Qantas 744 Depressurisation

[pasoundman]

flynerd wrote:
Consensus appears to be that the bottle in question is one of the newer composite bottles. This has an inner aluminum liner. The regulator would be expected to be brass.

During the 6 months the bottle had been in place I would expect it to have undergone many cycles through a wide temperature range.

The two materials at the threaded top of the bottle would be Aluminum and Brass. These have quite different coefficients of linear expansion (COE). Those being about 24 * 10^-6 per centigrade degree for aluminum, and 19 for Brass.

Cold Aluminum is very brittle and could become cracked over time in the circumstances.
The disparate COE of these metals at this critical joint may be a big factor.

I suspect that this experiment with lighter O2 bottles may be at an end very soon!

Now THAT is a very interesting analysis.

[pasoundman]

CVRs

In the old days of magnetic tape it's entirely understable how and why it was necessary to overwrite older recordings.

In these days of modern non-volatile semiconductor memory, could we not do a little better ?

[Litebulbs]

I was under the impression that a CVR recored the last 30 minutes of voice.

A. The voice recorder system preserves a continuing record of the latest 30 minutes of flight crew communications and conversations. The voice recorder receives inputs from the audio selector panels for the captain, the first officer, and the first observer, and from an area microphone in the flight compartment.

Pulled from a Boeing (not B747-400) MM

[HotDog]

Quote from New Zealand Airworthiness Schedule DCA/EMY/27A,24 May 2008
Oxygen Reserve Cylinders-Inspection & Replacement

To prevent oxygen cylinders exploding due to aging and deterioration of the
Aluminium Alloy 5283 (AA5283) cylinder shell material, identify the year of
manufacture of each affected P/N oxygen reserve cylinder made from AA5283 and
replace per the instructions in Eurocopter AS 350 Alert Service Bulletin No. 05.00.54.
Immediately after removing the oxygen cylinder from the aircraft, empty the cylinder
per the instructions in Intertechnique Service Bulletin (SB) GLD/GLF-35-150 dated 20
September 2006.
Note 3: Oxygen cylinders with P/Ns listed in this AD may only be used if the service life is
within the limitations of the compliance of this AD.
Note 4: Oxygen cylinders with P/Ns listed in this AD and which are held as spares are to be
inspected per the requirements of this AD. Identify the year of manufacture of the
cylinder and empty all oxygen reserve cylinders that have reached or exceeded 25
years after manufacture, per the instructions in Intertechnique SB GLD/GLF-35-150.
(EASA AD 2006-0286R1 refers)
Compliance: Within the next 15 days if the TIS cannot positively be established.
When operated in salt-laden atmospheric conditions:
At 15 years since manufacture, or within 15 days whichever occurs later.
When operated in normal climatic conditions:
At 25 years since manufacture, or within 12 months whichever occurs later.

[NSEU]

Cold Aluminum is very brittle and could become cracked over time in the circumstances.

Are we talking about pure aluminium or aluminium alloy? Remember that the entire fuselage is made of aluminium alloy

[Capt Kremin]

The CVR was probably still running after the aircraft came to a halt on the runway. It took some time for it to reach the gate apparently as it was towed in. I don't know how long after that it was powered down. It probably explains why the incident itself was overwritten.

[flynerd]

Machaca

If you have knowledge of any newly uncovered issues re COE/CTE between aluminium and brass please do share it with us.

No _new_ issues, only those that have existed for all time.
In the Navy, you learn to never tolerate iron/aluminum direct contact joints because of electrolysis.

I do not know what Al alloy is specifically used in these composite bottles, but the boundary interactions between the metals used in the bottle and the regulator would appear to be a real target for the investigators in this case. This despite the fact that they may have been standard practice for some time. This was not an exploding bottle ( my guess). This was a bottle whose regulator popped out in a real hurry. I am not addressing the walls of the bottle, but the brass/aluminum interface. If the bottle could ever be found at the bottom of the sea, I suspect it would be mostly intact with some superficial external damage.

