The Knee Defender, a gadget that blocks airplane seats from reclining, got a global boost after a scuffle between two passengers forced a United Airlines jet to make an unscheduled landing last weekend.

The gizmo’s website crashed Tuesday after traffic surged, and sales rose “substantially” for the $21.95 plastic clips that have been on the market since 2003, said the inventor, Ira Goldman.

While a product that interferes with another flier’s comfort may rub some people the wrong way, the issue is airlines’ legroom cutbacks, Goldman said. Carriers are shrinking space between rows — Spirit Airlines’ allotment is about 10 percent less than the industry standard — and using thinner cushions to squeeze more people into coach cabins.

“They don’t have Plan B for the fact that a lot of people, when they sit down in their seat at the gate, their knees already are hitting the seat back in front of them,” Goldman said.

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Goldman says he created the Knee Defender to stop seat backs from constantly banging against his knees.

It’s not meant for space hogs, Goldman said, but rather to serve as an “early warning” system for travelers who don’t want seat backs to unexpectedly collide with knees, babies on their laps or laptop screens.

Both people involved in the in-flight scuffle were in United’s Economy Plus section, an area in coach that gives people as much as 5 more inches of legroom for a fee or with elite frequent-flier status.

Goldman’s gadget is a pair of U-shaped clips that fit over the arms of the seat back tray table, blocking the passenger in front from leaning back. The Federal Aviation Administration prohibits the use of such devices during taxiing, takeoffs and landings, when tray tables must be stowed.

The agency said Tuesday in a statement that airlines can decide whether they want to allow such devices while cruising.

The four largest U.S. carriers — American, United, Delta and Southwest — all bar the use of the Knee Defender, spokesmen said. JetBlue discourages the use of the devices while not specifically barring them.

The U.S. industry’s standard is about 30 to 31 inches of space between rows. While Spirit passengers have to make do with only 28 inches, some of their seats aren’t susceptible to the Knee Defender. They don’t recline.

Goldman said he doesn’t know which airlines allow them and which don’t. He likened his invention to a radar detector, which may be legal and tolerated in many states, if frowned upon.

A 6-foot-3 entrepreneur and nonpracticing attorney, Goldman said he got the idea for the Knee Defender around 1998 while flying. He took out an umbrella and laid it across the arms of his tray table, noticing that it blocked the seat ahead of him from tilting back. He dusted off the idea a couple of years later at the urging of friends.

Soon, he said, he was asking acquaintances to test his invention in flight. One put down her tray table, took out 10 prototypes of his Knee Defender and started experimenting on the seat back of the person in front of her.

Another day, Goldman was going through an airport checkpoint and was asked to empty his carry-on luggage for a random screening. A Transportation Security Administration employee looked quizzically at him.

“‘What is this?’” Goldman recounted the TSA worker as asking. “I explained it, and he smiled and sent me on my way.”

Goldman said he sources the Knee Defender from a manufacturer in China. His only advertising expense was $1.25 to test some Google ads.

The product’s controversial nature generates its own publicity, said Goldman, who wouldn’t disclose annual sales or personal details such as his age. “But it keeps the lights on,” he said. “We sell these things.”

Goldman returned often to his theme that airlines’ legroom cutbacks are the problem, not his gadget or the people who use it. In his view, he is just calling attention to industry practices.

“I was the kid who says, ‘The emperor’s not wearing any clothes,’ ” Goldman said.