The US Court of Appeals has ordered the Federal Aviation Administration to review seat pitch guidelines on aircraft.
The court was hearing a case brought by passenger group Flyers Rights, which had asked for minimum sizes to be set.
The group had argued seat pitch had shrunk from an average of 35 inches in the 1970s to 31 inches, and even 28 inches in some cases.
The FAA had refused the request by Flyers Rights, claiming it ‘does consider seat pitch in testing and assessing the safe evacuation of commercial, passenger aircraft’.
However the court said the FAA had used ‘off-point studies’ with ‘undisclosed tests using unknown parameters’ to refuse the request.
The three-judge panel said: "That type of vaporous record will not do."
It criticised the FAA for refusing to disclose any data on tests and describing the hearing as: "The case of the incredible shrinking airline seat."
Judge Patricia Ann Millett wrote: "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 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."
Flyers Rights president Paul Hudson said: "We hope the FAA will now take it up as a proper rule making."
US rules state that aircraft manufacturers must prove a fully loaded plane can be evacuated within 90 seconds under low lighting conditions and with up to half the exit doors blocked.
