When the de Havilland DH 106 Comet entered service in 1952, it became the world’s first jet-powered commercial aircraft. Thanks to engines buried in the wing, the DH 106 still looks revolutionary today, and closer inspection of early versions reveals another major break with modern aircraft design: square windows.
Tragically, however, these contributed to the explosive decompression of two Comet passenger flights in 1954, each with the loss of all on board. The cause was discovered when another Comet airframe was immersed in water and subjected to repeated pressurization and depressurization. After about 3,000 of these cycles (which included 1,230 flights prior to tank tests), the cabin ruptured. “Examination of the failure provided evidence of fatigue at the point where the crack would be most likely to start, namely near the edge of the skin at the corner of the window,” concluded investigators.
Following these findings, passenger windows on subsequent Comets and all other commercial aircraft were built to an ovoid design that eliminated corners. Even so, the most structurally sound fuselage would do away with windows altogether—if airframe manufacturers were not beholden to passengers’ preference for a view.
This desire has seen passenger windows become steadily larger, culminating in the 19-in.-high apertures of the Boeing 787. These are at least one-third bigger than anything on rival aircraft, and while the venerable Douglas DC 8 also featured big portholes—a relic of the propeller age—they were spaced much farther apart than those on the Dreamliner.
For several decades, cabin windows have been made from stretched acrylic, a type of plastic. They consist of two panes: an outer one built to withstand four times maximum cabin pressure, and an inner, redundant pane that will perform the same job if the outer one fails—plus a scratch pane nearest the passenger.
Acrylic is much lighter than the glass employed in cockpit windshields, and unlike the polycarbonate used in some military canopies, it can be polished to restore clarity. However, acrylic’s impact-resistance is worse than polycarbonate’s, and it is susceptible to crazing, a network of very fine cracks on the surface of the material that reduces clarity. Similarly, its poor abrasion-resistance compared to glass makes it unsuitable for front aircraft windshields, which use wipers.
To solve some of these issues, PPG has developed a new material called Opticor, which it says is the first transparent aerospace plastic approved by the FAA in 50 years. Opticor is used in business-jet applications, and PPG says it has better resistance to fire, abrasion, chemicals and UV degradation than stretched acrylic. It also can be formed into complex shapes and tinted, a useful property to mitigate glare.
GKN Aerospace has an alternative called Crystal-Vue, a coating that offers improved abrasion and crazing resistance for conventional acrylic windows while also providing better optics, water repellence and resistance to damage during installation. “CrystalVue has extended the life of a passenger cabin window 3-4 times, avoiding the removal and polishing that uncoated windows would typically undergo every 2-3 years,” says a spokesperson for GKN.
Passenger windows also should have antifog capabilities. Condensation usually occurs when humid cabin air meets the cool inner pane of the window, so some business-jet windows may be electronically heated to prevent this. With good seals around the scratch pane, commercial aircraft can insulate the redundant windowpane from cabin air.
Patents such as those for Opticor and CrystalVue notwithstanding, much of the valuable proprietary technology in aircraft windows is found in the cockpit. This is partly because of the increased airworthiness demands for cockpit windows, such as a resistance to bird strikes, and partly because of their optical and aerodynamic requirements. Accordingly, there is less competition in the aircraft windshield market than for passenger windows, and three suppliers dominate: GKN Aerospace, PPG and Saint-Gobain Sully.
“We used to supply passenger windows, but the cabin window is a very simple product, and there is not enough added value or technology,” says Sylvain Mourlhon, aerospace sales director for Saint-Gobain Sully.
While stretched acrylic is widely manufactured, the chemically tempered glass used in cockpit windows is often produced to a unique formula that offers weight savings and superior mechanical resistance to other types of strengthened glass. PPG, for instance, developed its Herculite (now Herculite II) technology in the 1960s, while Saint-Gobain Sully has marketed its Solidion product since the 1980s. GKN’s competing Chemplex product was developed in the 1990s. Manufacturers also produce hybrid, glass-fronted plastic designs that offer weight savings.
