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Altitude
pairs in
Thousands
Percentage of
Altitude Deviations
by Altitude Pairing
Analysis of the ASRS database indicates that there
are far more clearance misinterpretations involving
the altitude pair of ten/eleven thousand feet than any
other altitude combination—fully 38 percent of the
sample data set. The next largest category accounted
for less than 5 percent of the total deviations in this
data set.
The sample data set on which this finding is based is
composed of 191 ASRS reports describing incidents
with the following characteristics: (1) an assigned
altitude was overshot or undershot, (2) a misinterpreted
clearance contributed to the occurrence, (3)
the event occurred between 1987 and 1990, and (4)
the deviating aircraft attained an altitude 1000 feet, (or
2000 feet above FL 290) above or below its assigned
altitude. The search was confined to ASRS Full-Form
records since only these contain all of the necessary
data elements.
The adjacent figure is based on an analysis of these
Pilots misunderstand the clearance, and
controllers misunderstand the readback due to
the similar sounding phrases of one-zerothousand
and one-one-thousand.
“I believe it is very easy to confuse one-onethousand
with one-zero-thousand, and viceversa.”
“I don’t know if the controller said 10,000
but intended to say 11,000 or if he said
11,000 and I thought he said 10,000.”
Readback/Hearback
Controllers fail to note incorrect altitude in
pilot readbacks. The old hearback bugaboo…
“Voice tape reading showed that the clearance
was to 11,000 feet, but readback by
[the] captain of 10,000 feet went uncorrected.”
“Controller said ‘Oh, I should have checked
your readback.’ ”
data. Each category relates to a pair of altitudes that
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