A couple who spent a little too long “making out” in the bathroom of a Frontier Airlines plane set off a security alert on Sunday, the 10th anniversary of the Sept. 11 attacks. When passengers noticed they had been in the bathroom for a suspiciously long time, crew members alerted the captain, and authorities dispatched a pair of fighter jets to accompany the flight into Detroit. On ordinary days, when calling in a military escort would be over the top, what are flight attendants supposed to do when they happen upon an attempt to join the mile-high club?

Knock, inquire politely, and barge in if necessary. Airlines don’t directly address this issue during flight attendant training, but, these days, few flight attendants tolerate sex onboard. They forbid able-bodied adults from entering the lavatory together. If passengers waiting in line suggest that something untoward may be happening in a bathroom, the crew member knocks and asks if everything is OK. If they get no response, flight attendants have the means to unlock and open lavatory doors. (Sunday’s mix-up was unusual, because flight attendants usually enter a lavatory and have a look around before notifying the captain of a security threat.)

Some aroused passengers don’t even bother with the lavatory, which forces flight attendants to tap them on the shoulder and request that they stop having sex in their seats.

There is one glaring exception to this protocol. Richard Branson, head of Virgin Atlantic, has pledged that his employees are not the type to “bang on lavatory doors when a couple slips in there.” The airline even installed double beds with privacy screens in some aircraft, for a “more intimate flight.” Branson’s antics have gotten him in trouble, though. Critics call him sexist, and U.S.-based flight attendant groups say he’s more concerned with brand management than stopping terrorists who might try to assemble bombs in the bathroom.

The Branson way is a throwback to the 1960s and 1970s, when flight crews were not only willing to look the other way, but may even have shared a laugh with post-coital passengers. According to flight attendant legend, crew members occasionally greeted couples returning from the bathroom with a glass of champagne and a cigarette and officially welcomed them to the mile-high club. While this level of acceptance faded, flight attendants remained relatively permissive for decades. Overly frisky passengers might get cut off from alcohol service but rarely faced more serious consequences. In a 1999 account, flight attendant Elliott Neal Hester explained that “most of my colleagues … tolerate and even chuckle at passenger audacity. Just as long as it doesn’t get too out of hand.”

The security panic that followed the Sept. 11 attacks seems to have driven the mile-high club underground. These days, flight attendants will virtually always break up a lavatory tryst or window-aisle romance.

Aspiring in-flight fornicators probably don’t have to worry about prosecution, as long as they cease and desist when ordered to do so. (They should, however, worry about hygiene. Health magazine counts airplane bathrooms among the 12 germiest places you’ll encounter on an average day.) Only a handful of passengers who have been caught in the act midflight have been referred to police. In September 2006, Carl Persing and Dawn Sewell ignored several requests to refrain from oral sex in the main cabin. According to an FBI affidavit, Persing “was observed with his face pressed against Sewell’s vaginal area. Sewell was observed smiling.” The couple acted out after being refused alcohol, and Persing promised a flight attendant he wouldn’t “go quietly.” In 1999, a British couple were charged after getting drunk in business class on an American Airlines flight and ignoring pleas to keep their hands to themselves. Far more often, however, plane sex goes unpunished.