A Frontier Airlines Airbus A321neo diverted to Chicago O'Hare on Wednesday 11 November after the tap in its forward lavatory sprang a leak. So much water came out that it ran back through the cabin13. The crew were worried the water would reach the aircraft's electronics and declared a mayday1. Flight 356613 was on its way from Denver to Detroit2. Frontier said nobody was injured23.

"Mayday, mayday, mayday"

In the radio recording broadcast by ABC News, one of the pilots calls Chicago Center with "Mayday, mayday, mayday". The explanation follows: "Our forward lav sink has sprung a leak and it's spraying so much water. It's flooding the cabin area in the back. We're worried about electronics. We'd like to divert to Chicago O'Hare."1 Footage from on board shows water soaking the aisle1.

A passenger told CBS News Chicago that a plumber travelling on the flight managed to shut off the water. By then too much had already run into the cabin, so the aircraft diverted to O'Hare2. Two passengers told ABC News they were grateful for the plumber, who helped stop the leak1.

In a statement to Paddle Your Own Kanoo, Frontier said the tap in the forward lavatory sprang a leak, "which caused water to flow along part of the aircraft floor. As a result, the captain requested an emergency landing at Chicago O'Hare." The flight landed safely3. ADS-B data from adsb.lol show the aircraft cruising at up to 35,050 ft. It was back on the ground in Chicago at around 13:50 local time, almost two hours after departing Denver5. According to CBS News Chicago the aircraft was then parked at gate M40 for maintenance. Frontier said a replacement aircraft would take the passengers on to Detroit23.

A new aircraft

Paddle Your Own Kanoo identified the aircraft as the A321neo registered N688FR3. According to ADS-B data, N688FR flew as flight 3566 from Denver to O'Hare on 11 November5. The FAA aircraft registry lists it as an Airbus A321-271NX built in 2026, with its registration certificate issued on 4 September 20264. N688FR had been on the register for less than ten weeks when the tap failed.

Why water is a risk for the avionics

According to Paddle Your Own Kanoo, the pilots feared the water could reach the electronics under the cabin floor3. Water in the avionics bay has been reported before. In 1999 the FAA wrote that the French authority DGAC had received reports of water dripping into the avionics bay of Airbus A319, A320 and A321 aircraft, where the aircraft's computers are located6. According to Paddle Your Own Kanoo, on the A321neo that bay sits under the cabin floor, just in front of the cockpit door3. Airbus analysis showed those computers might not be adequately protected from water ingress. Water could damage the computers' electrical connectors and cause the avionics computers to malfunction6.

By then the FAA had already issued two airworthiness directives. The first, from September 1998, required certain toilet rinse valves to be replaced. The second, from January 1999, required drip pans and additional drain gutters on the avionics racks, plus a rubber strip and new seal parts on the avionics compartment access door. In 1999 the FAA also proposed modifications to the protection above the electronics racks, the cockpit drain circuit and the wiring6. Neither Frontier nor the FAA has said whether any water reached the avionics bay on flight 3566.

Mayday or PAN-PAN

Crews have two internationally recognised emergency signals. MAYDAY, preferably said three times, signals distress. PAN-PAN signals urgency. A distress call has absolute priority over all other communications and imposes radio silence on the frequency, while urgency traffic has priority over everything except distress7. For another mayday into O'Hare, read how the crew of American Airlines flight AA618 handled smoke from a galley trash bin.

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What is not yet known

Frontier and the FAA have not said what caused the tap to fail. The number of passengers and crew on board has not been published. It is not confirmed whether the passengers reached Detroit the same day: according to Paddle Your Own Kanoo, the Frontier website listed the flight as cancelled3. CBS News Chicago gave the flight number as 3655. Frontier, the radio recording and the ADS-B data give 3566135.