[loaded dog]

What ifs

Is it not a possibly lucky event that whatever flew off the Oxy Bottle (Presumably the brass regulator?) actually hit the door handle and was therefore able to dissipate some of its energy into a moveable object...rather than hit the roof and possibly hole the aircraft from the inside? That door handle may have saved a bigger event?

Interesting thought.
Many people have also probably wondered about what would have happened if the liberated O2 found an ignition source but have tastefully kept their thoughts to themselves. At the risk of starting a bleak discussion, my understanding is that cargo fire systems are based on fire suppression only.

[Litebulbs]

It doesn't take a lot to thread a brass fitting, especially if you use a Boeing spanner!

[pacplyer]

Thanks for those great photo blow ups you guys. I continually marvel at your photo high-res skills. Composites are good stuff if they are not damaged. Carbon fiber by itself won't work good for tension loads, hence Kevlar is added to the pressure vessel of little aircraft like the pressurized lancair IV.

(To the best of my memory. Feel free to correct me.)

Click on this link to see what composite aviation oxygen bottles look like:

SCI

Note the ridges on the side of the bottles from this vender.

Breathable aviation O2 tanks are supposed to be green, I know. For some reason, in the 80's, our composite bottles were white and racked horizontal inboard of the frames in a cluster, not stored against the aircraft skin like you see in the previous space saving photos. But putting them in the side wall of the cargo compartment probably gives this -400 more lower cargo positions; I don't know.

But we are talking about millions and millions of flight hours over decades without a single known cylinder explosion. Kind of looks like a little charing on that last photo blow up. But no way to know which came first.....

Let's revisit the known Leak problem for a minute:
I think it's important to recall that the "shards" so far that we have been hearing about are not talking about actual bottle material, right? Just attached hardware like regulators etc which could have been leaking and snaked their way to a [fuel & ignition] source. So it doesn't necessarily matter whether the leak came from the crew or pax systems in that scenario, it only matters that the oxygen collected in the compartment down near the hull, got in contact with a [fuel & ignition source like a venting battery in the E&E compartment] and blew out the lower 42 skin while also buckling the floor upwards.

The intact missing oxygen bottle (which got sucked through the wall) is now being used by a chinese brain surgeon who bought it at the fish market having spotted it in the netting.

One thing's for sure: Just in case that's wrong, I'll have to start wearing my flack jacket when I stow away down there from now on.....

[NSEU]

There's something I don't understand about the CVR. It says this runs for two hours and then starts from the beginning and overwrites what's on it.

According to the report, it sounds like the QF30 had the newer type digital recorder with 120 minutes of recording (although some of the 744's may still have the older 30~40 minute tape type).

The erasure process doesn't wipe out the entire 2 hours. The tape or memory will always have the last 2 hours of the flight on it (or 30 minutes). CVR's on Qantas' 744's are operational whenever there is power on the #3 AC Bus. If the CVR circuit breaker was not pulled and power remained on the aircraft after landing, then the CVR would have continued running ... and the explosive event would have been overwritten.

[Barkly1992]

Loaded dog

Another hole (small) would have been redundant - the one in the hull was all that was required.

O2 does not burn.

Gees

[flynerd]

NSEU

Are we talking about pure aluminium or aluminium alloy? Remember that the entire fuselage is made of aluminium alloy

Yes mate, but the wall/frame of the AC might heat and cool a lot, and the only thing retarding its movement is the air around it.

The thermal conductivity of Al is about 120k (Btu per fDegree per square foot). For brass this figure is about 67k.

Thus the Al will cool quicker, and will shrink faster ( Al COE=24 as against Brass COE=19 ) around the regulator. Looking at the interface between the two there has to be considerable action/reaction at the joint. Think about how a soft little toadstool can push up through a bitumen road surface.
Now if the Al alloy used is purposely designed to contain a gas, then it could be quite different from one that only needs to behave well with a brass regulator. And I would expect the bottle alloy composition would be far removed from that used in an AC skin.

Boy, would I ever love to get the adjacent bottle and test cycle it -50C to +50C under pressure for another 1000 cycles. Thant said, I would LOVE for it to hold up and disprove my theory.

