9-11: United Flight 175; Transpoder Code 3321, an interesting anomaly.


Retrospectively, the attack against New York City was well planned and well executed.  Both hijacked aircraft, American Airlines flight 11 (AA 11) and United Airlines flight 175 (UA 175), took off within a few minutes of each other from the same airport, Logan in Boston, thus increasing the likelihood that both planes, if delayed, would be delayed the same amount of time.

The narrow exit corridor from Logan increased the likelihood that both planes would be on the same frequency at the same time.  It was possible, but not a given, that Marwan Al Shehhi, as a passenger on UA 175 could listen to the air traffic control communications from the cockpit and hear the voice of Mohammed Atta.

(See “9-11 United Airlines; Cabin Channel 9, a policy change,” for perspective.)

Al Shehhi, in the cockpit, knowing that Atta was flying AA 11 and knowing where to look, would have easily seen the fireball from the impact of AA 11. At that moment the transponder code on UA 175 changed to 3020, thus creating an unknown radar target in the sky, a Mode C intruder, for air traffic control.  That was tactical planning, well executed.  There was no need for a second code change, but there was one a minute later.

Why that second, tactically unnecessary, code change was made has never been and may never be explained. But happen it did.  For a possible explanation we turn to the hijackers’ preparation prior to the attack.

The Final Hours
(link updated Feb 7, 2014 to prosecution exhibit BS01101T for the Moussaoui trial)

The coordinated attack against the World Trade Center was swift, violent, and devoutly executed.  That devotion was revealed in the “The Last Night” instructions to the hijackers.  Those detailed, explicit instructions contained many Quran references.  Such devotion, in meticulous detail, to the final hours suggests that such devotion, detail, and meticulous planning also pertained to the planning for the actual attack.

Mohammed Atta and Murwan al Shehhi had countless quality hours together, measured in days, weeks and months prior to the attack against New York City to plan each and every detail and to do so in light of the Quran.

A Key Observation

A significant clue to that devotion and detail was found by a correspondent, Tom Lusch, during an extended email conversation about hijacker tactics and techniques.  Tom was of the opinion that the code change on United Airlines flight 175 was a matter of cockpit unfamiliarity on the part of the hijackers.  My view was different, based on an in-cockpit demonstration by the senior United Airlines pilot.  I thought the code changes to be intentional, but had no understanding of why the particular code, 3 3 2 1, came about.

Tom consulted an informed website and observed that Chapter 33 (The Combined Forces), Verse 21 of the Quran offered an explanation.  Here is a translation by Yusef Ali, take from the “Quranic Arabic Corpus.”

“Ye have indeed in the Messenger of Allah a beautiful pattern (of conduct) for any one whose hope is in Allah and the Final Day, and who engages much in the Praise of Allah.”

And one by Muhammad Sarwar from the same source:

“The Messenger of God is certainly a good example for those of you who have hope in God and in the Day of Judgment and who remember God very often.”

Those and other translations of that chapter and verse speak specifically to the “Day of Judgment,” the “Last Day,” the “Final Day.”

The correlation to the events of September 11, 2001, is chilling, especially so when we consider Mohammed Atta’s choice for his lead plane that day.

American Airlines Flight Eleven.

American Flight Eleven was, in this context, “America, Quran Chapter 1 (The Opening), Verse 1.”

“In the name of Allah, Most Gracious, Most Merciful”

And the choice of planes for Marwan al Shehhi was equally chilling

United Airlines Flight One Seventy Five

In the context of the Quran, United Flight One Seventy Five was “United [States] Quran Chapter 17, Verse 5, rendered in the Yusef Ali translation as:

“When the first of the warnings came to pass, We sent against you Our servants given to terrible warfare: They entered the very inmost parts of your homes; and it was a warning (completely) fulfilled.”

And in the Mohsin Khan translation

“So, when the promise came for the first of the two, We sent against you slaves of Ours given to terrible warfare. They entered the very innermost parts of your homes. And it was a promise (completely) fulfilled.”


So what do we make of this?  I asked Robbyn Swan, coauthor of The Eleventh Day, a correspondent with ties to scholars specializing in Islam to help.  Two scholars responded to her.  They both agreed that the hijackers were “rational actors” and “practical in the extreme.”  One doubted that they would “get carried away with such numerological mysticism.” The other observed that the change “increased the difficulty and complexity of the operation.”

And that latter observation is precisely the point.  The second code change from 3 0 2 0 to 3 3 2 1 was tactically unnecessary and while not difficult did add an additional task that wasn’t needed.  So why would Al Shehhi take the time to do that as he was preparing to turn UA 175 around and plummet into the World Trade Center from high altitude?  It was not the act of a “rational actor,” but someone motivated beyond the tactical necessity of the day.

The scholars and I are in agreement that the hijackers knew what they were doing and were rational and “practical to the extreme.”  One scholar made the additional observation that there are hundreds of verses that refer to “final judgment” or “final day.”  We do not know why that particular chapter and verse, but the demonstrable evidence is that Al Shehhi took a measured action to transmit that particular code.

And that leads us to the impact on air traffic control.

A Message for Air Traffic Control and History

The numbers of the code changes had no special meaning to air traffic control except that they did not belong in the scheme of things. An “intruder” is not unusual and typically resulted from pilot error or inattention.  The method of handling an intruder was simple; ask the pilot to recycle his transponder.

