The Germanwings plane that crashed in France did so during what should have been the safest part of the flight.
Yet, something went horribly wrong, leading to the death of 150 passengers and crew.
Although the investigation is at an early stage, here are some possible causes of the crash:
The jet averaged more than 5.3 flights a day over its 24 years. That is not unusual for a plane making short trips around Europe, but is on the higher end of what is normal.
Each take-off and landing cycle and the pressurisation and depressurisation associated with it adds stress to the ‘skin’ of the plane.
Aircraft that fly short, frequent routes go through more of these cycles than planes flying long distances.
In 1988, a 19-year-old Aloha Airlines Boeing 737-200 that had made frequent, short hops among the Hawaiian islands lost a large part of its roof. Corrosion and metal fatigue were to blame.
Flight 9525 reportedly descended at 1,200 metres per minute from its cruising altitude of 11,500m — twice the normal speed of a descent.
If there was some type of rapid decompression whether from metal fatigue or a bomb the pilots’ first move would be to get the plane down below 10,000 feet, where the air is breathable.
The masks that come down from the overhead bins provide about 10 minutes of oxygen. So, a descent rate of 3,000 metres per minute would get the plane down to breathable air just in time.
Mr Curtis said that initial rate of descent is consistent with what you would see in a decompression situation.
Airbus jets have one of the most sophisticated cockpits.
However, there have been issues in the past with some of the instruments there. The 2009 crash of Air France Flight 447 started because pilots received false air speed indications as it flew through a storm. Ultimately, pilot error led the plane into a stall, but only after they had received bad data.
More recently, a Lufthansa A321 — the slightly larger version of the A320 — dropped about 1,200m in one minute after the autopilot unexpectedly lowered the jet’s nose.
Following the November 2014 incident, the European Aviation Safety Agency issued a safety directive warning pilots about the possibility of an error with the angle of attack sensors on the plane.
Pulling up on the jet’s sidestick would not pull the jet out of the dive. The safety agency noted the only way to stop it was to turn off two of the three air data reference units in the cockpit. These units process the flight speed, up or down angle of the nose, and the altitude.
There is no indication that this crash is tied to any criminal act, but such actions cannot be ruled out yet.
The pilots might have accidentally put the plane into a dive or stall, and not been able to recover from it.
Separately, the pilots could have been so focused on getting the plane below 3,000m to a breathable atmosphere following decompression that they did not realise how close they were to the mountains.
July 17, 2014: Malaysia Airlines Flight 17, a Boeing 777, is shot down over the Donetsk region of Ukraine near the Russian border, killing all 298 aboard. The dispute over who is responsible for the disaster continues. The US and other Western nations have accused the pro-Russian separatists of shooting down the plane. But Moscow and the rebels have claimed Ukrainian forces were responsible.
April 10, 2010: A Tupolev 154 plane crashes in fog in western Russia. Among the 98 people killed is the Polish president Lech Kaczynski. A Russian military official said that air traffic control in Smolensk had tried to divert the plane to another airport because of inclement weather.
August 20, 2008: A Spanair MD-82 crashes on takeoff at Madrid’s Barajas Airport, killing 154 people.
August 16, 2005: Helios Airways Flight 522 crashes after apparently losing cabin pressure, killing all 121 people on board. The plane had left Larnaca, Cyprus, for Athens.
Pilots of two jet fighters saw two people moving around in the cockpit and the co-pilot slumped over the controls.
January 8, 2003: A Turkish Airlines flight crashes in fog on approach to Diyarbakir airport, southeastern Turkey, killing 75.
October 8, 2001: An SAS McDonnell Douglas MD-87 airliner collided with an eight-seater Cessna which had allegedly strayed on to the wrong runway at Milan’s Linate airport in thick fog, killing 118 people on both aircraft, and four people on the ground. The crash was blamed on human error, weather and the failure to install new ground radar equipment.
July 25, 2000: An Air France Concorde supersonic jet strikes a strip of titanium and blows a tire during takeoff from Charles de Gaulle airport. The tire scatters debris into the wing and ruptures the fuel tank. The jet bursts into flames and crashes into a hotel, killing 100 passengers, nine crew, and four people on the ground.
January 8, 1989: A British Midland Boeing 737 crashes onto the M1 motorway near Kegworth, Leicestershire, as it prepared to land at nearby East Midland Airport. Of the 126 passengers and crew aboard, 47 died and 74 were seriously injured.
December 21, 1988: Pan Am Flight 103 explodes in the evening skies above Lockerbie, Scotland, killing all 259 people on board and 11 more on the ground. Abdelbeset Ali Mohmed al Megrahi was convicted in connection with the bombing, and sentenced to life in prison. Al Megrahi died in May 2012, still protesting his innocence.
November 27, 1983: An Avianca Boeing 747 flying from Paris to Madrid crashes on its final descent, killing 181 people.
March 27, 1977: A KLM Royal Dutch Airlines Boeing 747 beginning its takeoff crashes in fog into Pan American World Airways Boeing 747 then still on the runway at the Los Rodeos Airport Tenerife. 574 die.
March 3, 1974: 346 people are killed when a Turkish Airlines DC-10 experiences decompression shortly after takeoff from Paris and slams into a park.
Germanwings said it was unclear what caused its Airbus A320 aircraft to crash in the French Alps, but that there had been no problems with the plane before takeoff.
“There were no anomalies on the plane,” the Lufthansa unit’s managing director Thomas Winkelmann said hours after the crash of the 24-year-old plane.
He said Airbus delivered the plane to Lufthansa in 1991, after which it flew exclusively for the German flagship carrier until it was transferred to Germanwings’ fleet last year.
That makes the aircraft older than the 11.5-year average age of Lufthansa’s fleet of 615 planes.
Winkelmann said: “That is acceptable because maintenance standards inside the Lufthansa group are known as very high worldwide.
“As long as you have your maintenance schedule in place and follow all the procedures together with the manufacturer, there is absolutely no issue with the age of an airplane.”
Routine maintenance was last performed on the A320 by Lufthansa Technik on Monday, and the last regular major round of maintenance was in summer 2013, he said.
The pilot at the Airbus’s helm has been flying Lufthansa and Germanwings for more than 10 years, added Winkelmann.
“We need to use all of the Lufthansa group’s resources, maintenance, flight operations, cockpit crews and all other experts, to jointly with authorities and with Airbus find out as quickly as possible what happened,” he said.
READ MORE: Germanwings: A continent in mourning
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