United Airlines Cabin Channel 9, Flight Deck, has long been available to passengers interested in communications from the cockpit to air traffic controllers. That line of indirect communication was likely available to hijackers Mohammed Atta and Marwan al Shehhi on September 11, 2011. As of January, 2014, the ability to listen to Flight Deck is no longer available on United Airlines flights, at least on Airbus 320 flights.
During the period Jan 21-29, 2014, I flew cross county on United Airlines fully expecting to listing to air traffic control conversations as I always have in the past. I flew Airbus 320 flights both ways.
On the outbound trip to the West Coast the Airbus had just been reconfigured inside. I asked a flight attendant where the plug for audio channels was and she pointed to the arm rest. Except there was no plug in, much to her surprise.
After looking a bit she consulted with the rest of the crew and reported back that there was no longer an in-flight capability for either audio channels or movies. She reported that United had made a corporate decision to no longer provide such service because the majority of passengers used their own electronic devices. The plane was wifi-capable, at a cost, of course.
On the return trip the Airbus had not been reconfigured and an audio plug was available, but not useable. When asked, the flight attendant responded that the in-flight audio and media equipment had been removed. That plane was also wifi-capable.
This United Airlines policy change ends an era. A quick web search suggests that there was at least a small segment of the flying population that routinely listened to Flight Deck and was unhappy with the decision to remove the audio channel equipment.
The hijacker pilots on 9/11 would have known of the existence of the Flight Deck channel because of their cross-country orientation flights in preparation for the attack. On 9/11, because of the narrow departure route out of Boston, American Airlines flight 11 and United Airlines flight 175 were on the same air traffic control frequency at the same time.
Whether or not Marwan al-Shehhi heard Mohammed Atta’s communications on frequency it is likely that the hijacker plan was that he could and that Atta’s communication, “we have some planes,” was a cue to al-Shehhi that Atta had cockpit control of his flight.