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results of this analysis with selected
findings of the 1982 ASRS study.
Looking at Reports
The objective of this review was to categorize
the types (i.e., undershoot or
overshoot) and frequency of crossing
restriction altitude deviations, and to
determine the types of human performance
errors that contribute to crossing
restriction altitude deviations.
Additionally, we looked at how and by
whom these deviations are detected and
corrected, and compared the number of
deviations for traditional versus glass
cockpit technology aircraft.
Reports selected for review in this
study involved Part 121 or 135 aircraft
in scheduled or non-scheduled air carrier
operations conducting Standard
Instrument Departure (SID) or Standard
Terminal Arrival Route (STAR)
procedures under Instrument Rules,
where the flight failed to level at or
cross a specified crossing restriction altitude
as instructed by ATC or as required
by a published procedure. Two
hundred full-form records, from December
1988 through February 1996,
were extracted from the ASRS Database
and reviewed. Of these, 172 met
the criteria for inclusion in this study.
A five-page coding form was developed
to extract pertinent information
from the data set.
What Doesn’t Matter
Of the 172 air carrier reports in the
study, 159 involved turbojet aircraft
and 13 involved turboprop aircraft.
We found no evidence that the day of
the week, time of day, aircraft type or
configuration, or weather factors
played a role in these altitude deviations.
Similarly, it did not intuitively
appear that crossing restriction altitude
deviations were more likely to
occur at any given ATC facility.
Finally, altitude crossing restriction
errors were detected by ATC and the
flight crew in approximately equal
proportions: 53 percent were detected
by flight crews, and 41 percent by ATC
controllers.
ATC-Assigned vs.
Charted Requirement
Where the required crossing restriction
altitude was assigned by ATC, the
flight failed to meet a crossing restriction
on a SID or a STAR in 66 percent
of events, while in 34 percent of
events the crossing restriction was a
charted requirement. The preponderance
of incidents in which ATC
assigned the crossing restriction
altitude may be attributable to diminished
time for climb or descent planning
and to breakdowns of communications.
The following report excerpt demonstrates
a communication problem:
- “We had assumed that while in radar
contact…we could safely descend to the
cleared level of Flight Level 70—apparently
Santiago Approach intended for us
to observe the arrival procedure altitude
restrictions, even though they had us in
radar contact and had cleared us to descend
to Flight Level 70. We both feel
that this incident was in part due to communications
misunderstanding.”
(# 294836)
And now for one that illustrates the
problems of reduced time for descent
planning:
- “ATC deviated from the expected
CIVET 1 Arrival. [We received an] unusual
crossing restriction not normally
used or expected during an approach into
LAX. Too many short-notice clearances
issued, with very little time between each
of them.” (# 304840)
12 Issue Number 10
Deviations Up—Going Down
Only 23 percent of altitude deviation
events in the data set occurred on
occurred on SIDs (climb), while a full
77 percent occurred on STARs (in
descent). One possible explanation for
this variation may be workload: in the
descent (STAR) phase of flight, flight
crews have a large number of tasks
and issues to contend with, including
obtaining ATIS, adjusting or planning
for changing weather conditions,
conducting company communications,
confirming gate assignments,
planning for terminal procedures and
runway configurations, traffic watch,
configuring the aircraft, or alerting
and communicating with cabin crew.
- “Number 1 Flight Attendant came
into the cockpit asking for gate connections
and giving a cabin write-up. Managed
to get distracted and forgot to reset
altimeters to the proper setting below
18,000 feet.” (# 306840)
It is also possible that on STARs there
is greater ambiguity about ATC expectations,
that is, when or where ATC expects
the flight to initiate descent.
Undershoots and Overshoots
A majority of altitude deviations—75
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