Which was done, except that UA 175 had become a Mode C intruder, 3 3 2 1. The intruder came to the attention of air traffic control. According to a Washington Post article on September 17, 2001, “a controller…shouted, There’s an intruder over Allentown.” (Lane, et al, “A Sky Filled With Chaos, Uncertainty and True Heroism”)

Any message intended by the code change was not for air traffic control. It was an enduring message for history.


9-11: UA175; 42d Street, the Allentown PA story


The female lead in the stage play “42d Street” was a young woman from Allentown, PA. She was sometimes referred to as “Allentown.” I use the 42d Street reference to remind me of the story of three planes in the skies over Allentown on the morning of September 11, 2001.

The Three Planes

It has long been known that two of the hijacked planes on 9-11, United 93 and United 175, crossed paths on 9-11.The location has never been given much thought, but it was over Allentown. What is not known is that a third commercial aircraft of note also crossed paths with both planes in the same time frame over Allentown.

The three planes were well separated in altitude, but closely spaced, in time, United 93 westbound, United 175 southbound and a third plane eastbound.  United 93 was still climbing, United 175 was at altitude, and the third plane was midway between the two, altitude-wise. None of the three aircraft were a danger to the others and none were aware of their proximity.

The third plane was Midwest Express Flight Seven (MIDEX 7) the centerpiece of Lynn Spencer’s book Touching History.

Midwest Express Flight Seven

I will tell the MIDEX-7 story in an article currently in draft. For now, let me just sketch in the details.

According to the pilot’s account as told to Spencer, the path of MIDEX-7 was intertwined with and integral to the final moments of the descent of United 175 into the World Trade Center, South Tower. The story as told was sufficient compelling that Darlow Smithson Productions wanted to make it a vignette in its “Voices From the Air” production for National Geographic.

I was the consultant for Darlow Smithson and they tasked me to pull together the audio communications supporting the MIDEX-7 story. I found little of substance in the collection of air traffic control files provided to the 9/11 Commission and reported to Darlow Smithson that the vignette should not be used.

So, what happened?  As with many eyewitness and participant accounts of the day, the pilot’s recall compressed time and conflated events. He had internalized what he recalled, but what he recalled is not quite what happened, a story for another day.

The MIDEX 7 story is an interesting one, but it is not the story that Spencer told. MIDEX 7 was still over New Jersey when United 175 impacted.

MIDEX 7 actually crossed paths twice with UA 175, but it is the first crossing that concerns us here. So, back to Allentown we go.

United 175

At the moment American Airlines 11 impacted the World Trade Center, North Tower, the transponder code for United 175 changed to 3020.  That one change was all that was tactically necessary to introduce a Mode C intruder into the national airspace system. Such an intruder, a transponding plane without an associated data block, is not uncommon and controllers deal with the situation by asking the pilot to recycle the transponder. That is what the controller did that morning, except nothing happened.

By that time, the transponder code had changed to 3321, an unnecessary and extraneous tactical move by the hijacker pilot.  I am drafting an article to help us understand why the second code change was made.

Thus, by the time UA 175 was asked to recycle its transponder the code had twice changed but not to the original code as instructed.  The plane was a dangerous intruder into the National Air Space System and that fact was noted by an air traffic controller  when United 175 was over Allentown, PA.

“A Sky Filled With Chaos, Uncertainty and True Heroism”

On Monday, September 17, 2001, Charles Lane, Don Phillips and David Snyder, Washington Post Staff Writers published an article with the title, above. It was about specific hijacked planes but the skies over Allentown suddenly became the focus.

The authors report that while the controllers were looking for American 11, “a controller glanced at another radar screen and shouted, “Look. There’s an intruder over Allentown.”

That was a reference to United 175 under its new code of 3321, a lethal intruder which made a broad U-turn over New Jersey and then plummeted at 6000 feet per minute in the direction of 42d Street. Intruder code 3321 impacted the World Trade Center, South Tower at 9:03 EST.

9-11: United Airlines; Cabin Channel 9, a policy change


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.

Recent Information

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.


9-11: UA175; Boston Center; Reaction in Real Time, Ground Stop


This short article documents for researchers and historians how Boston Center (ZBW) learned of the impact of a second airplane [UA175] into the World Trade Center and its immediate reaction to a request from New York Center (ZNY) to ground stop everything.

The Impact

ZBW learned of the second impact as it occurred, as recorded at the Severe Weather position Traffic Management Unit (NARA Batch 5, tape 3007988-s2, certification length 42 seconds). ZBW was on the phone with ZNY discussing the fact that ZNY was not taking any hand offs, any overflights. Wanda at the Air Traffic Control System Command Center (Herndon Center) facilitated the conversation. Thus, Herndon Center also learned of the second impact in real time. Here is that three-way conversation.

0902 ZBW ZNY second WTC struck

The Reaction

Within seconds, ZBW Traffic Management Unit complete a series of brief calls to airports in its area ordering a ground stop. There is an important aspect to the notifications. The traffic manager at ZBW started by declaring that all commercial traffic was ground stopped.  By the third call he had shortened his message to a requirement to “ground stop everything.” This is an example of how FAA facilities became confused as to whether or not the ground stop and the later order to land all commercial air traffic pertained to the military, law enforcement, and first responders. Here is that series of calls.

090340 ZBW Ground Stop Calls to Towers

I will add to this article as I find examples of FAA facilities asking for clarification.