After a Southwest Boeing 737 made an unscheduled landing Wednesday because of a cracked window, some passengers were, not surprisingly, alarmed. How, they wondered, could this happen? And how dangerous was it, given an incident two weeks ago when an engine tore apart on a Southwest jet, shattering a window and killing one passenger?

The incident on Wednesday, however, may not be the type of thing seasoned airline insiders fear. A commercial airplane’s windows are hardy, and each one is constructed so that a problem like Wednesday’s will not affect flight. Passengers were never in danger, mostly because aircraft windows, like most stuff on a plane, are constructed with built-in redundancies.

“Airplane windows are very strong,” said Richard Aboulafia, vice president for analysis at Teal Group Corp. “That’s why incidents like today’s are almost always completely inconsequential, and statistically almost never happen.”

The Southwest flight from Chicago Midway Airport to Newark, New Jersey, did not lose pressurization, and after the crack was spotted after passengers said they heard a pop, it landed in Cleveland, where maintenance crews planned to examine it. Pilots did not declare an emergency.

The Federal Aviation Administration said it will investigate, a spokesman for the agency said. An official with the National Transportation Safety Board said the agency is monitoring the incident.

The aircraft is involved in the incident was N713SW, a Boeing 737-700 delivered in 1998.

But two incidents within two weeks of each other involving windows aboard planes raises some questions about a part of an airplane we often take for granted. Here are some answers to questions travelers may have about windows.

How are Aircraft Windows Made?

The vast majority of commericial airplane windows are made by only a few suppliers, Aboulafia said. They include GKN Aerospace, a UK company that makes windows for Boeing, Airbus, Bombardier and Embraer. GKN manufactured the windows involved in Wednesday’s mishap, a Southwest spokeswoman said.

On its website, the company notes most aircraft windows consist of two plexiglass panes with an air gap between them, as well as a “scratch pane” facing the passenger.

The outer pane, GKN said, “is the primary structural window” and carries cabin pressure during flight. The inner pane, the company said, is redundant — existing only to hold cabin pressure if the outer pane is broken. It called that an “an extremely rare event.”

“The fail safety effectiveness of the two pane design is rigorously tested during window qualification,” the company said. GKS said its newest outer window panes were initially evaluated “for more than 100,000 flight hours, during five-year tests with 24 airlines, on 11 aircraft types.”

An FAA official also said in a statement that window failures are extremely unusual, and noted carriers perform inspections as part of their regular maintenance schedule.

What Does it Take to Truly Break a Window?

Wednesday’s window failure should not have happened, and investigators will want to learn what caused it. But as Southwest said in a statement, its aircraft windows have “multiple layers of panes,” and it takes a lot — like a piece of an engine crashing into the window at cruising altitude — to fully shatter not just one pane, but the whole thing.

That’s what happened on April 17, when a woman died after she was pulled out of the plane in a freak accident. It was much more of an engine failure than a window failure.

“It had nothing to do with the window at all,” Aboulafia said. “It just happened that there was an uncontained turbine failure. An uncontained turbine failure is incredibly unusual and is almost always going to do some damage somewhere.”

Normal wear-and-tear doesn’t usually affect window strength, said Robert Mann, an airline industry consultant.

“I can’t recall hearing of a passenger cabin transparency cracking,” Mann said. “They are multi-layered/laminated and designed to crack without shattering.”

Does Southwest Have a Window Problem?

It doesn’t look like it.

Most travelers probably would have never heard of the Flight 957 had it not come only a couple of weeks after Southwest’s engine failure. That engine failure was a big deal, and it is interesting it came just two years after a similar incident. It makes sense the FAA is requiring airlines to be more proactive in inspecting certain CFM56 engines, and that the NTSB is investigating. The NTSB has said a fan blade likely failed in the April 17 incident.

The window crack is more likely be a fluke. But that doesn’t mean Southwest’s bookings won’t suffer. The airline projected it would lose between $50 million and $100 million of revenue this quarter because of fallout from the April 17 incident. Given all of Wednesday’s news coverage, the airline might face a short-term revenue hit from this window problem as well.

But passengers would be well-served to book another flight rather than drive.

“If you drive instead of taking a plane you might as well call the life insurance people and double your policy,” Aboulafia said.

Photo Credit: A window pane on a Southwest Airlines Boeing 737 failed Wednesday, scaring passengers. Pictured are some of the carrier's 737s. Bloomberg