Qantas' Boeing 747-400 is not, by any means the first aircraft to survive damage to its pressure hull. The damage, in this case, however, is minor compared with what others have suffered and still landed safely.
In 1989 a United Airlines 747-100 climbing out of Honolulu bound for en route for Auckland suffered the dramatic loss of its forward freight door in flight over the Pacific, and in separating the door ripped away a large part of the pressure hull at cabin level.
The year before that, also over the Hawaiian islands, an Aloha Airlines 737-200 lost the upper part of its entire forward fuselage just behind the flight deck while it was in the cruise at 24,000ft.
The damage to the pressure hull on both sides extended below the cabin floor line. The aircraft diverted to Maui airport and landed safely. One stewardess had been lost, but everyone else on board survived.
The cause was corrosion resulting - over the years - from Hawaii's humid, salty atmosphere, combined with the fact that this particular 737 had flown more trips than almost any other aircraft in aviation history at the time the damage occurred.
Sudden decompressions, for whatever reason they occur, are one of the most frightening in-flight events passengers can encounter because of the drill the crew has to follow to stay safe following the event. The event starts with a bang caused by the structural failure that has breached the pressure hull. The pressure drops dramatically hurting peoples' ears and causing an instant temperature fall that often causes mist to form in the air momentarily. Then the crew puts the aircraft into a steeper descent than the passengers will ever have experienced before, which leads many to believe the pilots have lost control. Actually they are getting the aircraft to a height where the ambient air pressure is sufficient for people to breathe normally before the limited supply of emergency oxygen to the passengers' masks runs out.
The workload for the crew at this time is so high they don't have time to send a calming message to the passengers until the situation has stabilised.
So what caused the Qantas event on 25 July? Since I'm blogging, we can enter the realms of intelligent speculation, but remember, that's what it is:
Most sudden decompressions caused by structural failure are caused by unnoticed pre-exising damage to the hull. The damage can been caused by impact which weakens the hull but does not breach it. This weakened area can, as the aircraft flies again and again, develop cracking which - if not discovered - can result in a local failure. The pre-existing damage could equally have been caused by corrosion that goes unnoticed, but the result can be precisely the same.
Finally, although at this point there is no public evidence to corroborate the theory, an explosion could have caused the damage. The location of this hole in the pressure hull, on the starboard side just forward of the wing, is in the wall of the freight bay where luggage is usually stored. All bags loaded into aircraft holds now are individually x-rayed, so if there has been a security breach the systems will have to be checked.
Whichever the originating cause of this relatively small breach in the 747's huge hull, the passengers were not at risk.
How can I say that with any confidence? Look at the pictures to see what aeroplanes have survived before, and then compare those to the damage to the Qantas aircraft's hull. It's amazing how much damage a modern aircraft can take and still survive.
Or to put it more accurately, it's not amazing, it's what they're designed to be able to take.