Planes can safely fly with cracks
Aeronautical engineers say cracks on a plane are normal though, if left unrepaired for too long, could become dangerous.
How much can a superjumbo really take?
With Airbus in the spotlight over wing cracks on its A380s, it may be reassuring to know that in stress tests, a jumbo jet's wings can be flexed about 7.6 metres upwards ... the equivalent of two storeys.
That's just one of the tests modern passenger planes - such as the Boeing 787 Dreamliner and the A380 - go through to ensure they meet the safety requirements of aviation authorities before they are delivered to airlines.
In the early days of aviation, stress testing wings meant men standing on them, or loading them up with sand bags.
Today, it involves hydraulic rigs that fill up a hanger and bend the wings to extreme angles.
The wings have a standing jig, and they will bend the [prototype] wing continually for years to test the strength of the wing, for example, in turbulence
Other tests include:
* stalling the aircraft hundreds of times
* freezing it at a temperature of -42 degrees Celsius
* flying it at the lowest-possible speeds and almost as fast as the speed of sound
* landing it in heavy cross winds and in wet conditions
* dragging its tail on the ground
* carrying heavy loads
"The modern jets ... are not designed to be thrown around the sky during barrel rolls, but they are built very strong," said Captain Barry Jackson, the president of the Australian and International Pilots Association and 32-year-veteran pilot, who flies the A380 for Qantas.
"The wings have a standing jig, and they will bend the [prototype] wing continually for years to test the strength of the wing, for example, in turbulence.
"There's strict requirements on passenger aircraft and they are more reliable, very strong, very sturdy, able to withstand severe turbulence and hopefully not too many heavy landings."
Captain Jackson said pilots train every three months in a simulator to mimic situations such as engine failure, engine fires, cargo fires and volcanic ash in engines.
Can't predict every situation
And while not all situations can be predicted, the reliability of today's aircraft - coupled with well-trained pilots taught to cope with any emergency - go a long way in ensuring passengers arrive safely at their destinations, Captain Jackson said.
One example of an unexpected - and potentially catastrophic - situation was when British Airways flight BA38, a Boeing 777 aircraft, lost all power and crash-landed at London's Heathrow airport just short of the runway in 2008.
A Heathrow worker told the BBC then the landing was so good despite the dire situation that the pilot Peter Burkill deserved "a medal as big as a frying pan".
"That was a good outcome because of the strength of the aeroplane," Captain Jackson said.
"Both engines failed and the plane ... landed heavily, so much so that one of the landing gears collapsed into the wing."
Captain Jackson, who has been flying for 32 years, including 28 years with Qantas, said the classic example was of Qantas flight Q32, which made an emergency landing at Singapore's Changi Airport on November 4, 2010, after "monumental engine failure".
"That's the perfect example of the damage that an aircraft can withstand and also how it's dealt with.
"The number one and number four engines were [in] degraded mode and manual thrust had to be used, which is obviously not normal. So the pilot had to have flight training to be able to handle that and also there were 58-odd messages to deal with the damage that was caused to the aircraft."
Stress testing is 'not enough'
The difficulties of ensuring a commercial jet is safe to fly is illustrated in the delays it took for the A380 and Boeing's Dreamliner to come on to the market.
Just yesterday, Japan Airlines announced it did not expect to get delivery of the Dreamliner by the end of February after a new manufacturing glitch involving "incorrect shimming", Reuters reported.
Shims are materials used to close the small gaps between joints.
The A380 - called Europe's "21st century flagship" - was first commercially flown in 2007. But Airbus faced questions after the QF32 incident and has been ordered by European air safety officials to inspect all 68 jets in service after airlines such as Qantas reported finding hairline cracks in its wings.
Steve Re, the technical representative of the Australian Licensed Aircraft Engineers Association, said it appeared there were some stresses induced in the A380's fittings when they assembled the aircraft.
And while the Airbus might be able to fix the cracks, the modifications would transmit the stresses to other parts of the aircraft, he said.
"You can have stress testing of production models ... and a stimulated flying experience, but you can see from the A380 that the testing obviously didn't pick up the cracks.
"Only time will tell whether the stress tests will mimic real-life conditions," he said.
How did the A380 wing cracks happen?
Mr Re said the wing cracks may have been caused by fittings - for example of uneven thickness - being pulled together without shims.
"It's a standard aircraft practice that if you've got a gap, you fill that gap with a small piece of metal - what's called a shim, so you reduce the fit-up stress.
"So you are not bending the other component when you clamp it together," he said.
"I don't think they were employing that practice of shimming those joints. They were just pulling the fittings together, which was inducing the stress."
Airbus spokesman Stefan Schaffrath told Reuters the damaged L-shaped parts, which fixed the wing skins to their underlying frame, were "not a primary load-bearing structure", Airbus spokesman Stefan Schaffrath said.
He said that cracks were discovered in a "handful" of the 4000 brackets on each aircraft, but that "the safe operation of this aircraft is not at stake".
Need constant upgrading and inspections
Mr Re said the experience of the Boeing 747, which was first flown by a commercial airline in 1970 and is still being flown by some airlines today, showed that parts were constantly being upgraded and modified, and that there needed to be a regular inspection regime.
"An aeroplane as it flies is continually being stress tested, which is why there is this continuous inspection routine. As the aeroplane's get older, more and more things will emerge."
And new ways of servicing and inspecting the carbon-composite Dreamliner would also be needed as it had cutting-edge technology not used before in the industry, Captain Jackson said.
"In a standard alloy aeroplane ... when a catering truck runs into it, the dents and scratches are fairly obvious.
"With a carbon fibre aircraft, those dents and scratches are not that obvious ... and we are learning more and more about how to inspect those aeroplanes."