An A320 aircraft was in cruise when the ANTI ICE R WINDOW ECAM alert triggered. The flight crew had previously installed a sunshade, purchased online by one crew member, on the right sliding cockpit window. 

The flight crew did not observe any immediate problems on the right windows after the ECAM alert triggering. However, after a short period, the right sliding window became hot and started to become distorted.

The flight crew checked the COCKPIT WINDSHIELD / WINDOW CRACKED and COCKPIT WINDSHIELD / WINDOW ARCING procedures from the QRH, and decided to initiate a descent and perform a diversion. They pulled the circuit breaker connected to that window (ANTI-ICE/WINDOWS R).

During final approach, when passing through 1 000 ft, the flight crew heard a crack in the right window. They landed safely at the diversion airport.

Post-flight inspection found severe damage to the right sliding window. The sunshade that was covering the window also displayed heat damage (fig.1).

The damaged window was removed from the aircraft and sent to the windows manufacturer for investigation. 

Detailed inspection by the manufacturer revealed that “the window experienced excessive temperature at the inner ply. The excessive temperatures had caused thermal relaxation of the inner ply to such an extent that the inner ply had fully relaxed back to expose the interlayer over a large area.”

The windows manufacturer also described that “Non-destructive thickness measurements taken of the outer ply in conjunction with visual assessment concluded this ply to be undamaged. As such the window can be considered to remain failsafe.” The structural integrity of the window was therefore still maintained.

The heating film adjacent to the active(*) temperature sensor was found to be inoperative. This failure on this type of window can cause the window heating system to incorrectly estimate the temperature of the window, resulting in an increase in temperature of the window areas with a functional heating film.

(*) On this type of window only one sensor is active at a time. The second sensor can be activated by maintenance if the other sensor fails.

The sunshade applied on the window acted as a thermal barrier, preventing natural cooling of the inner side of the window by convection of the cabin air on the window surface (fig.4). 

The combination of the overheating, caused by the faulty heating system with the thermal barrier effect created by the sunshade resulted in the temperature of the inner ply to go above the acrylic material’s glass transition temperature. This caused the inner ply to be affected by a shrink-back effect causing the damage.

Cockpit windshields of all Airbus aircraft are made of glass. Cockpit side windows of A220, A300, A310, A330, A340, A350, and A380 aircraft are also made of glass, with the exception of A320 family aircraft that can have side windows made of glass or of stretched acrylic. 

Regardless of the material they are made of, two structural plies ensure the integrity of the window thanks to a fail-safe concept: each structural ply is able to sustain two times the maximum differential pressure on its own. A supplementary protective outer ply is used on glass windows to protect the structural plies against FOD damage (fig.5).

A heating film provides anti-ice/anti-fog capability to the window. Its location may change depending on the type of window.

(fig.5) Typical cockpit windows structure

The event described earlier in this article illustrates the thermal barrier effect of a sunshade. However, suction cups used to attach different types of equipment on the windows can also act as a thermal barrier. This can affect the local cooling of the covered area. When combined with a loss of the temperature regulation of a window, this can cause significant damage to the window (fig.6).

Even if glass windows are not at risk of shrinking, equipment acting as a thermal barrier still creates a local overheat that may have an impact on the service life of the window. Overheat can cause bubbling of the interlayer next to the heating film, causing damage to the window heating system or affecting visibility.

Suction cups may not be strong enough to maintain the equipment in place in the case of vibrations. Equipment falling from a cockpit window may obstruct cockpit controls and affect the safety of the flight, particularly in critical flight phases like takeoff and landing.

Non-certified equipment used on cockpit windows may affect visibility and cockpit operations, like, for example, the use of the oxygen mask, roller blind operation, or accessibility to emergency equipment. It can also affect the opening of a sliding window during an emergency evacuation.

In general, flight crews should avoid attaching any equipment to the inner surface of the cockpit windows. This practice is advised to prevent the risks previously described.

Flight crews should use the sun visors and roller blinds installed in the cockpit. They reduce the quantity of light entering the cockpit, while still enabling sufficient cooling of the window’s inner side. Sun visors and roller blinds are designed so that their filtering capability enables direct vision of the sun through them. 

Due to the complex shape of the cockpit windows, sun visors and roller blinds cannot cover their entire surface. This results in sunlight leaks inside the cockpit. If the protection against excessive light provided by the existing devices is considered to not be sufficient, flight crew can consider using sunglasses as an additional safeguard. However, they should make sure to use the appropriate type of sunglasses to prevent reducing the visibility of the flight instruments.

(fig.9) Sun visors of an A350 aircraft

Airbus developed specific cockpit mounts for A320 family, A330, A340, A350  and A380 aircraft. These mounts can be installed as an option on newly built aircraft or in retrofit on in-service aircraft (fig.10). They are attached to the window's frame and were tested to safely hold tablets in position in every condition. They are EASA and FAA approved, and designed taking into account their integration in the cockpit, and ensuring compatibility with normal and emergency operations (use of oxygen masks, accessibility to flashlight, emergency evacuation, etc…).

Operators wishing to install these mounts should contact their Customer Support Director.

Supplemental Type Certificate (STC) alternative products are also available on the market, providing approved solutions attached to the window’s frame that can safely support tablets and prevent damage to the windows.