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Ames Research Center�
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National Aeronautics and

Permit No. G-27
NASA
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BULK RATE
Number 189 February 1995
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 From NASA's Aviation Safety Reporting System�
A Frequency of Digits
Among the most famous one-liners from the black-and-white
movie era was W.C. Fields’ sly flattery of Mae West, as he
admired her hand: “Ah, what symmetrical digits you have!” Yet
digital symmetry–identical or very similar number elements
used in radio frequencies–is a less amusing subject to many
pilots, especially when similar digits are assigned to ILS
frequencies for parallel runways.
The limited number of ranges available for ILS frequency
assignments is undoubtedly one reason that repetition of
numbers, or use of similar numbers, occurs. Regardless, a recent
report to ASRS from an air carrier pilot illustrates the problems
that can ensue when a flight crew fails to verify navigation radio
settings because of high workload during the final approach:
■ We had been deviating around cells since we had reached the
southern tip of Florida. We were given an approach clearance for
the ILS to 9R. The First Officer [F/O] was flying the aircraft and
I was concerned with a cell that had been on the field and at the
time was located at the boundary of the airport. I had looked at
the F/O's radios and had seen the proper inbound course dialed
in and a frequency of 110-something. Before we got to the initial
approach fix, the Approach Controller told us to turn right 20
degrees and intercept the 9R localizer. It was at this time I realized
that my F/O had tuned in 110.3 instead of 110.9, the
localizer for 9R. We had initially been on the localizer for 9L. Fortunately,
approaches were not being made on that runway at the
time. I think a major contributing factor to this incident is the fact
that both localizer frequencies for landing in the same direction
start with 110 (110.9 for 9R and 110.3 for 9L). Interestingly
enough, both localizer frequencies landing to the west start with
109 (109.1 for 27R and 109.5 for 27L).
Our reporter wrote that he had never previously encountered
this situation. However, a cursory look through the approach
plates for a number of major metropolitan airports revealed quite
a few instances in which parallel runway ILS frequencies are
similar.
Some parallel runway ILS frequencies have the same first three
digits, and only the decimal digits differ:
33R/L 111.95/111.7 Baltimore-Washington Int’l
11R/L 110.3/110.7 Minneapolis-St. Paul
8R/L 109.9/109.3 Atlanta
One ILS frequency was found with identical decimal digits, and
nearly identical first three digits:
2R/C 111.75/110.75 Nashville, TN
Dallas-Ft. Worth has an interesting situation with its parallel
cluster “Right” ILS runways assigned the same first three digits,
and its parallel cluster “Left” ILS runways also assigned the
same first three digits:
18R/L 111.9 / 110.55 Dallas-Ft. Worth
17R/L 111.35 / 110.3 Dallas-Ft. Worth
Miami International appears to be the only airport with a double
whammy–its easterly parallel runways are assigned frequencies
with identical first three digits, and the reversed, westerly parallel
runways are also assigned frequencies with identical first
three digits.
Frequencies with similar or identical number elements can certainly
play a role in incidents such as the one above reported to
ASRS. However, careful selection and visual verification of radio
frequencies (especially with electronic displays), combined with
verbal callouts of the navaid frequency, go a long way toward
preventing these types of incidents.
From Angels to GRUMPs
CALLBACK’s readers occasionally like to embellish on stories
and safety tips offered here, and this was the case with our December
‘94 issue (#187), which inspired several letters of interest.
In response to our “Guardian Angels” item about a Cherokee Six
internal fuel drain actuator that was accidentally activated by a
passenger, one pilot shared this more uplifting experience:
✍ I owned a converted Twin Comanche. I had been to Florida,
from where I filed IFR in lousy weather back to North Carolina.
Level, at altitude and in clouds, I shifted to the outboard tanks,
and shortly thereafter the right engine went dead, windmilling.
The fuel gauge showed a full tank. I shifted back to the main
　
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