Aircraft Windows and Windshields

The windows of an airliner are not simply holes for looking through. On a pressurized airplane every window is part of the pressure hull and carries the cabin pressure load like the skin around it, and every flight adds another load cycle. The flight deck windshield has harder work still: it must stay clear of ice and mist and stop a bird at cruising speed without letting it through.

The two kinds are built differently for those jobs. Flight deck windows are heated laminated panels, usually of toughened glass; cabin windows are acrylic panes arranged so that one can fail without the cabin losing pressure. Both are examined in ATPL airframe and systems, and type-rating courses add each type's window heat controls, failure indications and checklists. Window heat is also covered, with probe heat and rain protection, in probe heat, window heat and rain protection.

Loads on windows

Pressurization stretches a fuselage in two ways: hoop stress tends to expand its cross-section and axial stress to lengthen it. Every window is a cut-out in that stressed skin, so it interrupts the load path and concentrates stress at its edges. A doubler, a reinforcing plate around the opening, compensates; where the skin is machined from solid, the metal around the window is simply left thicker. Smooth radii at the corners keep the stress peak down, because fatigue cracks start at stress concentrations, and each pressurization cycle adds to the fatigue. The de Havilland Comet accidents of 1954, in which fatigue cracks at window corners led to the failure of the pressure cabin, made this a central design issue (see structural design and design loads).

The window panel itself carries the cabin pressure differential, combined with the aerodynamic pressure of the airflow and with temperature effects, including the gradient between the cold outside and the warm cabin. The FAA rule for large airplanes, 14 CFR 25.775(d), bases the design of windshields and windows in pressurized airplanes on continuous and cyclic pressurization loads, the inherent characteristics of the material and the effects of temperatures and temperature differentials, and requires the panels to withstand the maximum cabin differential combined with critical aerodynamic pressure and temperature effects after a failure. Its advisory circular, AC 25.775-1, has that fail-safe strength shown after any single failure in the installation or associated systems. The fail-safe requirement dates from a 1962 amendment of the Civil Air Regulations, carried forward into Part 25, adopted after fail-safe designs had prevented depressurization in many windshield failures.

View full schematic

If a window fails completely, the cabin loses pressure within seconds. A departed window, door or section of fuselage is the typical cause of a rapid decompression, with a bang, a rush of air, a sudden mist and an immediate emergency descent (see decompression).

Flight deck windshield construction

Flight deck windows on pressurized airplanes are usually made of laminated glass: toughened glass panes bonded to each side of a clear vinyl interlayer. The vinyl is the key to bird-strike resistance: it stretches and deforms to absorb the energy of the impact. A transparent, electrically conducting coating on the inside surface of the outer glass pane carries the heating current, and the panel is bolted to the frame through its edge. The Boeing 737 NG FCOM describes the same arrangement on its most common variant: glass panes laminated to each side of a vinyl core.

FAA guidance describes typical windshields and cockpit side windows more generally, as laminates of at least two structural plies, of thermally or chemically toughened glass or of acrylic or polycarbonate, with facing plies, adhesive interlayers, protective coatings and embedded heater films or wires. Toughening puts the surface of the glass in compression, which matters because glass is much stronger in compression than in tension and fails once the tensile stress grows too high. Its strength also rises with the rate of loading, as in a bird impact, and scratches, chips and gouges reduce it.

At each pilot's side, an opening direct vision (DV) window slides back along a track. It lets the pilot see out if the windshield is fouled or its demisting fails, it can be opened in flight once the airplane is depressurized, and on many types it is the flight crew's emergency exit. The A320's two sliding windows are its cockpit emergency exits, each with an escape rope stowed above it; a visible red ring below the release button shows that the window is not locked, and the maximum speed with a window open is 200 kt. The Boeing 737's two No. 2 windows open for evacuation, with an escape strap stowed above and aft of each, and the EASA type certificate data sheet lists the two cockpit side windows of the ERJ 190-300 (E190-E2), 483 by 508 mm, as flight crew emergency exits. The A220 is an exception: its flight crew exit is an overhead hatch.

Cabin window construction

A passenger window is a fail-safe design in miniature. Two structural acrylic panes, an outer pane and a middle pane, sit in a rubber seal within a metal frame, and each can carry the full pressurization load on its own; if one fails, the other keeps the cabin pressurized. An inner pane on the cabin side carries no pressure load. A small vent hole in the middle pane lets the air between the structural panes equalize with the cabin and lets moisture escape, so in normal flight the outer pane carries the load; as FAA guidance puts it, such designs are usually arranged so that the fail-safe pane is not loaded unless the main pane has failed. The NTSB's report on Southwest Airlines flight 1380 describes the Boeing 737 window this way. Its outer pane was designed to the pressure requirements of 14 CFR 25.775(d), which did not require it to withstand impact, and in all 33 outer pane fractures that Boeing had recorded in the 737 fleet by August 2019, the middle pane carried the load.

