Oxygen in the Skies

Modern aircraft must pressurize oxygen inside the cabin to create a comfortable environment for passengers. Decades of engineering has gone into making this system safe. (Photos.com)

‘Aviation is fascinating to people because airplanes are beautiful to look at, because they go fast, because speed is exciting, because flight for those who haven’t studied is fascinating and a mystery. Air crashes are very rare nowadays, and they are getting even rarer. But just because airplanes are getting better and smarter, and more reliable, all the difficulties mankind has ever faced with—weather phenomena, extremes of weather— these are all unknowns. Don’t ever settle into a modern airplane and think nothing can go wrong.’—David Learmount, operations and safety editor of Flight International magazine

Airplanes are an incredible engineering accomplishment, but producing tremendous airborne structures is only part of the design challenge involved in safe air travel. Engineers must also consider how to keep passengers safe and comfortable as well.

At high altitudes oxygen levels are thin, so modern aircraft must also serve as life support vessels that pressurize oxygen inside the cabin. This requires that the fuselage overcome tremendous pressure changes as it soars into the sky to protect the passengers inside. If an aircraft cannot withstand these pressure changes, an ordinary flight can become a nightmare.

The Comet

In 1952 the Comet ushered in the Jet Era as the British built plane took its maiden voyage from London to Johannesburg. However, this engineering marvel of the day would bring tragic consequences.

In 1953 the Comet crashed twice shortly after take-off killing a total of 54 passengers. The following year, the Comet disintegrated mid-air above the Mediterranean Sea killing all 35 on board. Investigators discovered that the Comet’s design could not withstand pressurization of high altitudes and the stress blew its fuselage to pieces. The Comet fleet was grounded.

Scientists scrutinized the Comet design by placing the craft in a large tank that was repeatedly filled and emptied of water to examine how the plane responded to changes in pressure. Their test revealed the devastating consequences of metal fatigue—the weakening of metal structures due to prolonged stress and load.

Investigators noticed fatigue cracks around the Comet’s square windows giving rise to further cracking that traveled throughout the structure. Through repeated tests, the exterior of the airplane became so stressed that the cracks eventually connected and under the force of changing water pressure the fuselage was torn apart.

The Comet design was not sufficient enough to keep the necessary pressurized oxygen in the cabin. But precisely because of the Comet, modern airliners are now designed with rounded windows to avoid pressure buildup at edges and corners. In addition, extra rivets are now used to control possible cracks that might occur elsewhere on the fuselage.

Pushing an Aircraft Beyond its Lifespan

In 1988, Aloha Airlines Flight 243 took off from Hilo on its way to Honolulu, Hawaii. At an altitude of 24,000 feet and 23 minutes into the flight, the Boeing 737 experienced an enormous explosive decompression causing the entire top half of the plane’s skin, extending from just behind the cockpit to the fore-wing area, to tear off.

The National Transportation and Safety Board’s (NTSB) investigation and laboratory tests revealed cracks around the rivets on the fuselage—classic signs of metal fatigue.

By this time, airplane design had improved considerably since the Comet disaster 35 years before. However, the life span of a Boeing 737 is 20 years and the manufacturer warns that the craft is only good for about 75,000 flights. While the Aloha aircraft had only served for 19 years, it had amassed a staggering 89,000 single flight trips.

Because of the relatively short distances among the Hawaiian Islands, aircraft serving the area must fly more frequently. This forces the fuselage to experience more breathing—expansion and contraction—creating more pressurization stress than craft serving in any other area. The moist tropical air of the region also contributes to corrosion which further weakens the structure of aircraft.

To prevent the development of cracks and their spread through the fuselage, modern airliners are designed with a series of columns and rows of rivets that form 10-inch squares. If cracks do form, this design helps keep them within controlled areas, preventing the spreading or joining of cracks that could endanger the integrity of the aircraft.

The fuselage is designed to withstand numerous cycles of expansion and contractions (or pressurization) during its life of operation, but even a well-built structure can only tolerate so much.

In the case of the Aloha Flight 243—a craft pushed beyond its life span—so many cracks had developed that the rivets were not able to control them. These cracks eventually joined together, causing a large section of the plane to fly off in mid air.

Luckily, the crew of Aloha Flight 243 was able to ground the crippled plane safely and save the lives of those on board.

The Faulty Door

In 1989, just a year after the Aloha 243 incident, another Boeing 747 takes off from Honolulu International Airport on its way to Auckland, New Zealand, carrying 337 passengers and 18 crew members. As the aircraft approached 23,000 feet, almost 16 minutes into the flight, a cargo door under the front of the plane separates from the fuselage. The missing door creates a sudden depressurization causing the floor in the main cabin to cave in. Two rows of seats filled with passengers are sucked out of the craft and ejected into the sky.

Unlike passenger doors, which are virtually impossible to open during flight, cargo doors rely on a locking mechanism to ensure the integrity of the fuselage. The NTSB investigation into flight 811 revealed a faulty locking mechanism.

As the plane kept climbing, the pressure on the door increased, eventually separating it from the structure. As a result of the incident, the NTSB instructed that all Boeing 747s were to outfit their cargo doors with non-faulty locks.

The goal of safely containing pressurized oxygen at high altitudes has always been a challenge for engineers. A single flaw in the design or manufacturing of an aircraft can lead to devastating decompression which can spell disaster for the people aboard.

Planes flying today are 100 times safer then those in the 1950s. Accidents, though extremely tragic, have led to technological breakthroughs making air travel safer and more efficient than it’s ever been.

The Airbus 320, for example, has an excellent safety record. The strength of this narrow-body commercial passenger owes much to the design flaws of the past.

The 320 uses extremely strong titanium rivets to hold the fuselage in one piece, and as many as 3000 rivets are used to join separate parts on each wing. In addition, this craft is covered with an aluminum skin as thick as a coin, protecting passengers from the dangerous elements found at high altitudes.