If you think someone should do something about the cramped legroom on airplanes, you've got a friend in a US federal court.

The US Court of Appeals in Washington, DC, on Friday ordered aviation regulators to consider setting minimum standards for the space airlines give passengers.

"This is the Case of the Incredible Shrinking Airline Seat," Judge Patricia Ann Millett wrote on behalf of the three-judge panel. "As many have no doubt noticed, aircraft seats and the spacing between them have been getting smaller and smaller, while American passengers have been growing in size."

The court found in favour of Flyers Rights, a nonprofit advocacy group, which had argued that steadily shrinking legroom and seat size created a safety hazard and the Federal Aviation Administration should impose new restrictions.

The issue of airline passenger legroom has boiled over this year as some carriers said they plan to add more seats to planes.

American Airlines in May announced it would shrink the space between most rows to 76 centimetres on its newest Boeing 737 Max jetliners, while later dropping a move to cut the distance in some rows to 74 centimetres in the face of criticism from employees and customers.

Flyers Rights argued that the average seat width has narrowed from approximately 47 centimetres in the early-2000s to 43 centimetres in the early-to-mid-2010s.

In recent decades, the distance between seat rows, known as "seat pitch," has gone from an average of 89 centimetres to 79 centimetres, and as low as 71 centimetres at some airlines, the group said in the suit.

While the FAA has contended its standards for safely evacuating an aircraft are adequate, US politicians have grilled members of the administration and airline executives on the issue at several hearings this year, and some have drafted legislation to address the issue.

The court said the FAA had used "off-point" studies and "undisclosed tests using unknown parametres" to justify its initial refusal to review the rules. "That type of vaporous record will not do," the court said.

The combination of less legroom and larger passengers has created a safety hazard, Flyers Rights argued, making it more difficult to exit a plane in an emergency and heightening the risk of deep vein thrombosis, a potentially fatal condition of blood clots in the legs that has been associated with longer flights.

"We're really gratified," Paul Hudson, president of Flyers Rights, said in an interview. "We hope the FAA will now take it up as a proper rulemaking."

Airlines for America, a trade group for the large US carriers, said in an email it was not involved in the suit and referred to FAA for comment.

Evacuation tests

The FAA said in an emailed statement that the agency "does consider seat pitch in testing and assessing the safe evacuation of commercial, passenger aircraft. We are studying the ruling carefully and any potential actions we may take to address the court's findings."

The long-term impact of the court rules remains unclear. It stopped short of ordering FAA to create new rules, so the agency could conduct a review and decide not to act.

In a statement last May, the agency said it had already conducted evacuation tests on the smaller seat configurations to ensure they are safe. The agency has no rules on seat width or the distance between rows, relying instead on the evacuation standards.

After American's initial announcement it was shortening the distance between seats, the agency said Boeing in 1998 successfully performed evacuation tests on the 737-400 with seats spaced 71 and 74 centimetres apart. As a result, the agency had certified up to 189 passengers aboard the 737 Max that American is buying.

Maximum capacity

Even as emergency evacuations have become significantly safer in recent decades, a debate continues to rage on how US and other leading aviation regulators around the world certify the maximum number of passengers allowed on an airliner.

In part because full-scale evacuation tests have resulted in serious injuries, the FAA and other agencies have in some cases allowed manufacturers to substitute computer simulation and more limited tests.

US regulations require that Boeing, Airbus and other manufacturers prove that a fully loaded plane can be evacuated within 90 seconds with half the exits blocked and in low lighting conditions.

Hudson, who has served on various FAA advisory panels in recent decades, has argued that the FAA's requirements aren't adequate to protect safety.

Hudson's group initially filed a petition to FAA in August 2015, asking the agency to create rules on seat size and spacing. The agency turned it down in February last year.

In its response to the suit, the FAA cited earlier evacuation studies on seat rows placed as close as 71 centimetres apart to argue existing rules were adequate to protect safety. However, the agency declined to release those studies to Flyers Rights or to the court, arguing they contained proprietary information from manufacturers.

"The problem here is that the administration has given no reasoned explanation for withholding the tests in their entirety, and it has declined to file them under seal or in redacted form," Judge Millet said in the ruling.

If it decides to deny Flyers Rights's petition again, "the administration must provide appropriate record support for its decision." Millet said.

Bloomberg