.
At 83 per cent in this year's first three months, American ranked behind one of its main competitors - Delta Air Lines, at 86 per cent - but before another, United Airlines, at 81 per cent, according to the Department of Transportation.
In trying to improve that score, Isom is tackling not only coffee makers but also other seemingly mundane nuisances, like spills on cloth upholstery, that can delay the ground crew's ability to prepare the cabin for the next flight. (If you start seeing more synthetic-leather seats in economy, their superior ability to be wiped is why.)
"One delay at the beginning of the day can impact hundreds, if not thousands, of passengers and their belongings," Isom said in an interview. "It's not only the most customer friendly,'' he said. "It's also the most financially sound way to run an airline."
How can something as seemingly minor as a balky coffeepot delay a flight?
The Federal Aviation Administration requires coffee makers to have safety features like circuit breakers and insulation around the wiring to protect from electrical fires. So when a coffee machine starts misbehaving, maintenance crews must inspect it to ensure there is no fire hazard.
In some cases, a broken machine can be quickly replaced by a spare from a maintenance warehouse. But if the coffee maker cannot be easily repaired or replaced, mechanics will disable it by turning the water off, shutting down its power source and recording it as inoperative in the aircraft's logbook.
"You can't just put Mr. Coffee in an airline," said Jeff Lowe, president of Aviation Fabricators, a certified repair station in Clinton, Missouri. "You have to do all kinds of engineering and analysis and provide test results to the FAA to get approval."
Other special features include latches and locks to ensure that the coffeepot does not shake loose during turbulence, as well as special electrical circuitry that is compatible with an airplane's power source. All those elaborately engineered features mean there is more that can go wrong.
Because they operate at higher altitudes, aircraft coffee makers must also be designed to heat water to a lower boiling point than household machines. And even the water is complicated in an airliner.
Marcos Jimenez, an engineer at Zodiac Aerospace who has developed patented coffee maker technology, said there were two main types of machines: those that use water from an airplane's water reservoir and those that require a flight attendant to pour filtered, bottled water into the machine.
Most commercial airlines use machines hooked up to a water tank.
"Because it's in a tank, they have to take particular care to make sure the water is not growing bacteria and whatnot. So they treat it with chemicals, kind of like a pool," he said.
These chemicals, along with minerals in the water, can cause residue to build up in the tubing and other parts. Clogs can cause the machine to break down over time, especially if maintenance crews don't clean them thoroughly at least once a month, Jimenez said. "I don't drink the coffee unless I know the water's coming from a bottle."
Delta and United acknowledge that coffee makers can cause trouble, though they do not seem as focused on it as American is.
"We do have some delays due to coffee maker problems from time to time, but it's not a prevalent cause," said Jonathan Guerin, a United spokesman. He noted, however, that United's maintenance crews recently increased the frequency of checks on coffee makers specifically to reduce minor delays.
American, though, is working with B/E Aerospace, a manufacturer of aircraft cabin products, to replace malfunctioning coffee makers on many of its planes, at thousands of dollars each.
American's Boeing 737s, which make up about a third of its fleet, typically have three to five of the appliances on board. Larger planes like the Boeing 777, which usually flies international routes, may have more than a dozen coffee makers.
If the overhaul is effective, it might cut down on the number of complaints on Twitter by fuming passengers like Darren McGrady.
McGrady, a personal chef based in Dallas, boarded a 6:45 a.m. flight in April, only to find the departure delayed by a broken coffee maker.
After he tweeted his unhappiness, the American Airlines corporate Twitter account quickly replied:
"We'll get the coffee maker up and running so we can have you on your way!"
A few minutes later, the American Airlines Twitter account gave him an update:
"It's all set now! You'll be wheels up soon."
"You can sit and wait on an airplane for the dumbest things,'' said Lowe at Aviation Fabricators. But in hindsight, he took a philosophical view.
"They have to wait for some guy in maintenance to say it's OK,'' Lowe said. "And frankly, that's probably the way it ought to be."
The New York Times