Investigators believe intense heat from film floodlights concentrated on the exterior of an Airbus A321neo caused the aircraft’s windows to suffer thermal damage, before a number of window panes fell from the jet during a flight from London Stansted.

The Titan Airways aircraft had been conducting a positioning service to Orlando on 4 October, ahead of a multi-day charter operation. Nine passengers were accompanying the aircraft which also had 11 crew members.

As the A321neo climbed off runway 22 the passengers – seated in the forward cabin – felt that the interior was colder and noisier than it ought to be.

When the aircraft reached 10,000ft and the seat-belt sign was switched off, the loadmaster went to checked the noise – which he described as particularly loud. In the vicinity of the overwing exits he noticed a dislodged window pane, and informed the cabin crew and pilots.

Titan A321neo window incident cabin photo-c-AAIB

Source: Air Accidents Investigation Branch

One of the dislodged fuselage windows photographed during the flight

The pressurisation system was operating normally, and there were no abnormal cockpit indications, but the pilots levelled at 14,000ft while other crew members went to inspect the window.

After the inspection the crew decided to return to Stansted, where the jet (G-OATW) landed without further incident after a 36min flight.

None of the occupants was injured but a check of the left-hand side of the fuselage found that two cabin windows were completely missing, and a third had its inner pane dislodged while its outer pane was gone. A fourth pane was also discovered to be protruding from its retainer and no longer flush with the fuselage.

Titan A321neo filming incident-c-AAIB

Source: Air Accidents Investigation Branch

Floodlight arrays illuminating the A321neo’s windows the day prior to the incident

According to the Air Accidents Investigation Branch, the aircraft had been the subject of a filming exercise the day before, during which high-power floodlight arrays were directed externally at the central cabin windows to simulate sunrise.

Six sets of lights were initially used for around 5h 30min on the right side of the fuselage before being moved to illuminate the left side for about 4h.

Investigators found that the lighting array used, branded as Maxibrute 12, should be positioned at least 10m from the subject – but says they were “likely” to have been positioned closer, at distances of 6-9m.

“The windows appear to have sustained thermal damage and distortion because of elevated temperatures while illuminated,” says the inquiry.

Titan A321neo window incident-c-AAIB

Source: Air Accidents Investigation Branch

Missing and dislodged windows on the left side of the Titan A321neo

Each window has an inner pane and outer pane – manufactured from stretched acrylic – and a seal, fixed to a retainer in the fuselage window frame. The retainers of the affected windows were correctly installed and in good condition, but a corresponding ring of foam material on the cabin liner was found to be partially melted.

“Visual examination of the damaged window panes revealed that they were deformed and shrunk,” adds the inquiry. “The deformed panes no longer formed an effective interface with the rubber seals.”

Examination of the jet found that the underside of the left-hand horizontal stabiliser had been punctured, with small pieces of acrylic found inside, while a broken outer window pane was recovered from a rapid-exit taxiway during runway inspection.

Investigation of the incident, with a view to future prevention, is continuing but the inquiry points out that loss of window integrity could have resulted in “more serious consequences” if the damage had been more extensive and occurred at a higher altitude.