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The navigation unit is being sent off to maintenance, because
we have had problems of this sort once before.
The reporter found out later that the problem was not the
equipment, but resulted from the Annapolis VLF station
being taken off the air and its frequency assigned to the
Hawaii VLF. The FAA has recently published a NOTAM
regarding deviations associated with VLF navigation.
OMEGA/VLF users should consult the manufacturers of
their OMEGA equipment for information on how to avoid
navigation errors due to frequency reassignments. In
addition, the AIM refers OMEGA/VLF users to the U.S.
Coast Guard for a taped status report on OMEGA stations
at the following telephone number: 703-313-5906.
Additional OMEGA status information can be obtained
“live” on a 24-hour basis from Coast Guard Navigation
Information Service briefers at 703-313-5900.
Aircraft à la Modes
was no longer using the ABC crossing restriction for
computation. The Controller stated that it was not a
problem, and…the landing was accomplished without
further incident.
The cause of this incident was…overconfidence in the FMS
to present valid descent profile information. Correction:
Always double-check the FMS data against other available
navigation data to insure that your programming is
correct and that the aircraft is following accurate FMS
guidance.
A preventive measure for this type of mode error is a lineby-
line review of the “legs” page against the clearance and
the charts, after any change has been made to the FMS.
The reporter also makes a good point about overdependence
on automation. Many other pilots who have
reported their “automation woes” to ASRS echo this
advice.
Modes represent the different behaviors, or functions, of a
given machine system. The more functions a system has,
the more modes it has. Many everyday products have
multiple modes. For example, an electronic wrist watch
has modes for time, alarm, timer, and chronometer, each
accessed by a different combination of control inputs.
Other examples of multi-modal systems include television
remote controls, telephone answering machines,
automobile cruise controls, and, of course–the aircraft
Flight Management System (FMS).
In both highly automated aircraft systems and simpler
consumer products, multiple modes are increasingly
“mapped” to fewer controls and displays–sometimes to a
single control device. This is one reason that working
with an FMS system requires flight crews to have a
thorough understanding of the various mode interactions.
Control inputs to the FMS must bring about the desired
result in the lateral and vertical modes—or mode errors
and confusion will result.
An air carrier crew’s experience reported to ASRS
provides an example:
n We were cleared direct to ABC fix and the remainder of
the arrival. I started the airplane direct to ABC with the
FMS, then inserted the crossing restriction of 10,000 feet
on the “legs” page…and selected the arrival for use by the
FMS. What I failed to notice was that when I inserted the
arrival, the computer dumped the crossing restriction I
had inserted just a few moments earlier. Descending
through 17,500 feet, Approach Control asked if we would
make the crossing restriction, and it was immediately
obvious that we would not, as the descent line we were on
The OMEGA navigation system, which uses the VLF (Very
Low Frequency) communications band, is operated and
maintained by the U.S. Navy. At one time VLF was stateof-
the-art technology for long-distance underwater
communication with submarines, and many aviators are
still using the VLF communication signals in combination
with OMEGA navigation signals.
The entire OMEGA system is scheduled for shutdown by the
Navy in September 1997. Some airborne OMEGA users
were taken by surprise when the Navy recently shut down
one VLF station and reassigned its frequency to another
station. A charter pilot provides an example of how
navigation was affected:
n We were below the altitude where we could receive the
VOR, so we were navigating solely by OMEGA/VLF. Center
informed us we were about 20 degrees off our heading. The
next sector informed us we were 20 miles west of our
Omega: (The Beginning of) The End
Editor’s Note: Several research papers on the subject of
human interaction with automation and modes are available
free on written request from the NASA Ames Aviation
Operations Branch. Requests should be directed to: AFO
　
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