[Machaca]

Carleton composite cylinder and valve assembly (part number B42365-1):

Specifications:

Operating Pressure...0 to 1850 psig @ 70 deg F (0 to 128 barg @ 21 deg C)
Burst Pressure.........6290 psig (434 barg)

Proof Pressure.........3085 psig (213 barg)

Media....................Gaseous Oxygen per MIL-PRF-27210

Materials
...Hand Valve..........Brass & stainless steel
............................Oxygen compatible silicone O rings

...Cylinder..............Aluminum alloy, 6061-T6 liner
...........................T700 Carbon Fiber overwrap

Internal Volume......115 cubic feet (3.26m3) Oxygen @ 1850 psig (128 barg), 70 deg F (21 deg C)

Weight.................19.6 lbs (8.9 kg) maximum empty
...........................29.5 lbs (13.4 kg) maximum full

Cylinder Certification... DOT-E 11194 & DOT-CFFC

Operating Environment:

Temperature......... -65 to 160 deg F (-54 to 71 deg C)

Shock.................. 6 g operational
20 g crash

Vibration ............. 9.5 Grms each axis

Product Interfaces:

Outlet/Fill Port.......CGA-540
..........................903-14NGO-RH ext thread

Safety Outlet Port..MS33649-05 (AS5202-05)

[Barkly1992]

Loaded Dog

What sort of ignition source - a barbeque or something? Your comment just does not make sense. If there was a fire onboard the oxygen would just help to keep it alight for a short while - no doubt go out once the cabin ventilated.

Silly speculation with no reason to raise such hypothesis.

[flynerd]

Brass - Aluminum

Thanks for the bottle specs Machaca. Looks like they are not using brass-in-aluminum for that bottle.

Looks like we still need to wait on what the ATSB tell us about the bit they recovered from the galley.

[loaded dog]

Barkly1992

Point taken grumpy. I had already deleted previous post.
Turns out to be another oxygen irony
Firstly: backup system places demands upon itself (as rumour has it)
Secondly: Redeems itself by creating hole in hull that reduces potentially dangerous enriched oxygen area.

[pacplyer]

Interesting thought.
Many people have also probably wondered about what would have happened if the liberated O2 found an ignition source but have tastefully kept their thoughts to themselves. At the risk of starting a bleak discussion, my understanding is that cargo fire systems are based on fire suppression only.

Point taken grumpy. I had already deleted previous post.
Turns out to be another oxygen irony
Firstly: backup system places demands upon itself (as rumour has it)
Secondly: Redeems itself by creating hole in hull that reduces potentially dangerous enriched oxygen area.

Exactly loadeddog,

I think we're all thinking the same thing after that last enlarged photo showing [possible] black charing:

[Hypothetically speaking only:]
The oxygen [and unknown fuel source like hydrogen from a battery] ignited from a spark (perhaps in the adjacent (e&e) electrical bay) caused an explosion yielding the sidewall, causing the decompression.

The decompression snuffed out the fire before it could it could get burning significantly. The debris we see hanging out was sucked into place after the tank, sidewall and fairing fell away in the slipstream; hence, it shows no evidence of scorching and fooled the chit out of us for days.....

Plausible?



pac "Sherlock" plyer



.

[lomapaseo]

I think we're all thinking the same thing after that last enlarged photo showing black charing:

The oxygen ignited from a spark (perhaps in the adjacent (e&e) electrical bay) caused an explosion yielding the sidewall, causing the decompression.

The decompression snuffed out the fire before it could it could get burning significantly. The debris we see hanging out was sucked into place after the tank, sidewall and fairing fell away in the slipstream; hence, it shows no evidence of scorching and fooled the chit out of us for days.....

Plausible?

pac "Sherlock" plyer

You can enter your horse in the race if you want, but you don't need to immagine that you see charing in a photo to speculate that partially confined oxygen between a blanket and a side wall could ignite for a second and cause a local overpressure. Just use the "could have" and the ripped out lower rivet line.

Maybe we can get "Myth Busters" to run this experiment

[Buster Hyman]

With trepidation & clutching my Degree in Aviation Engineering from Corn Flakes University...I stray from reporting on one punters experience to asking a technical question....

Would not a fire trigger the alarms in the cockpit, or are we talking a "flash" fire here?