The interphone and P.A. system were linked to the CVR...

§ 25.1457 Cockpit voice recorders.
(a) Each cockpit voice recorder required by the operating rules of this
chapter must be approved and must be
installed so that it will record the following:
(1) Voice communications transmitted from or received in the airplane
by radio.
(2) Voice communications of flight
crewmembers on the flight deck.
(3) Voice communications of flight
crewmembers on the flight deck, using
the airplane’s interphone system.
(4) Voice or audio signals identifying
navigation or approach aids introduced
into a headset or speaker.
(5) Voice communications of flight
crewmembers using the passenger loudspeaker system, if there is such a system and if the fourth channel is available in accordance with the requirements of paragraph (c)(4)(ii) of this section.
(6) If datalink communication equipment is installed, all datalink communications, using an approved data message set. Datalink messages must be recorded as the output signal from the communications unit that translates
the signal into usable data.


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Note that this is the requirement for CVRs that was effective on January 1, 2012.

The interphone and P.A. system were linked to the CVR...

§ 25.1457 Cockpit voice recorders.
(a) Each cockpit voice recorder required by the operating rules of this
chapter must be approved and must be
installed so that it will record the following:
(1) Voice communications transmitted from or received in the airplane
by radio.
(2) Voice communications of flight
crewmembers on the flight deck.
(3) Voice communications of flight
crewmembers on the flight deck, using
the airplane’s interphone system.
(4) Voice or audio signals identifying
navigation or approach aids introduced
into a headset or speaker.
(5) Voice communications of flight
crewmembers using the passenger loudspeaker system, if there is such a system and if the fourth channel is available in accordance with the requirements of paragraph (c)(4)(ii) of this section.
(6) If datalink communication equipment is installed, all datalink communications, using an approved data message set. Datalink messages must be recorded as the output signal from the communications unit that translates
the signal into usable data.


You are not allowed to view links. Register or Login

How many times did Cooper use the interphone ?  Once?  Everything's OK ... ?  I thought it was Tina who did all the talking ?

If not mistaken, Cooper used the interphone at the start of the flight, then used the PA at 8:05..

I think Cooper used the Interphone at 8:05 also.  But the cockpit crew apparently used the PA when they tried to contact him at that time.

After Tina was sent to the cockpit they spoke with Cooper. it's in the transcripts..

7:42 HJ advised (by inter-phone) he was having difficulty getting steps down...

True.  Apparently the only way that Cooper could talk directly to the cockpit was by means of the Interphone.  But since he probably wasn't close to the Interphone all the time, the crew probably used the public address system to get his attention.

Made some radio jumps last Saturday taking different payloads on each jump. Note the leg pouch and other asymmetrical attached gear. On radio jumps I usually open my canopy right after clearing the plane to give maximum hang time for working ham radio contacts. Yesterday I decided to extend my freefall time to further examine the effects of asymmetrical air loads on jumper stability. As I approached terminal velocity the leg pouch exerted very noticeable roll and yaw forces that I had to counter to remain stable. At night with no clear horizon reference I think a tumble would have occurred. I still think Cooper’s best bet would have been to go down the stairs backwards and pull. He’d get an open canopy in a few seconds. No worries about freefall stability. If you look closely at the open canopy photo you can see my long wire trailing HF antenna streaming behind me.
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377

I do think Cooper intended to jump shortly after takeoff. Can you imagine his anxiety as that opportunity quickly passed? Everything was changing and his control of the situation was vanishing.

I prefer leisurely flights when I jump. I miss the slow rides up in DC 3s and heavily loaded Twin Beeches. I occasionally slumbered during the long rides. Once, at the World Free Fall Convention I dozed off in a slow climbing DC 3. Half an hour later, as we turned onto jump run, I was jolted awake by a gorgeous Japanese women jumper giving me a wet kiss and yelling “my husband is coming, you have to leave right now”. Laughter erupted as I bolted upright and stumbled towards the door. For the rest of the week jumpers would shout “my husband is coming” when they saw me. Embarrassing.

377

That's hilarious !  I tell ya, I am THE biggest coward when it comes to flying, heights, roller coasters...you name it.  I have often thought that there may not be a dollar amount that could get me to jump.  Now, a big wet kiss from a beautiful woman.....hmmm...

I do think Cooper intended to jump shortly after takeoff. Can you imagine his anxiety as that opportunity quickly passed? Everything was changing and his control of the situation was vanishing.
377

You seem to forget what is holding this hijacking together - its the threat of a bomb in closed quarters. If C bails early in a more urban environment not only is the risk higher he can be surrounded, but his bomb is neutralised and offers no protection. His bomb is central to the whole plan... and dictates when and where he will bail.

His personal protection relies on the bomb and clearing the plane of people. Only then is he free to conduct his escape without interference. The further he gets away from an urban area, the better. He does not promise he will even take the bomb with him. This creates the idea that he has left the plane but his bomb is still active on the plane. This keeps people from even going to the back to look! Meanwhile Cooper is gone/has bailed. None of that can happen if he bails early ... and Cooper knows it. 

The 'tell' I focus on is Cooper's psychology. It is key to his plan and his thinking process. His bomb is a fundamental element in his plan. His choice of Tina versus Flo or Hancock is another critical 'tell'. Nyrop has guaranteed cooperation. Why change what has become a working strategy?  Bailing early changes what is coming together as a working plan! Why change a working scenario? He wont and he doesn't, by choice. He's a smart guy who stays with what is working . . . bailing early would only change the whole dynamic. Cooper cannot afford to change what has come together.     

What Cooper did and did not do - his choices - tells us a lot about Cooper's thinking process. Bailing early was never part of his plan. There is nothing that favours him doing that, and much that favours him not doing that!  Cooper boarded the plane with the skeleton of a plan. When things came together he chose to stick with what was working ... he had no rigid plan to bail early. He worked the situation as it developed...