Acrylic is sensitive to sustained stress and to chemicals, which is why the form of the material matters. Stretched acrylic is acrylic sheet stretched biaxially during manufacture. The FAA's advisory circular on windows and windshields, AC 25.775-1, explains that stretching aligns the polymer chains in layers parallel to the direction of stretch, which improves resistance to crazing, slows crack growth and raises tensile strength, and makes the material less sensitive to notches, at the cost of making it harder to form.

Bird strike requirements

Transport airplanes are certified against bird strikes, and the windshield has its own rule. Under CS 25.775(b) and 14 CFR 25.775(b), the windshield panes directly in front of the pilots must withstand without penetration the impact of a 4 lb (1.8 kg) bird at the design cruising speed VC at sea level.

The rest of the airplane has a separate case. In the FAA's Part 25 it sits in the damage-tolerance rule, 14 CFR 25.571(e), which uses the same 4 lb (1.8 kg) bird at VC at sea level or at 0.85 VC at 8,000 ft. Part 25 adds an empennage case, 14 CFR 25.631, with a heavier 8 lb (3.6 kg) bird.

Heat is part of the bird-strike defense. The conducting coating that keeps ice off the windshield also keeps the glass pliable, and so more resilient, because cold glass is more brittle and fails at lower impact energies. On the Boeing 737 the temperature controllers keep windows 1 and 2 at the correct temperature to ensure their maximum strength in the event of bird impact.

Even a windshield that holds can disable a crew. The FAA's Aeronautical Information Manual notes that windshield strikes have left pilots confused and disoriented, unable to communicate and with aircraft control problems, and advises reviewing emergency procedures before flying where birds concentrate (see bird strike and foreign object damage).

Window heat

Window heat on transport airplanes is electrical. The conductive film within the laminate is a resistance element: a control sensor in the glass measures its temperature, and the controller removes or reduces the power when the glass reaches its operating temperature and reapplies it as the glass cools. A separate overheat sensor, set higher, cuts the power and lights a warning if the controller fails; some systems lock out until the switch is cycled. Overheating can delaminate or crack the panel or burn out the conductive film. Warm conditioned air ducted onto the inside of the windshield completes the demisting.

On the 737 the green ON light goes out when the window reaches the correct temperature, so it may not come on at high ambient temperatures. The A320's distinction between the two failures reflects their roles: the windshields are critical for visibility in ice and mist and for bird-strike resistance, while the side windows are not flight-critical.

Cracks, delamination and procedures

A windshield crack can follow a bird strike or an overheat of the heating system. Delamination is the separation of the plies of a laminated panel, which overheating can cause. Acrylic panes suffer crazing, a network of fine cracks over the surface. AC 25.775-1 attributes it to surface tensile stress above a critical level sustained over time, or to exposure to organic fluids and vapors such as solvents, and notes that stretched acrylic panels are usually retired for loss of transparency from crazing before their strength is seriously degraded. Volcanic ash scores windshields, and hoar frost on a cold-soaked airplane descending into moist air can blank them out.

A cracked windshield can fail completely at speed. The response taught for a bird strike that cracks the windshield is to keep control, slow down to reduce the load on the panel, check for other damage, such as engine indications, declare an emergency if needed and return to land. The load on a window is the cabin differential plus the aerodynamic pressure, so speed and, where the checklist calls for it, cabin differential pressure are the two levers. Certification allows for this: in the FAA's fail-safe test of a failure obvious to the crew, the pressure may be reduced after the pane fails to account for the crew action in the flight manual, and the failed panel must then hold that pressure for the rest of the flight. Each type's checklist sets the actions for a cracked panel; window heat faults, such as an overheat light or a heat failure caution, have their own procedures and minimum equipment list conditions.

Installation is as critical as design. AC 25.775-1 has the fail-safe case cover single failures in the installation and its systems as well as in the panes themselves, such as broken fasteners, cracked mounting components and malfunctions of the windshield heat.

Cabin windows can be lost to external damage. On Southwest Airlines flight 1380 in 2018, part of a failed engine's fan cowl struck the fuselage of a Boeing 737-700 next to a cabin window as it climbed through FL320, the window departed within seconds and the cabin depressurized rapidly; one passenger was fatally injured. It was the first known complete loss of a passenger window in the 737 fleet.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.