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to altitudes, airspeeds, headings, frequencies,
transponder codes, flight
numbers or various other flight elements.
ATC instructions full of numbers
are often delivered rapid-fire,
received and read back by a pilot in a
noisy cockpit, then heard back and accepted
by a harried and hurried controller.
At some time in their careers,
most pilots have been waylaid by all
the numbers in a clearance such as,
“Aircraft 46261, cleared for takeoff
runway 26, wind 250 at 16 knots, turn
left heading 210, climb to 2,600 feet,
squawk 1216, contact departure on
126.2.” (Another article on “the
number crunch” can be found in
ASRS Directline #2, Fall 1991, “One
Zero Ways to Bust an Altitude.”)
Letters may pose a problem, too. B,
C, D, E, G, P, T, V, and Z are the largest
group of letters that can be easily misheard.
Other similar-sounding letters
include M and N, I and Y, F and S, and
A, J, and K. Proper use of the phonetic
alphabet can eliminate much of the
confusion of similar sounds. Still,
similar-sounding words may be misunderstood,
especially when transposed,
for example, Delta Alpha and
Alpha Delta, and Kilo Echo and Echo
Kilo.
Take a Number, Please
Where do aircraft numbers come
from? FBOs, general aviation aircraft,
and most non-scheduled air taxis use
the aircraft tail numbers or N-numbers
(the numbers or the number-and-letter
combination) as their radio
callsigns. Most scheduled air taxis and
commuter and air carrier airlines use
flight numbers rather than N-numbers
as their callsigns.
The FAA does not assign the flight
numbers used by most air carriers and
commuters. If it did take on this task,
it would probably have as much difficulty
as the air carriers themselves. Assignment
of flight numbers is typically
a function of an airline’s marketing
department. Sometimes it appears that
Marketing chooses the quickest, easiest
method of assigning numbers to
newly-created flights. Consequently,
some flight numbers have only one
number that is different, some have
numbers that are transposed, some
just happen to sound similar, even
though they may contain few, if any,
of the same numbers. This problem
appears to be increasing, as airline
mergers and buyouts have led to operators
blending flights under the
same carrier name, but with a decreasing
pool of available flight numbers.
For example, air carrier A buys commuter
B and gains 30 new flights per
day. The easiest way to assign flight
numbers is to take a block of unused
numbers, say, 4101-4130, and assign
those numbers in order of departure
time. Often odd numbers are assigned
to one compass direction, and even
numbers assigned to the opposite direction.
This can apply to North-South
flights and to East-West flights. So, for
our fictitious air carrier A, SFO-PDX
flights could be 4101, 4103, 4105, etc.,
and PDX-SFO flights could be 4102,
4104, 4106, etc. If air carrier C, which
may also have undergone mergers,
also has flights to and from PDX or
SEA at approximately the same times
using the same or similar block of
numbers, there is a potential for major
confusion while these aircraft are sharing
the same airspace.
The FAA does assign aircraft tail
numbers. Usually the numbers are assigned
at random. However, owners or
operators of corporate, FBO, or air taxi
aircraft may request specific or “personalized”
N-numbers, like many state
vehicle license plates. The result can
be a whole family of aircraft with similar
or similar-sounding numbers and
letters. These aircraft look very impressive
sitting all in a row on the ramp,
but their numbers can create a nightmare
for both pilots and controllers if
several of the aircraft depart in the
same direction at the same time.
8 Issue Number 8
What is Similar?
“Similar” generally means having a
resemblance to one another or to
something else, or like but not completely
identical.
What seems or sounds similar to
one person may not sound similar to
another. Hence, a pilot may be utterly
confused about which aircraft a controller
is giving instructions to, while
the controller is frustrated and impatient
with the pilot’s hesitation or failure
to comply with an instruction.
What sounds similar to a pilot in a
noisy cockpit may not appear to be a
problem to a controller looking at a
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