How unusual were MH370's final ATC exchanges? There are 2 noteworthy occurrences in the ATC radio transcript within just minutes of a 3rd noteworthy, quite unprecedented occurrence in aviation history:


At 01:07:55 MH370 unnecessarily reported maintaining FL350 a second time
At 01:19:29 MH370 famously only replied "Good Night, Malaysian 370" without reading back the frequency for the handover, although this is an ATC instruction that should be repeated (see below)
At 01:20:33 MH370 "goes dark" (losing XPNDR, ACARS and SDU).


This temporal proximity caught my attention and left me wondering if the 2 ATC transmissions cited above are out of the ordinary (and thus a possible precursor of what was to come).



background information:


  What are some important things controllers usually expect you to read back?
  • Clearances: headings, vectors, altitudes, transponder codes, and radio
  frequencies. (a288)





  « The last radio transmission “Good Night Malaysian Three Seven Zero”
  was spoken by the PIC. However, he did not readback the assigned
  frequency, which was inconsistent with radio-telephony procedures. »
  (safety investigation report, p.431)




questions


Please cite what (if any) ATC rules have not been followed.  
From your experience as flight controllers or pilots, how unusual is that ? Is this rather unheard of or occurring fairly regularly ?  
Is the handoff procedure described below by the 767 Captain technically correct and does the missing step 3 permit the conclusion that step 2 was not executed either ?



  @Edward_767, a 767 captain, has closely studied the audio recordings
  of the MH370 radio transmissions. He is convinced that the second
  altitude call at 01:07:55 was NOT made by the Captain, but by the
  First Officer, possibly because the captain was not in the cockpit at
  that time. His theory is that the first officer made the call because
  he was not sure whether the captain had made the previous altitude
  call at 01:01:14.
  
  He makes another interesting observation regarding the final exchange
  where MH370 is handed off to HCM. He states that the typical radio
  procedure would be to  
  
  
  receive the new frequency (in this case, 120.9 MHz)  
  dial it into the radio as the standby frequency  
  read back what frequency was dialed in as part of the confirmation of the handoff  
  listen for the final transmission  
  hit the radio switch which swaps the active and standby frequencies  
  and call ATC on the new frequency.  
  
  
  In the transmission that acknowledged the handoff to HCM, the captain
  did not read back the frequency. Ed believes this is because the new
  frequency was never dialed into the radio, as the captain never
  anticipated making a call to HCM ATC. Ed believes the missing
  frequency during the acknowledgement of the handoff is yet another red
  flag.
  
  The time between the 2 transmissions would be less than 15 seconds. It
  was 13 seconds at MH370’s previous controller handoff, where the new
  frequency was read back. At the handoff between Kuala Lumpur and Ho Chi
  Minh, the pilot acknowledged the handoff, but never read back the new
  frequency, and the call to Ho Chi Minh was not made. That leaves a window
  of only about 15 seconds for the abnormal event that caused the turn back.




ATC transmission log (full transcript incl. taxiing here)
I have not heard the actual audio in context; I've only read the transcripts.

However, from the transcripts, I see the timing between some of their calls as a bit confusing and unusual.

01:01:14 | MH370 | Malaysian 3 7 0 maintaining level 3 5 0
This is slightly odd, because 11 minutes earlier they were told to climb to 350.   I do not think it is common to report when you have finished your climb.  ATC can see you leveling off at your assigned altitude.

Because of this unexpected, slightly odd call, ATC responds with:01:01:19 |  ATC  | Malaysian 3 7 0
("You called me.  ATC is here; I hear you loud-and-clear; Go ahead: What is your request?")

There is no response for 6 and a half minutes. This could reasonably be because MH370 has no request, and doesn't need anything from ATC, or perhaps they are having radio difficulty and did not hear ATC's call?  The most extreme interpretation would be that there is currently something happening in the cockpit that is distracting them.  Perhaps a lack of oxygen? Or some other distraction?

After 6 and half minutes, MH370 makes the same call again:
01:07:55 | MH370 | Malaysian...3 7 0 maintaining level 3 5 0

Again, this reads like the preface to a request.  Perhaps they didn't hear ATC last time, and are calling in again with a request? Perhaps they're suffering from hypoxia, and don't even remember that they already reported in?

ATC quickly responds the same way again:
01:08:00 |  ATC  | Malaysian 3 7 0
(I'm still here, I still hear you.  Go ahead with your request.)

Silence again for 11 minutes, before ATC gives them a normal instruction to change to the next frequency.  MH370 responds normally by acknowledging the instruction. At this point, we have confidence that their radio is working, and can probably rule out technical problems for the earlier lack of a request.

Technically, they should read back the frequency, but it is not at all unusual for a pilot to just acknowledge when he is confident he copied it right, such as a crystal clear transmission, or a route he's flown many times and a frequency he knows well.  Or, the lack of confirmation could indicate hypoxia or another mental impairment. Or that he has no intention of copying the frequency or changing to it.

My biggest point of concern and confusion is that MH370 twice reports their altitude, which sounds like they're calling ATC to make a request, and then says nothing more for 6 and 11 minutes.  The significance of this is not at all clear.  Either hypoxia or radio/electrical problems would explain this.  A cockpit struggle does not seem apparent, but some sort of distraction in the cockpit could cause them to start a conversation but never complete it.

My personal opinion, without much supporting evidence, is mental impairment, potentially due to failed pressurization, carbon monoxide, or other contamination like engine-oil in bleed air, which has been known to cause neurological damage.