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.