Over the last few weeks, several commercial flights around the world were forced to make emergency landings due to window damage. The majority of such incidents are not catastrophic, but a window-related malfunction can potentially be very dangerous. Considering the immense stresses aircraft windows endure as a matter of course – from temperature extremes to cabin pressure – the importance and criticality of windows as an aircraft component should not be taken for granted. So how often do cracks in passenger windows or cockpit windshields occur?

Shattered windshields

On April 25, 2018, a Flybe Flight BE801 left Newquay Cornwall Airport (NQY) in the UK and was headed to London Gatwick Airport (LGW) when the pilot was forced to turn back at 12,000 feet after the outer layer of the cockpit window shattered shortly after takeoff. A photograph of the damage taken by one of the 92 passengers on board the aircraft, an Embraer 195, showed the crack covering much of the main window in the cockpit.

Flybe – a regional airline based in England – issued a statement on the incident, saying: “We can confirm that the aircraft operating the flight landed safely at Newquay without incident having returned from airborne following a crack appearing in the outer layer of its windscreen.” The airline also said that the captain “took the necessary action”.

On May 6, 2018, a JetBlue flight traveling from Puerto Rico was forced to divert paths and make an emergency landing in Fort Lauderdale, Florida (U.S.), after an outer layer of the windshield of the cockpit shattered mid-flight.

Flight 1052 took off from Luiz Munoz Marin International Airport (SJU) in San Juan, Puerto Rico, at 10:29 a.m. AST and was traveling to Tampa, Florida. The aircraft, an Airbus A320, was diverted to Fort Lauderdale in south of Florida instead, and landed at Fort Lauderdale International Airport (FLL) at 12:52 p.m. EDT.

In a statement, the New York-based low-cost carrier said that the flight diverted “in an abundance of caution following a report of damage to one of the outer layers of the cockpit windscreen,” adding that the flight landed safely and passengers were accommodated on another plane bound for Tampa. The cabin did not lose pressurization during the incident.

A flight attendant told passengers on the plane that, "it happens I won't say frequently, but I've actually had this happen before," adding that, “there's multiple, multiple layers in the windscreen, and it's the outer layer that shattered." And JetBlue is right.  The cockpit windshields on airliners are usually constructed of multiple layers to withstand the immense thermal, aerodynamic and mechanical stresses and extremes of flight.

According to the U.S. Federal Aviation Administration (FAA) the windshield and window panels “must be capable of withstanding the maximum cabin pressure differential loads combined with critical aerodynamic pressure and temperature effects for intact and single failure conditions”. The “fail safe” concept in the design means that in the case of a failure of one component, it will not lead to a catastrophic failure of the whole structure.

Damaged passenger windows

Southwest – a major U.S. airline based in Dallas, Texas – has recently made headlines with two incidents involving damaged windows. On April 17, 2018, Southwest Flight 1380 traveling from New York LaGuardia Airport (LGA) to Dallas Love Field (DAL) in Texas, made an emergency landing at Philadelphia International Airport (PHL) in Pennsylvania, after the Boeing 737-700 suffered engine failure mid-flight.

Debris from the engine shattered a window of the aircraft, causing depressurization and killing one passenger and injuring several others. There were 143 passengers and five crew members on board. The U.S. National Transportation Safety Board (NTSB), which is conducting an investigation into the incident said the passenger's death was the first fatality in an accident on a U.S. airline since 2009.

More recently, on May 2, 2018, Southwest Flight 957 traveling on a domestic route from Chicago Midway International Airport (MDW) in Illinois, to Newark Liberty International Airport (EWR) in New Jersey, was forced to divert to Cleveland-Hopkins International Airport (CLE) in Ohio, after a window pane cracked mid-flight.

None of the 76 passengers on board the Boeing 737-700 were injured in this incident. Southwest also assured that the cabin did not lose pressure as there are “multiple layers of panes in each window”. The FAA confirmed only that a passenger window was broken and said it will investigate the incident to determine the cause of the crack.

Commenting on the latter incident, Richard Healing, a former National Transportation Safety Board (NTSB) member who now leads Air Safety Engineering consulting firm, told Bloomberg it is “very rare” for a passenger cabin window to have such issue. According to him, cockpit windows crack more often than those in the cabin. This corresponds to the FAA’s data, which suggests that passenger window failures are rare. For instance, according to the agency, these types of failures are rare in 737's - only 26 times in the aircraft's worldwide service history (it entered service in 1967).

In any case, Healing pointed out that, “The window performed, the backup function worked as intended,” with the multiple layers preventing cabin depressurization. Indeed, the window HAS to perform. The structural integrity of multi-layered windows is attributed to proper design, manufacturing and the right maintenance procedures. This is important as the safety factors necessary for glass components in aircraft windows are significantly higher than for other materials used in aircraft construction.

Whatever may be the cause of a window failure inflight, the actions and precautions taken by the flight crew in such potentially dangerous and complex situations is crucial in getting the aircraft safely on the ground.