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FL250: A Descent Odyssey
HAL: “Good evening... Everything is running
smoothly.”
We were at FL250 when
Center cleared us to cross 30
miles west of ABC VOR at
17,000 feet. The First Officer was
flying on autopilot and dialed in
17,000 feet in the altitude alerter
then started programming the
FMC for the crossing restriction.
I dialed in ABC on my VOR.
Realizing that we were fairly
close to the idle power descent
profile, I mentioned this and selected Level
Change. There was no intersection for the crossing point so
the First Officer had to build it, which takes time.
HAL: “I am putting myself to the fullest possible use,
which is all that any conscious entity can do.”
When the FMC finished thinking, it indicated that we were
well below profile, so the First Officer hit VNAV which
brought the descent back to 1000 fpm. That didn’t make
sense so I looked at the descent profile which verified what
the First Officer had indicated. My VOR readout and the
FMC did not agree, but I did not realize what was wrong
at the time. I advised [the First Officer] that we were pretty
close to the profile and once again selected Level Change.
The First Officer was as confused as I was, but accepted
the idle power descent profile... I realized in hindsight that
he had no idea what I was basing my concern on. Passing
FL200, I concluded that we would make the restriction
based on the VOR information, but that it would be close. I
called 10 miles which probably caused more confusion
since the FMC indicated that we were significantly farther
away. In deference to me, the First Officer increased the
descent speed up to our previously assigned limit speed to
hasten the descent... We crossed the restriction point at
17,400 feet... We were very close but not perfect. It took a
while, but I finally realized that the First Officer had
constructed the crossing waypoint correctly but had
inserted it after the next intersection instead of before it.
The FMC assumed that we were going to fly to the preexisting
intersection then back to the crossing point, which
added a number of flying miles to the crossing point and
led to the descent profile being in error. Unfortunately the
error was caused by a reliance on modern technology
which is wonderful but relies upon correct inputs.
HAL: “This sort of thing has cropped up before and
it has always been attributable to human error... I
think you ought to calm down...and think things
over.”
After the flight, we had an extensive conversation in which
I showed the First Officer how to verify that constructed
intersections are inserted correctly. We also discussed why,
when there is any confusion as to who is correct, the
appropriate solution is to follow the most conservative
course of action. If it turns out that it was unnecessary and
we are way early on the crossing profile, nothing is lost but
a small amount of fuel. Controllers could help minimize
this potential problem when dealing with modern FMCequipped
aircraft by giving crossing restrictions based on
predefined intersections that would likely already be in
our database. Any time you have to construct a crossing
point, it takes a lot more time and introduces a significant
opportunity for error. If there is any doubt, take the most
conservative of all the options and do not hesitate to ask
ATC for help.
HAL: “I feel much better now.”
While all of these reports deal with Airline Crew
or ATC Controller issues, the need to maintain
situational awareness applies to all pilots.
Overreliance on any technology can lead to
complacency. As with all aids to flight and
navigation, from coupled autopilots to hand-held
GPS units, system knowledge and situational
awareness are key factors in safe airmanship.
CCAALLLLBBAACCKK From NASA's Aviation Safety Reporting System
Number 282 March 2003
A Monthly Safety Bulletin
from
The Office of the NASA
Aviation Safety Reporting
System,
P.O. Box 189,
Moffett Field, CA
94035-0189
http://asrs.arc.nasa.gov/
February 2003 Report Intake
Air Carrier / Air Taxi Pilots 1757
General Aviation Pilots 526
Controllers 34
Cabin/Mechanics/Military/Other 114
TOTAL 2431
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B757-200 fuel scavenging discrepancy
Absence of markers on a closed runway
MD88 E&E compartment water damage
ASRS Recently Issued Alerts On…
Safety in the Cabin
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