While most would just grumble about the loss of a few inches of personal leg space on an airplane, the use of a $21.95 device that prevented another passenger from reclining and thus encroaching upon the person behind them sparked a fight that actually grounded the plane.

The incident that occurred Sunday on a United Airlines plane involved the Knee Defender, a gadget that attaches to the user’s tray table, pressing against the seat in front and preventing any backward motion.

“For those of us who have to squeeze ourselves into the limited airplane legroom space of a coach seat offered by many airlines, a seat in front of us that is poised to recline is a collision waiting to happen – with our knees serving as bumpers,” the website Gadget Duck, which sells the patented device, wrote of the product that has been on the market for more than a decade. “With Knee Defender, the ‘Tall Guy’ – tall men and tall women, both – can now use a simple, convenient, pocket-sized device to help defend against most flying seatbacks. And because Knee Defenders are adjustable, you can generally set them to provide only as much protection as you need.”

Image source: Gadget Duck

The Knee Defender device, which prevents the person in front of you from reclining their chair, started a fight that caused an airplane to make an unplanned landing to kick off the people involved. (Image source: Gadget Duck)

Knee Defender also provides a template “courtesy card” that its customers can give the person ahead of them, warning them the device would be in use.

The Federal Aviation Administration does not take a stance on the device but leaves it up to airlines to set their own rules. United Airlines, along with many other airlines, does not allow the device.

A "courtesy card" users of the Knee Defender can give the person seated in front of them. (Image source: Gadget Duck)

A “courtesy card” users of the Knee Defender can give the person seated in front of them. (Image source: Gadget Duck)

The dispute on United Flight 1462 from Newark, New Jersey, to Denver started when a 48-year-old male passenger in row 12′s middle seat used the Knee Defender to prevent the woman, also 48 years old, in front of him from reclining so he could comfortably use his laptop on the tray table, according to a law enforcement official with knowledge of the situation but not authorized to speak.

A flight attendant asked this passenger to remove the device but he refused. The woman then stood up, turned around and threw a cup of water at him, the official said. That’s when United decided to land in Chicago. The two passengers were not allowed to continue to Denver.

The Knee Defender compared to a house key to give you an idea of their size. (Image source: Gadget Duck)

The Knee Defender compared to a house key to give you an idea of their size. (Image source: Gadget Duck)

O’Hare International Airport spokesman Gregg Cunningham said no arrests were made when the passengers were escorted off the plane and not allowed to continue onto Denver with their fellow travelers.

It should be noted that both passengers involved were sitting in United’s Economy Plus section, which already allows them four more inches of legroom than standard coach class.

Gadget Duck did not immediately respond to TheBlaze’s request for comment about its product.

“If the airlines will not protect people from being battered, crunched, and immobilized – very real problems according to health care professionals, medical studies, government agencies, and even some airlines – then people need options to protect themselves,” Gadget Duck wrote on its website.

The Associated Press contributed to this report.