A British Airways Airbus A321 flying from London Gatwick to Faro in Portugal turned back to Gatwick on Tuesday 13 October after a passenger window was reported cracked. Flight data provider AirNav Radar posted that "a passenger window is cracked"1. British Airways said the aircraft diverted "following a minor technical issue" and landed safely1. The flight was BA2660, operated by the A321 registered G-EUXD23.

The window was in the passenger cabin, not on the flight deck23. No injuries have been reported4.

What the flight data shows

Open ADS-B data from adsb.lol shows G-EUXD, flying as EFW16PH for the subsidiary BA Euroflyer, taking off from Gatwick at 08:06 UTC (09:06 UK time) and climbing to 35,000 ft54. About 43 minutes after takeoff, over the sea off the south coast of Brittany in France, the aircraft started to descend. At 08:50 UTC the transponder switched to 7700, the general emergency code, and the aircraft turned back towards England5. According to The Aviation Herald, the crew descended to FL180 over the next 15 minutes2. The data matches that: the aircraft levelled at 18,000 ft from about 09:07 UTC5.

The data does not show an emergency descent of the kind flown after a sudden loss of pressure. The descent from 35,000 to 18,000 ft took about 17 minutes, an average of around 1,000 ft per minute5. At 09:21 UTC the transponder went back to a normal code. The aircraft landed at Gatwick at 09:39 UTC (10:39 UK time), just over an hour and a half after takeoff5. There was no diversion to another airport: the aircraft returned to where it departed. Emergency services met it on arrival as a precaution, LBC reports6, and Gatwick Airport confirmed the safe landing, according to the local paper The Argus7.

Which window it was

AirLive reports that the damage was to a window on the right side of the cabin near row 33. The Aviation Herald says a passenger reported a cracked window near seat 3F, that cabin pressure was not lost and that the oxygen masks did not drop2. AirLive also says cabin pressure was maintained3. British Airways has not confirmed the window and has not said which pane was cracked14.

LBC describes the aircraft as a 22-year-old A321-231 and says it is expected to be inspected to establish the extent and cause of the damage6. No cause has been published.

How a cabin window is built

On the Airbus A321, each cabin window assembly consists of two panes of stretched acrylic held in a rubber seal, the UK Air Accidents Investigation Branch explains in a report on a separate A321 event8. A vent hole in the inner pane lets cabin pressure into the gap between the two panes. According to the AAIB, both panes can carry the full differential pressure across the fuselage, providing redundancy if either one fails8.

On the cabin side there is a third pane, attached to the cabin trim: the scratch pane, the one passengers can touch. The AAIB describes it as sacrificial, protecting the inner pane, and not part of the load bearing structure8. To a passenger, a crack in the scratch pane looks much like a crack in a structural pane, but it means something very different. Whether the crack on G-EUXD was in the scratch pane, the inner pane or the outer pane has not been disclosed.

From inside the aircraft that distinction is not always possible to make. In the event the AAIB investigated, an A321 that had a loose window shortly after takeoff from Stansted in 2023, the crew did not know whether only the outer pane or both panes were affected. That crew also returned8.

Flight deck windshields: laminated and heated

A flight deck windshield is built differently. The Airbus A320 windshield examined by the US National Transportation Safety Board consisted, from the outside in, of a glass layer with a conductive anti-ice coating, urethane and vinyl interlayers, a second glass layer, another vinyl layer and a third glass layer9. On a Boeing 757 windshield, the middle and inner glass panes are the structural panes, and the heating film sits on the inner surface of the outer ply10. The NTSB says certification testing shows that windshield can carry a differential pressure of 19.2 psi with one structural pane fractured, and that Boeing requires it to hold 11.8 psi for at least 15 minutes with all panes fractured10.

The heating itself is a known source of cracks. In both NTSB investigations, electrical arcing in the windshield heating system led to the cracking910. On a Sichuan Airlines A319 in 2018, arcing ended with the right windshield being blown out of its frame in cruise; the European Union Aviation Safety Agency later proposed repetitive inspections of Airbus windshields11.

The US certification rules for transport aircraft require windshields and windows in a pressurised aircraft to withstand maximum cabin differential pressure after any single failure. They assume that after a failure obvious to the crew, the differential pressure is reduced in line with the operating limitations, so the flight can continue safely with a cabin altitude of no more than 15,000 ft12. The windshield panes in front of the pilots must also withstand the impact of a four-pound bird12.

What the checklist asks

Airbus has a dedicated procedure for a cracked flight deck window. In 2011 Airbus revised it, the NTSB reports on the authority of the French investigation bureau BEA: the crew checks with a pen or a fingernail whether the inner ply is cracked. If it is not, the fail-safe design allows the flight to continue without other restriction9. If the inner, structural ply is cracked, or if that cannot be determined, the principle from the certification rules applies: lower differential pressure, which means flying lower. The exact altitude and pressure limits differ per type and are set out in the manufacturer's procedures. The crew of the A320 examined by the NTSB could not determine how many plies were affected, followed the quick reference handbook and diverted to Chicago9.

Which procedure the BA2660 crew applied for the cabin window is not known. British Airways has not explained why the crew chose 18,000 ft or a return to Gatwick. Whether a mayday or a pan-pan was made on the radio is not confirmed either: AirNav Radar and AirLive refer only to the 7700 squawk13, and we have not found a public recording of the radio traffic.

Two days later, on 15 October, a United Boeing 767 also turned back to London Heathrow after squawking 7700.