Qantas 747′s sudden decompression: it could have been worse

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.

 

 

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Nine passengers were lost instantly the failure occurred, but the aircraft landed safely back at Honolulu. The problem was damaged and defective cargo door latches.

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.

 

 

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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.

 

 

 

5 Responses to Qantas 747′s sudden decompression: it could have been worse

  1. Gabriel Okolski 25 July, 2008 at 5:56 pm #

    While it is true that modern aircraft have extremely wide damage tolerances, and that in its post-decompression state, the Qantas 747 was not in much immediate danger, it’s important to remember that the damage could have been a lot worse. The hole released a good deal of material toward the wings and #3 and #4 engines, and the passengers are lucky that no uncontained engine failure resulted and no apparent damage to the control surfaces resulted.

    Yes, after the fact, the aircraft was still in fine flying condition, but the explosive decompression, like many other decompressions in aviation history that did not end well (Turkish Airlines 981, Japan Airlines 123), could have easily been catastrophic.

  2. Ben Jaensch 27 July, 2008 at 5:58 am #

    Gabriel Okolski,

    while the hole may have released material overboard, it was a long way from #3 engine and a very long way from #4 engine. At cruise speeds the material would travel straight aft, possibly threatening the leading edges of the wing but no the higher tailplane.

  3. David Nicholas 28 July, 2008 at 1:51 pm #

    How nice to read David Learmount’s concise and calm analysis. I managed to avoid listening to any the dreadful Chris Yates’ output on this event (and feel much the better for it). What I want to know is why David L seldom appears on TV now – perhaps for exactly that reason – he is unsensational and undramatic whereas the “hundreds terrified in jet plunge” fraternity seem to have a need to have their fears fed with more of the same dross…..

  4. Iliya Maximov 21 August, 2008 at 8:48 am #

    Exactly the same thing I wrote on my blog the day after the incident. It’s pity that “normal people” do not consider what happened as “the perfect example of a failsafe construction”.
    I’m a bit concerned about the oxygen bottle theory. Do you find it possible and did you come across any Boeing bulletins related to the Qantas incident?

  5. David Nicholas 26 August, 2008 at 9:54 am #

    So it’s happened again. An emergency descent following loss of cabin pressure (this time involving Ryanair), and it’s headline news (along the lines of “hundreds terrified in jet plunge”) and I have switched off all media until they get Chris Yates out of the way….
    Surprisingly, the most measured report I heard this morning was from Michael O’Leary himself, succinctly explaining that with the flight and cabin crew wearing oxygen masks as well, the explanatory cabin PA had to wait until the aircraft had completed its emergency descent (this in response to “many complaints” that “nobody told us what was happening”). Isn’t this precisely why the emergency briefing is given before takeoff (I wonder how many of the passengers paid attention to it)?
    There are not that many mainstream media hacks to bring up to speed on the basics of aviation procedures and safety. How about Flight International sponsoring a media symposium to explain the basics with the aim of minimising the abject nonsense (much of it speculative and alarmist) trotted out by the “Travel Editors” of most of our media organisations? So many people fly now that there is widespread public anxiety and even panic after serious accidents that even a precautionary turnback can make the headlines. Ahhhh for the days of “By Our Air Correspondent”……….