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percent—were altitude undershoots
(failure to reach the assigned altitude—
usually on descent). This
indicates that flight crews may have
been late in planning or execution of
the procedure.
- “The Captain began programming
the FMC when we should have started
down to Flight Level 190. Afterwards, the
Captain commented that he always tells
new copilots to begin the descent before
programming the FMC if there is any
doubt as to meeting the crossing [restriction],
and he was upset that he had tried
to program the FMC first.” (# 112925)
Point of Detection
In over half of all events in the data
set (51 percent), the error was detected
before reaching the required or specified
altitude. In 28 percent of events,
the error was discovered at the required
or specified crossing restriction
altitude. In 17 percent of events the
error was discovered after passing the
required altitude.
In those events where the error was
discovered at or before the required
crossing altitude, climb or descent
rates may have been sufficiently high
to preclude recovery before the deviation
occurred.
How Much Did We Miss By?
1. Point of Detection: The magnitude
of the altitude deviation at the
point of detection averaged 2,400
feet, with a median of 1,500 feet.
2. Point of Maximum Excursion: The
altitude deviation magnitudes at the
point of maximum excursion were
examined using methods employed
by the 1982 ASRS study, and were
found to be exponentially distributed,
with a mean deviation of
approximately 2,500 feet. The mean
for crossing restriction deviations at
point of maximum excursion was
substantially larger (approximately
1,400 feet greater) than the mean
for undifferentiated altitude deviations
(1,080 feet) reported in the
1982 ASRS study on altitude deviations.
The median for the point of
maximum excursion was 1,500 feet.
Controller Actions
ATC did not intervene, or was not
required to intervene in order to avoid
airborne conflict in 43 percent of
incidents in the data set. (This supports
the research team’s subjective
assessments of incident severity.) In 60
percent of incidents (100 of 168), the
flight continued the climb or descent,
with ATC concurrence.
- “We were given descent clearance from
Flight Level 230 to 13,000 feet by ATC, on
the MINEE 1 Arrival (MCA). We read back
‘Descending out of Flight Level 230 for
13,000 feet’ and dialed 13,000 feet in the
altitude select and began the descent. ATC
then told us to contact Approach. We
checked in with Approach and stated we
were descending to 13,000 feet. As we
passed through 14,700 feet, Approach
asked us if we were level at 15,000 feet, we
replied ‘Negative, we are descending
Issue Number 10 13
Table 1—Human Errors
Based on 233 Citations from 171 of 172 Reports
Human Errors CitationsPercent
Exercised poor judgment 43 25.1
Neglected to cross-check data 42 24.6
Delayed implementing procedure 41 24.0
Misunderstood clearance 35 20.5
Other (unspecified) 32 18.7
Forgot clearance 15 8.8
Did not read, or mis-read chart 14 8.2
Not stated or ambiguous 9 5.3
Did not hear clearance 1 0.6
Looked at wrong chart 1 0.6
TOTALS 233 136.4%
Note: Multiple citations are possible in this category, thus the total
number of citations exceeds the number of reports.
through 14,700 feet for 13,000 feet.’ We
also said we would stop the descent and
return to 15,000 feet, if necessary. Approach
replied, ‘No, descend and maintain,
13,000 feet.’ We then advised them we
were given 13,000 feet by ATC and had
checked in with him stating we were descending
to 13,000 feet. Approach then
said 13,000 feet was O.K.” (# 297750)
Advanced and Traditional Cockpits
There were slightly more (61 percent)
advanced cockpit (EFIS and/or NAV
control) than traditional cockpit
aircraft in the data set. This compares
to 51 percent advanced cockpit versus
49 percent traditional cockpit air
carrier aircraft in the entire ASRS
database for the same time period.
It was expected that advanced cockpit
aircraft would be more likely to be
involved in crossing restriction altitude
deviations due to the greater
complexity in programming descents
and descent crossing fixes. While we
did see this pattern, the difference in
numbers between advanced and traditional
cockpit aircraft was not large.
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