This weekend, a Southwest Airlines Boeing 737-300 was involved in an explosive decompression incident in the US, which resulted in an emergency landing.
The pilots took the plane into a controlled descent and landed safely, but many passengers were understandably frightened. The United States FAA (Federal Aviation Administration) has announced that it will issue a safety directive to airlines on April 5.
But what really happened? Which airlines in and around Australia fly similar aircraft? And what happens next for the airlines, the planes and for passengers? Here's the full story.
The recent Southwest Airlines incident
On Friday afternoon US time (Saturday morning in Australia), part of the ceiling blew off a Southwest Airlines flight between Tucson and Los Angeles, which are just over 700 km apart.
That's what the airline business calls an "explosive decompression". Since the air pressure in planes flying at altitude is higher than the surrounding atmosphere, the air inside presses outwards, and rushes out if the integrity of the aircraft skin is compromised. Any interruption to the smooth skin of an aircraft is compounded by the speed of the air flowing past the plane.
As a result, a hole estimated as roughly four feet by one foot opened up in the ceiling of the short-range Boeing 737-300 airplane, and passengers rushed to put on the emergency oxygen masks.
The plane descended quickly (which is what's supposed to happen, so that the air becomes thick enough for passengers to breathe) and made a safe landing at the Yuma, Arizona airport and military base.
Southwest Airlines and the Boeing 737
Southwest is one of the USA's biggest airlines, carrying the most domestic US passengers of any airline in a fleet of 547 Boeing 737 aircraft.
Its low-cost model and the fact that it's not a member of any of the global airline alliances mean that Australian business travellers very rarely fly the airline, but it's a staple of US leisure travel.
The Boeing 737 is the most prolific plane in the sky, with over 6600 built since the original version was launched back in 1967. It's a small, short-range jet aircraft with two engines that has undergone three full generations of redesigning, meaning that the 737s rolling off the production line today only look like the original 1960s era planes on the outside.
The particular model involved in the Southwest incident is the 737-300, which is the second generation version, produced between 1984 and 2000. The plane in question was delivered in 1996.
In our region, the only large operator of the 737-300 series is Air New Zealand, which has 15 of the planes on its domestic mainline routes.
Qantas, however, also has second-generation planes: the 737-400 fleet, which it put up for sale yesterday. Qantas' planes are all older than the Southwest Airlines 1996-vintage plane that was involved in the incident over the weekend. Once Qantas sells the 737-400s, it will only have newer third-generation 737-800 planes.
Virgin Blue used to operate second-generation 737-400 aircraft, but has since replaced them with third-generation 737-700 and 737-800 planes.
Why did the Southwest Airlines incident happen?
Southwest, Boeing, and the US National Transportation Safety Board (which investigates aircraft accidents) sprang into action immediately after the incident. The Southwest 737-300 fleet was grounded while the planes were checked, with cancellations and delays for passengers on Southwest's flights.
The 737-300 fleet is still being checked, but initial reports from Boeing, Southwest and American authorities suggest that "aircraft skin fatigue" may be the cause.
Skin fatigue occurs with age, and tiny cracks can develop in a plane's metal skin. If they're not fixed, the cracks can develop into larger cracks.
Eventually the pressure of the air inside the aircraft and the friction of the air outside the plane will lead to failure and bits of the plane will peel away.
That's what happened on a large scale to an older Boeing 737-200 during a flight in Hawaii in 1988. That aircraft, although also a 737, was nearly 30 years older than the Southwest plane involved in the incident over the weekend.
A small section of the roof peeled off on Aloha Airlines flight 243. A larger section of the plane's ceiling, stretching from behind the cockpit to just in front of the wing, was then torn off by a combination of the pressure inside and the wind speed outside the plane. A stewardess was killed as a result of being pulled out of the aircraft by the decompression, and several passengers were injured.
That accident, to an aircraft built in 1969, led to changes in aircraft design, manufacturing and maintenance in the late 1980s and early 1990s.
Modern aircraft are now more resistant to explosive decompression. When an oxygen cylinder blew up on Qantas flight 30 in 2008, creating a hole in the side of the plane 2m wide and 1.5m high, the aircraft made a safe emergency landing with no injuries to passengers or crew.
Similarly, a different Southwest 737-300 plane (which is the same model as the one involved in this weekend's incident) experienced an explosive decompression in 2009. A 14-inch by 17-inch hole was torn in the ceiling of the aircraft near the tail, again with no injuries.
What are local airlines doing about the problem?
In the days since the incident over the weekend, small cracks have been found on some Southwest aircraft inspected by the airline, Boeing and the US airline safety authority NTSB.
Australian Business Traveller contacted both Qantas and Air New Zealand this morning with detailed questions about what steps the airlines had taken in response to the Southwest incident over the weekend.
The two companies are the only major airlines in the region to use older 737 planes -- though both are in the process of replacing them with newer 737 and A320 aircraft from Boeing and Airbus.
Qantas has yet to respond to our questions, but an Air New Zealand spokesperson told us: "All Air New Zealand aircraft are regularly checked as part of their standard maintenance routine. Air New Zealand follows all relevant directives from authorities and aircraft manufacturers."
Airworthiness Directives are issued by national safety authorities like CASA in Australia and the FAA in the United States, and instruct airlines to take action to fix a problem with aircraft.
The US FAA safety body has announced that it will issue a directive to airlines flying older 737s to inspect the aircraft for problems similar to the Southwest plane.
"The FAA will issue an emergency directive tomorrow that will require operators of specific early Boeing 737 models to conduct initial and repetitive electromagnetic inspections for fatigue damage. This action will initially apply to a total of approximately 175 aircraft worldwide, 80 of which are U.S.-registered aircraft," the FAA announced in a statement today.
FAA directives are usually followed by other national safety bodies, including CASA in Australia and the New Zealand CAA.
For the most urgent directives, airlines have to ground planes until the checks and any necessary changes have been made.
About John Walton
Aviation journalist and travel columnist John took his first long-haul flight when he was eight weeks old and hasn't looked back since. Well, except when facing rearwards in business class.