In 2018, Congress said FAA within a year had to issue regulations establishing minimum dimensions for passenger seats - including minimums for seat pitch, width, and length - "that are necessary for the safety of passengers."

Seat pitch - the distance from one seat back to the next - on low-cost carriers Spirit Airlines and Frontier Airlines is among the industry's tightest at 28 inches (71 cm) in coach class. The average for other mainline economy seats is around 30 to 31 inches.

"Currently there are no minimum seat dimensions," said Michael Kirkpatrick, a lawyer for FlyersRights.org. "Maybe they're going to codify the current seat dimensions and status quo for all conditions required for safety, then maybe there'll be a challenge on the merits. But the point here is that there is no regulation now on seat pitch, width or length."

The Justice Department said the FAA's "examination of existing evidence has not yet demonstrated a safety need for minimum seat dimensions" but the FAA continues to "diligently examine" the issue.

The government argues the FAA does not need to issue rules if the regulations "are not required to protect passenger safety," while the flyers group said without judicial action "FAA will continue to treat the statutory requirement as a low priority that it can ignore indefinitely."

In August, the FAA sought public comment and has already received 11,700 comments. The comment period closes on Nov. 1.

Current rules say airlines must be able to evacuate passengers within 90 seconds and do not set seat size requirements.

In July 2018 the FAA said it would not regulate seat size. Airlines' margins could suffer if they had to reconfigure planes and eliminate seats.

FlyersRights says U.S. airline seat pitch has shrunk by 3 to 7 inches since 1970 while seat width has decreased by over an inch.

(Reporting by David Shepardson in Washington; Editing by Matthew Lewis)

By David Shepardson