Depressurisation event at MEL today?

[tuapekastar]

Well, as someone who was there, for what was probably the most 'major' depressurisation, world wide, in recent years...pitch never exceeded -3°, and bank, about 5°, increasing to 25° when we decided to turn off track.

The aircraft don't wallow about the sky...though perhaps they would if driven by reporters.

Thanks for this and the subsequent explanatory post. A pity reporters/TV graphic artists don't arm themselves with even just a hint of information before presenting ridiculously inaccurate scenarios.

 

[oz_mark]

I'm confused. The AC units on all 737s are internal recycle units. Can someone who actually knows, tell me, how this could have caused depressurization. I am presently in QLD with a commercial pilot who is also asking the same question.

Simple. You pressurise with conditioned air (which includes bleed air to balance what you are losing, but bleed air is rather warm, so you need to cool it down)

Even if you could just recirculate, the environment would become a bit unpleasant

 

[AaronBradford]

Haven't seen in mentioned yet, but just wanted to point out that the aircraft in question was the 734 VH-TJH.

 

[Slats7]

Actually what is entertaining is reading some of the comments from readers. It seems most are sick of the QF bashing!

+2319011!!

 

[jbryan5]

Like jbryan5, I work to keep PartnerJohnM PG, but nothing so exciting for me on a PG status run so far - and I'm doing some very soon.

As they forewarn in the safety briefing: "In the UNLIKELY event of cabin depressurisation...".

If something like this were to happen to me, I know that I'd want to be on QF and not on a potentially dodgy LCC or third world carrier.

Not taking away from the seriousness of the matter, but can jbryan5 answer the niggling question: did the masks really deliver oxygen or just muffle the screams?

The cabin was remarkably quiet. I was in 6c with a spare seat in b. So i was focussed on a suduko when the masks dropped. I was surprised but not alarmed as the plane was in control, no lurching or rolling. Realised it was a controlled descent. The loop message made it clear it was an emergency and that oxygen was needed. Again, the masks are fairly flimsy so I'm sure to have heard screams if there were any. None in the front half of plane!!

After leveling, there was palpable relief that we had leveled and were not hurtling further down. Smooth descent, good flying. Then the crew were given the clear signal with belts sign still on. That was when murmurings came from way back and crew plus a pax from business (assumed doctor or medical person) went back to calm/treat/deal with panicked pax. I noticed the woman in 6a had damaged glasses...assumed they had broken in her tight grip while descending and not knowing what was happening.

I surprised myself how calm I remained, but my heart was beating fast, blood pressure rose and thoughts of mortality and loved ones went through my head. It makes you realize how precious and fleeting life is.

To your point though, I felt more comfortable to be on Qantas when this event occurred. It was the most harrowing, but have had some other dodgy flights (perhaps there is a whole thread on this somewhere in AFF) like a Lauda air flight Spain to Portugal which landed at totally wrong angle and bounced and dipped down the runway. Or an AA flight into RDU that took 3attempts to land.

 

[NM]

I'm confused. The AC units on all 737s are internal recycle units. Can someone who actually knows, tell me, how this could have caused depressurization. I am presently in QLD with a commercial pilot who is also asking the same question.

As jb747 mentioned previously, an aircraft is not a completely sealed unit. Air leaks out and is not completely contained within the cabin. Pressurisation is maintained when the air leaking out is replaced by new air coming in. The new air coming in is "pumped" in by the air conditioning "packs". While its true that on aircraft like the 737 that the AC system does recirculate cabin air, it is also adding new air from the engine bleed system.

Simple. You pressurise with conditioned air (which includes bleed air to balance what you are losing, but bleed air is rather warm, so you need to cool it down)

The reason the new air is "hot" and needs to be cooled is not because its been heated by the "fire" in the engine, but due to the diabatic process of pressurisation. Bleed air is taken from the low or intermediate stage compressor of the jet turbine and is compressed to a very high pressure that results in it being very hot (up to 200 degrees C). Its then expanded and cooled for delivery to the cabin. Bleed air is used by more than just the cabin anvironmental control system.

So the air being fed into the cabin by the AC packs is a combination of recirculated cabin air and "fresh" compressed air from outside. The cabin pressure is maintained by controlling amount of "fresh" air being added against the air leaving the aircraft through leaks and the out-flow valve. Even with the outflow valve(s) closed, without the AC packs operating its only a matter of time for the cabin pressure to drop (also referred to as cabin altitude increasing) as air leaks from the cabin.

Cabin pressure is generally measured/reported as an equivalent altitude, such that the air pressure inside the cabin is the equivalent of being at a certain altitude. Cabin altitude is generally maintained around 6,000 to 8,000 feet. The lower the cabin altitude the more comfortable it is for passengers but the bigger the pressure differential between inside and outside. Note that 8000 feet above sea level is about the same as being at the north rim of the Grand Canyon - lots of people live at such altitudes every day, with Mexico City being around 7,400 feet above sea level. Most people can breath quite happily up to 10,000 feet or more which is why an aircraft with a pressurisation problem will descend to around 10,000 feet. Oxygen masks will automatically drop when the cabin altitude reaches a point somewhat above 10,000 feet (around 14,000 feet - jb747 may be able to tell us the exact threshold).

So for the oxygen masks to have dropped in this incident, the outflow of air from the cabin obviously caused the cabin altitude to increase above the threshold before the aircraft had descended below the threshold.

The outflow value is very important to ensure the cabin is fully depressurised before the doors are opened on the ground.

 

[oz_mark]

The reason the new air is "hot" and needs to be cooled is not because its been heated by the "fire" in the engine, but due to the diabatic process of pressurisation. Bleed air is taken from the low or intermediate stage compressor of the jet turbine and is compressed to a very high pressure that results in it being very hot (up to 200 degrees C). Its then expanded and cooled for delivery to the cabin. Bleed air is used by more than just the cabin anvironmental control system.

Such as the anti-icing systems on many aircraft - so it helps to be hot air!

 

[nlagalle]

Back when i was skydiving, 14,000 feet was the ceiling we could jump from without using oxygen for the trip up.