Commercial aircraft windshields are typically composed of three layers, also known as plies, of glass or plastic. The inner two are part of the fuselage structure and are each around 8 mm (0.3 in.) thick, while the outer ply, about 3-mm, serves as a barrier to protect against foreign object damage and abrasion. The outer ply often includes a hydrophobic coating.
A cockpit will comprise 4-6 transparencies, each weighing about 35 kg (77 lb.), on an Airbus A320. Thermally tempered glass would be about one-third heavier than that designed for aerospace, notes Mourlhon—an important point since aircraft started featuring curved rather than flat panes in the cockpit. Curved glass improves aerodynamics, but it also necessitates larger panes to meet visibility criteria for pilots.
On top of resisting bird strikes of up to 350 kt., cockpit windows must de-ice from -50C (-58F) and prevent fogging inside the cabin. In the past, this was accomplished with thin wires of a design similar to those in rear car windows, but the main manufacturers now use a coating of indium tin oxide. Just nanometers thick, this coating sits between glass plies and is completely transparent. Nonetheless, it is electrically conductive and can thus transmit heat.
Aside from airworthiness criteria, cockpit window designers must contend with different installation requirements. On Airbus aircraft, windows are clamped into the metal fuselage, which allows the airframe to move around them. This means that the glass is not load-bearing, which improves its reliability with respect to issues such as delamination. Boeing windows, on the other hand, are bolted into place, which can require isolation from loads, complicating their design. However, Brent Wright, global platform director for transparencies at PPG, notes that PPG windshields “can be integrated into the aircraft fuselage structure, enabling aircraft designers to forgo the heavy structures previously required to isolate the windows from damaging loads.”
Like many other component manufacturers, aircraft transparency suppliers rely on the aftermarket for the bulk of their profits. Mourlhon notes that while sales are split pretty evenly between OEM and aftermarket provision, margins are slight for the former. “The windshield business is mainly driven by the aftermarket,” he says.
Cockpit windows are on-condition items, meaning that repairs are conducted only when a fault occurs, not at regular intervals. Common reasons for maintenance include moisture ingress through imperfect seals, which can in turn damage heating elements, and foreign object damage to the outer glass ply. If cracks appear, they cannot be repaired on the airframe “because of the stress on the glass,” says Mourlhon. Instead, damaged windshields must be returned to the manufacturer, where the glass plies will usually be debonded, repaired and relaminated. Meanwhile, often within a matter of hours of the failure, a spare is dispatched to the operator.
The average aircraft goes through three or four windshields in its lifetime, so a growing global fleet naturally means a bigger aftermarket. Recognizing this, GKN launched its Transparencies Aftermarket Service (TAS) in late 2015. Initially operating from the company’s transparencies manufacturing facility in Garden Grove, California, TAS is intended to expand to Europe and Asia, offering window repair, framing and exchange services for both passenger and cockpit windows. PPG and Saint-Gobain have their own repair facilities, along with global distribution points and aircraft-on-ground services.
Strides have been made toward predictive maintenance. PPG has deployed a new technology on business jets to warn of an impending failure of heater circuits. “The intelligent window circuit intervenes to prevent a heater burnout, thereby allowing the flight to continue to its destination and enabling the operator to replace the window in a more scheduled manner,” says Wright.
Windows of Opportunity
Advances in organic LED technology have led to speculation that future aircraft will do away with cabin windows altogether and rely instead on screens integrated into curved cabin walls to relay panoramic vistas to passengers. However, organic LED technology is still far too unreliable for such use in commercial aircraft anytime soon, and GKN notes that “the customer experience of being able to view through windows seems to continue to be a high priority.”
A change closer at hand could be electrochromic windows of the type supplied to the 787 by PPG. Installed between the passenger and the two structural windowpanes (supplied by GKN), these dispense with the mechanical blind found on all other commercial aircraft and instead employ an electrochemical reaction to darken a conductive gel sandwiched between transparent layers. Thus passengers and crew can dim and brighten cabin windows at the touch of a button.
Combined with larger windows, this technology lends a futuristic feel to the 787’s cabin, but some in the windows business are skeptical of it because, for instance, the technology does not allow windows to be fully darkened, as blinds do.