[ventus45]

Aircraft Oxygen systems.

From FAR § 91.211 Supplemental oxygen.

(b) Pressurized cabin aircraft.

(1) No person may operate a civil aircraft of U.S. registry with a pressurized cabin—

(i) At flight altitudes above flight level 250 unless at least a 10-minute supply of supplemental oxygen, in addition to any oxygen required to satisfy paragraph (a) of this section, is available for each occupant of the aircraft for use in the event that a descent is necessitated by loss of cabin pressurization; and

(ii) At flight altitudes above flight level 350 unless one pilot at the controls of the airplane is wearing and using an oxygen mask that is secured and sealed and that either supplies oxygen at all times or automatically supplies oxygen whenever the cabin pressure altitude of the airplane exceeds 14,000 feet (MSL), except that the one pilot need not wear and use an oxygen mask while at or below flight level 410 if there are two pilots at the controls and each pilot has a quick-donning type of oxygen mask that can be placed on the face with one hand from the ready position within 5 seconds, supplying oxygen and properly secured and sealed.

(Note this bit)
(2) Notwithstanding paragraph (b)(1)(ii) of this section, if for any reason at any time it is necessary for one pilot to leave the controls of the aircraft when operating at flight altitudes above flight level 350, the remaining pilot at the controls shall put on and use an oxygen mask until the other pilot has returned to that crew member's station.

<end of FAR extract>


The timing of the first "maintaining FL350" and the second "maintaining FL350" may indicate the interval when the mask was put on, if it was indeed put on.

When you think about it, if a planned event, it would be wise to "ensure" that the mask mike was working, "before" the expected hand-off from KL to HMC. You would not want to be fumbling about then if you had already de-pressurised, now would you.

A forensic audio spectral analysis of the ATC tape "might" be able to confirm that, irrespective of whether or not the two transmissions were made by the same person, or not.  

They might have to do a lot of tests with many test subjects, both pilots and non-pilots, both with mask off, and with mask on, and also both pressurised and de-pressurised, to build up a spectral data base, to enable them to determine that though.

Depending on the "quality" of the ATC tapes, it would be an obvious thing to do, now wouldn't it.

OK, they (the investigators) have already done it, and they have the results, they just aren't telling us. I wonder why ?


ADDED 54 MINUTES LATER

OK, perhaps they may not have got anything very useful.
I did suggest they needed to do the tests both mask on and mask off, and also pressurised and de-pressurised.
That is a 2 x 2 matrix of 4 conditions.
Here is the reason for the FOUR tests.
We have to test for the "NULL" condition at the second "maintaining FL350", to see if it is either the same, or different, at the hand-off.

Consider the time-line:
Time 00:50:08 (16:50:08 utc) MH-370 was cleared to the planned cruising altitude of FL350 by Lumpur Radar.
Time 01:01:17 (17:01:17 utc) MH-370 reported maintaining FL350.
Time 01:07:29 (17:07:29 utc) Last ACARS message from MH-370.
Time 01:07:56 (17:07:56 utc) MH-370 reported maintaining FL350 for the second time.
Time 01:19:26 (17:19:26 utc) Lumpur Radar instructed MH-370 to contact Ho Chi Minh Air Traffic Control Centre, Vietnam:
"Malaysian Three Seven Zero contact Ho Chi Minh 120 decimal 9 Good Night".
Time 01:19:30 (17:19:30 utc) One of the flight crew members replied:
"Good night, Malaysian Three Seven Zero."
Time 01:20:31 (17:20:31 utc) Radar (primary or secondary ?) shows MH-370 at IGARI.
Time 01:20:36 (17:20:36 utc) Mode-S symbol for MH-370 dropped off the radar display.
Time 01:21:13 (17:21:13 utc) Last SSR symbol for MH-370 dropped off the radar display.
Time 01:37:00 (17:37:00 utc) Next scheduled 30 minute ACARS that was never sent.

There is ONLY a 27 second window (time interval) between the sending of the last ACARS at 01:07:29 (17:07:29 utc) and the "second - maintaining FL350" at 01:07:56 (17:07:56 utc).
If it was a "planned event" - this may be the key.

He would know that the cruise ACARS was scheduled for about that time, so he would be waiting for it, he would have the screen up and be watching for it to go out, ie, be transmitted. 

Only then, after it had gone, could he act.  
He could not de-pressurise any earlier, otherwise ACARS would report it.
So he now has 30 minutes before MAS Flight Following might even suspect something.

He then dons the mask, and checks oxygen flow, and then checks the mike, transmit "maintaining FL350" again.

TWENTY SEVEN SECONDS is just the rite amount of time to do that, slowly, methodically, and carefully.

So, 4 seconds later, at Time 01:08:00 (17:08:00 utc) he (whoever it is) is ready.

Let the games begin, any time from now....................