Implications of September 11 Flight Transponder Activity

Aidan Monaghan | May 14, 2017



Abstract



Introduction

Perhaps the most significant consequence of lost September 11 flight SSR data to ATC was a circumstantial impression of accused hijacker flight deck takeovers. As can be shown, aircraft Mode S transponder SSR information can be caused to vanish from ATC radar displays by other documented means. Coincidentally or not, the only September 11 flight with a continuously operative transponder (United 175) was also the only flight with a transponder able to warn away commercial flights in serious danger of collision thus facilitating its subsequent impact with World Trade Center 2 (WTC 2). This capability was enabled by its Mode S transponder dependent Traffic Collision Avoidance System (TCAS). The TCAS feature warns aircraft flight crews of mid-air collision risks posed by similarly equipped nearby aircraft.

From the Federal Aviation Administration (FAA):

The four September 11 flights reportedly contained what are known as Mode S transponders:

Just prior to year 2000, a major modernization of the FAA’s entire Air Route Traffic Control Center computer system in the continental United States was scheduled to be completed.

From the FAA:

In the case of UA 175, although its transponder-broadcasted flight ID number reportedly changed several times following its alleged hijacking, the transponder itself continued to transmit unlike the other three flights, making it known to the TCAS of nearby aircraft. Of the four wayward September 11 flights, only UA 175 experienced serious near mid-air collisions with other non-wayward commercial flights. These were Delta Flight 2315, US Airways Flight 542 and Midwest Airlines Flight 7 (5).

Again, UA 175 was the only September 11 flight with a transponder that continued to operate following its unauthorized course change, helping it avoid at least one mid-air collision with another TCAS equipped flight and thus facilitating its own impact with WTC 2.

Referring to UA 175 and US Airways Flight 542:

Delta Flight 2315 and US Airways Flight 542 were TCAS compatible Boeing 737s. Midwest Airlines Flight 7 was a TCAS compatible DC-9. The role played by TCAS during UA 175's conflicts with the Delta and Midwest flights is unclear.

Interestingly, the FAA's "Host" ATC computer system provided an ability to anticipate route conflicts like those experienced between UA 175 and Delta 2315, US Airways 542 and Midwest 7, based on filed aircraft flight plans:

UA 175’s transponder was reportedly unique to at least the United Airlines fleet, but could nevertheless be turned off. Interestingly, much of the source document containing this fact regarding September 11 flight transponders was redacted.

According to a safety bulletin published by the International Civil Aviation Authority (ICAO), a given Mode S transponder broadcast can be suppressed or “jammed” by another transponder broadcasting via an ICAO 24-bit aircraft address identical to the one already assigned to the given aircraft, causing the given aircraft’s SSR flight information to disappear from ATC radar displays, not unlike the disappearance of SSR information for all but one of the September 11 flights. The same circumstances can also cause a given aircraft’s transponder to become invisible to the TCAS systems of other aircraft.

To date, only military aircraft seem to be capable of mid-flight changes of the 24-bit aircraft address that is contained within its Mode S transponder broadcasts:

Interestingly, just prior to the September 11 attacks, French aviation authorities investigated unexplained transponder signal jamming that caused ATC SSR returns of commercial Boeing 767s to drop from radar screens.

Yet despite September 11 flight transponder signal losses, ATC was still able to track these flights (just as with the afore mentioned flights in France a year earlier), complicating official assumptions that September 11 flight transponders were manually turned off by hijackers to evade detection and interception by U.S. air defenses.

Regarding ATC coverage of American Airlines flight 77 on September 11, 2001:



Conclusion

All references to deactivated September 11 flight transponders within the 9/11 Commission Report infer that the losses of SSR ATC data were due to hypothesized manual transponder deactivation by accused hijackers following the alleged seizures of the respective flight decks.

However, these references do not support the contention that September 11 flight transponders were manually turned off by hijackers and no additional evidence has been provided. Despite the reported losses of September 11 flight transponder signal, ATC should have been able to track the flights.



References: