United grounded 5 planes because it couldn't turn off their 'No Smoking' signs, breaking a strange FAA rule

• United Airlines "briefly" grounded its Airbus A321neos due to a peculiar rule.
• It needed an exemption from requirements flight crew be able to turn off "No Smoking" signs.

United Airlines passengers faced delays on Monday after five jets were grounded due to a peculiar rule about "No Smoking" signs, an airline spokesperson told Business Insider.

Earlier in the day, United wrote to the Department of Transportation requesting its Airbus A321neo aircraft be exempted from a requirement that flight crew be able to turn the signs on or off.

The rule exists even though smoking was banned on all US flights 24 years ago.

In its letter, United said four of the five Boeing models it operates are also equipped with the signs "hardwired" to stay illuminated at all times.

Those jets — the 737, 757, 767, and 777 — were granted an exemption back in 2020 by the Federal Aviation Administration.

However, it looks like United forgot to get an exemption for the A321neo before it entered service last December. The airline ordered 70 of the jets in 2021, and another 60 last year.

They use its updated "United Next" interior, which is also being retrofitted on other single-aisle jets.

"We are removing our five Airbus A321neo aircraft from service while we seek FAA approval for the 'No Smoking' sign to remain automatically illuminated rather than operated from the cockpit," United said in a statement to BI.

No flights were canceled as United managed to replace the A321neos with other aircraft types.

The FAA told BI it was "working to quickly resolve a non-safety issue that United Airlines discovered with some of its Airbus A321neo aircraft."

Around four hours after those statements, a United spokesperson said the FAA gave it permission to keep operating the jets while it evaluates the exemption request.

There were a "handful of delays" as the A321neos were "briefly out of service," the spokesperson added.