DALLAS — The arrest of a United Airlines pilot this week for allegedly drinking too much before entering the cockpit is the latest in a series of incidents involving airline pilots and alcohol.

The United pilot, Erwin Vermont Washington, was about to take off from London's Heathrow Airport for Chicago with 124 passengers on board. Instead, he was removed from the aircraft, suspended by his airline and now faces up to two years in a U.K. prison if convicted on criminal charges. He is the third U.S. pilot arrested in 13 months on alcohol-related charges.

Monday's arrest raises more questions about what goes on in airplane cockpits. It follows the distracted flying incident in the U.S. last month, where Northwest Airlines pilots overshot Minneapolis by more than 100 miles because, they said, they were using their laptop computers.

In May an American Airlines pilot was arrested at Heathrow and charged with being under the influence of alcohol. Another United pilot was arrested on the same charge in October 2008. And a Southwest Airlines pilot was suspended in January after allegedly showing up for his flight in Ohio reeking of alcohol. He's back on duty.

In 2008, 13 pilots violated the Federal Aviation Administration's alcohol-related rules. Pilots can't fly if they have a blood-alcohol level of 0.04 percent or higher, half the legal driving limit in most states. They are prohibited from drinking any alcohol in the eight hours before reporting for work, a provision known in the profession as the "bottle-to-throttle" rule.

British law is even stricter with a 0.02 percent limit. That level can be reached with about one regular beer.

The number of incidents involving alcohol is tiny compared to the thousands of flights each day around the world. But when it happens it's usually up to passengers or crew members to spot a pilot who isn't fit to fly.

The FAA checks pilots' backgrounds for alcohol-related offenses such as drunken driving, But Barry Sweedler, a former National Transportation Safety Board official who worked on alcohol-related issues, says the FAA does little enforcement. "They rely on other people to find the bad apples," he says.

U.S. regulators have approached the issue by encouraging pilots with a drinking problem to identify themselves and seek treatment. They are tested periodically and can regain their license, usually in about a year. Sweed?ler estimates that there are hundreds of airline pilots who are alcoholics and take part in a federally sanctioned treatment program that includes periodic monitoring.

"It's the guy who thinks he can get away with it that's scary," Sweedler says. "I'm sure there are pilots over the limit who are flying every day."