How Airplane Windows Are Designed To Withstand Enormous Stresses, And How They Can Fail
A troubling string of incidents involving aircraft window failures has caught public attention over the past few weeks, including one where the cockpit windshield of a Sichuan Airlines A319 blew out at 30,000 feet, and the tragic death of a passenger on a Southwest 737 when an engine failed, ejecting metal shrapnel that shattered her window, causing her to be sucked partway out of the plane.
It’s raised questions for many over the safety of aircraft windows. Here’s what you should know about the rules, standards and methods of design, and testing and manufacturing of these critical aircraft structures.
Effectively, airplanes are not so different from submarines. Both involve structures that have to withstand a stark difference in pressure and temperature outside and in. In both cases, the outside environment is not hospitable to life. A tiny flaw or weak point can quickly spread and lead to tragedy.
Airplanes face other challenges. They operate at very high speeds in an environment where other flying objects—birds or debris—may pose a threat.
All of these risks are considered in the rules and standards governing the manufacture of aircraft structural parts and components.
The guidelines on various accepted methods of composition and stress testing of aircraft windows can be found in the FAA’s AC (Advisory Circular) 25.775-1. For anyone who, like this writer, really likes to deep dive into things like chemical treatments, material loads and other stress tests, this document is good reading material.
In summary, the AC makes clear that aircraft windows undergo the same degree of stringent testing as aircraft fuselages or engines. They are all built to be tough.
One of the world’s oldest manufacturers of aerospace transparencies (windows) is PPG PPG +0.52%, headquartered in Pittsburgh. It was founded as a specialist glass, paints, coatings and materials manufacturer in 1883, and their first aircraft windows were installed in 1926 on the Ford Trimotor. They have placed high-performance panes on planes ranging from commercial aircraft to fighter jets and business jets and specialize in flight deck windshields.
labs developed an Opticor Advanced Transparency Material with advanced impact and crack propagation resistance properties, which can be used in the manufacturing of both windshields and passenger windows.
Brent Wright, PPG global business director for aerospace transparencies explains, “On the cockpit there are two primary purposes for the windshields. Number one, protect the crew from the outside harsh environment and number two allow the crew to see outside and in doing so. Getting to that, to protect the crew from the outside environment, the windshield has to be designed to be structurally capable and safe.”
Unlike many passenger cabin windows, flight deck windows are built with multiple plies of glass or stretched acrylic material. There are two structural plies, each capable of withstanding the pressure differential in the cabin. On the outside, there is a thin outer ‘face ply’ which also has a de-icing element.
“On your car, in the back window, you’ll see those lines for defog or defrost, and the aircraft window has something similar,” Wright explains. “It has a film on the outer glass that is electrically heated. Power goes to the film and the film heats up and deices the window on the outside.”
Besides the wires that heat the window using electricity, there are also sensors to measure resistance and to control the heat—so that the window never becomes too hot or too cool for operating limits. The window heating system makes the necessary adjustments at varying altitudes and in varying weather.
Like engines, fuselages, and other external structures, windows must undergo bird strike tests. Bird strikes are a serious problem for aviation, because we encroach on their airspace and have built airports near natural habitats and migratory routes.
“Commercial aircraft are rated to withstand a birdstrike of a four-pound bird and anywhere from 250 to 350 knots,” Wright says. “That, in turn, is driven by the flight envelope of the airplane and where the birds are in the airspace speed limits. For example, in the U.S. below 10,000 feet the speed limit is 250 knots. The aircraft may exceed that [speed] on descent and you build in a little more than the minimum for safety requirements.”
Besides pressure, temperature control, and bird strike testing, aircraft windows area also tested for chemical resistance to things like hydraulic fluid or jet fuel as well as abrasion including rust or rain erosion.
“The windows have to withstand all of that,” Wright says. “Not only that but they may be tested in different contentions…where one of the structural plies is damaged, to determine and certify the fail-safe aspect of the design.”
