Remote Possibility
Sam Aurelius Milam III
Could
the commercial airliners that were hijacked on September 11, 2001 have
been hijacked by remote control? The first time that I ever heard
that theory, it was justified by reference to performance limits.
The justification went something like this. The Boeing 757 and 767
aircraft are controlled by computers. Even when an airplane isn't
being flown by the "autopilot" and the pilots are actually in control,
the computers on the airplane convert the pilot's inputs into electrical
signals that are sent through wires to actuators that actually operate
the airplane. The software in the computers contains performance
limits that prevent things like stalls and excessively steep glide paths
or excessively sharp turns. The advocate of the remote hijacking
theory, whose identity I have long since forgotten, claimed that the airplanes
that hit the World Trade Center towers far exceeded the programmed performance
limits during their final moments of flight. That, he claimed, proved
that the airplanes were not being operated by people inside the cabins
of the airplanes. In normal circumstances, the software would have
prevented such performance. He claimed the existence of anti-hijacking
code programmed into the software that would allow someone with the right
equipment and password to unilaterally assume remote control of the airplanes
during a hijacking. He claimed that the software was written to bypass
the programmed aircraft performance limits during such remote operation
and, instead, allow the aircraft to perform to its actual limits.
Thus, the airplanes were under remote control.
It
was an interesting theory. I've been able to verify some parts of
it. However, it has a couple of flaws. The first is that I
can't find any indication that the factory software contains the anti-hijacking
provisions. However, I have more to say on that subject, later in
this article. The other flaw relates to the allocation of priority
between the pilot and the software. Unlike Airbus, with which the
forgotten advocate of the theory might have been familiar, Boeing designs
the control systems so that the pilots can always override the software
driven controls by exerting "brute force" through existing mechanical linkages
that are redundant with the software-driven controls. For example,
muscle power applied to the yoke can overcome contrary software instructions
to hydraulic servos, allowing the pilot to manipulate the actual flight
control surfaces of the airplane. It was described in one document
that I read as being similar to driving a car with the power steering unit
out of service. That design philosophy gives the pilots of Boeing
aircraft ultimate control over such things as throttle settings and flight
control surfaces. There is, however, a remedy to that particular
flaw in the theory. It's entirely feasible that modified or more
powerful valves, pumps, or servos could have been covertly installed.
Then the pilots would not have been able to overpower the software-driven
controls. So, the theory still remains a possibility.
How
about that software? The Boeing 757 and 767 aircraft are, indeed,
software-controlled airplanes. The technology even makes it possible
for the airplanes to be flown by computers from shortly after takeoff through
the landing. Not only that, the computers run on software that is
loadable. Systems that are controlled by loadable software are the
Aircraft Communications Addressing and Reporting System (ACARS) (757, 767),
the Airplane Condition Monitoring System (ACMS) (Flight Data Recorder System)
(FDRS) (767), the Airplane Information Management System (AIMS) (757, 767),
the Cabin Telecommunication Unit (CTU) (757), the Digital Flight Data Acquisition
Unit (DFDAU) (757), the Electronic Flight Instrument System (EFIS) (757,
767), the Engine Indication and Crew Alerting System (EICAS) (757, 767),
the Enhanced Ground Proximity Warning System (EGPWS) (757, 767), the Flight
Control Computer (FCC) (757), the Flight Management Computer (FMC) (767),
the Flight Management Computer System (FMCS) (757), the Interactive Videodisk
System (IVS) (767), the Interactive Videodisk System (video) (IVS) (767),
and the Satellite Communication System (SATCOM) (757, 767). The functionality
of those systems can be changed by simply load-
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