Like other aircraft components, not every part undergoes destructive testing—that would mean that no product would ever make it to the end of the line—but a new design must undergo all manner of tests before the methodology for manufacturing that part is approved. Raw materials and lots also have to have a sample part that is tested and destroyed in some way, to demonstrates that the build that follows complies with the standards set in the specification.
Passenger Cabin Windows
While passenger cabin windows don’t have as much structural load resistance, forward force resistance, or the same visibility provisions as cockpit windows, they do have to preserve the pressure conditions in the cabin and they do have to have fail-safes. Most passenger windows are built from separate panels with airspace between them, including a fail-safe panel which can protect the interior even if the external panel fails.
Passenger windows on Aeromexico Dreamliner aircraft, Amsterdam
Some modern aircraft cabins—like the ones on the Boeing Dreamliner—have multi-ply transparencies. They are mounted on a composite fuselage which means they are built to withstand greater structural loads.
Some aircraft windshields are fastened with bolts. Others use a clamping system, but Wright says both methods are equally reliable.
“The Airbus A320 and A340 are clamped-in design, without bolt holes. Boeing aircraft, on the other hand, are typically bolted-in designs. Both of them, just by virtue of the thousands upon thousands of flight hours that we see, are perfectly suitable for the application,” Wright says. “There is a trade-off, however. If you want to make the window an integral part of the aircraft structure, then a bolted in design is the way to go because it transmits the aircraft loads right through window. The window becomes a structural part of the front of the fuselage. The alternative is to isolate the window from any possible loads being transmitted by the aircraft. What happens then is that to you have to have a heavy metal fuselage build-up, or frame, around the window to isolate it.”
When windows are damaged, they have to be replaced, except in certain cases of very minor damage—light scratches or scuffs—which must be repaired by experts.
Wright says that pilots have a lot of say in deciding when a flight deck window needs to be swapped out. The typical service life of a flight deck window is ten years, nearly of the rated service lifetime of most aircraft, which is around 25 years. The majority of the aircraft flying today are much younger than that. With airlines buying more planes each year, the global fleet is expected to stay young on average—under 15 years.
When things go wrong
It may seem strange to say, but a cracked passenger window that prompted an emergency landing of a Southwest Airlines plane May 2 failed properly. With critical components, designers must engineer in failure mode performance. A backup system has to take over when one system fails. The fail-safe pane on this window ensured that when one pane cracked, it did not lead to a collapse of the other inner pane or to cabin depressurization. This is not an operating condition, and the pilot did the right thing by diverting the plane, but aircraft windows are designed to endure these unusual circumstances.
The failure of a flight deck winshield on a Sichuan Airlines flight that sucked a co-pilot halfway out of the flight deck is still pending investigation. All we can do is speculate. But that incident may not have been caused by a window failure.
As reported by Flight Safety Australia, the industry newsletter published by Australia’s Civil Aviation Safety Authority, the sudden cracking and explosive failure of the co-pilot’s windshield is similar to a previous incident in 1990. In that case, investigators found a failure in maintenance where a mechanic used undersized bolts to fasten a new windscreen.
Of course, what frightens people are events like the tragic death of Jennifer Riordan, who was pulled through her window after an engine failure on a Southwest Airlines LUV -0.53% flight April 17, which also led to cabin depressurization and an emergency landing. An event like this extremely rare.
“Obviously people will say what can prevent that—would this material have helped or would that situation have helped. I don’t know what happened with the engine. I suspect that that fragment of blade was moving at a ballistic speed,” Wright says. “It would have been awful hard to withstand the threat there.”
National Transportation Safety Board (NTSB) records show only 29 incidents involving aircraft windows on aircraft operated by commercial airlines over the past decade.
The reason aviation maintains such a strong safety record, though, is that every incident is studied closely by independent investigators, by regulators and by manufacturers. The investigation will consider all possible scenarios, and identify a root cause. Any changes that need to be made to avoid something like this happening again will be made.