Cockpit
automation is to use automation to build intelligent agents that automate operator
tasks.
"To command
effectively, the human operator must be involved and informed. Automated
systems need to be predictable and capable of being monitored by human operators.
Each element of the [cockpit] system must have knowledge of the other's intent."
This is the spirit of the guidelines developed by Billings for human-centered
automation. (Billings, C. E. (1997).
Pilots generally
use the VNAV function during the climb and cruise phases of flight.
In a survey of 203 pilots (questionnaire at the end of this post) at a major U.S. airline,
McCrobie et al., found that:
- 73%
of pilots used VNAV in climb phase;
- 20%
used the function in descent; and,
- 5%
use the function in approach.
The VNAV
function (also known as the PROF function) accounts for the majority of reported
human factor issues with cockpit automation.
Vakil & Hansman's review of Aviation Safety Reporting
System (ASRS) reports,
an anonymous incident reporting data-base
for pilots, found that 63% of pilot-cockpit
interaction issues were in the control of the coupled vertical/speed trajectory
of the aircraft performed by the VNAV function.
Each
new generation of aircraft has increasing levels of flight deck automation that
have improved the safety and efficiency of airline operations. The full potential
of these technologies has not been fully realized however. A case in point is the potential to improve operations during
the workload-intensive descent and approach phases of flight. The Vertical Navigation (VNAV) function of
the Flight Management System (FMS) serves as an intelligent agent during these
phases by automatically selecting appropriate
targets (e.g. altitude, speed, and
vertical speed) and pitch/thrust
control modes to satisfy the objectives of each leg of the flightplan. This
decision-making logic is complex and has raised several sets of human factors related
concerns.
A
cognitive engineering analysis of the NASA Research VNAV function
(representative of the PROF function on Airbus aircraft and the VNAV functions
in modem Boeing airplanes) identified that the current design of the user interface
for the VNAV function violates two basic principles of cognitive engineering for
interfaces between operators and complex automation:
1. The
VNAV button is overloaded in descent
and approach phases of flight.
Selecting
the VNAV button results in the engagement one of six possible VNAV commanded
trajectories.
2.
Flight Mode Annunciator (FMA) for the VNAV function is overloaded in descent and approach phases of flight.
The
same FMA is used to represent different trajectories commanded by the VNAV
function.
Overloading of user-interface input devices and overloading
of display feedback are well known sources of operator confusion.
These
principles are considered to contribute directly to the difficulties pilots have
in learning, understanding, and predicting complex automation behavior.
The airlines
are effectively relying on the pilot community
to discover and informally communicate to each other ways of using the function
in all flight regimes. This is reflected in a series of surveys that found that
pilots request additional training on VNAV and other FMS functions over all other
aircraft systems.
The VNAV function
provides three automated features:
1. VNAV automatically selects altitude targets and speed targets according to pilot MCP
entries and the altitude and speed constraints in the FMS flightplan.
2. VNAV automatically selects pitch and thrust control modes to fly to the targets.
For
example during descent, VNAV chooses between a FLCH descent, a vertical speed
(fixed rate-of descent), and an FMS path descent. In the case where VNAV
selects vertical speed control mode, VNAV also selects the vertical speed
target.
3. For
the descent and approach, VNA V automatically provides an optimum path that is
used as the reference for all automated altitude/speed target and control mode
selections.
Automated selection of
VNAV targets
A study
of the soft-ware of contemporary VNAV functions, Sherry & Poison, found
that the typical VNAV function automatically chooses the active altitude target
from a possible list of sixteen, and chooses the active speed target from a possible
list of twenty-six. Pilots are generally familiar with only a small set of these
targets that occur most frequently and are self-explanatory.
For example, the VNAV altitude target is almost
always the pilot entered MCP altitude. In rare cases, when the MCP altitude has
been raised above a constraint altitude in the climb phase of the FMS flightplan
(or lowered below a constraint altitude in the descent phase of the FMS flightplan),
the VNAV function will capture and maintain the constraint altitude (and not the
MCP Altitude). Hutchins describes scenarios
in which pilots became confused with the relationship between the MCP altitude
and the FMS flightplan altitude.
The
remaining altitude targets automatically selected by VNAV cover "comer
cases" and are rarely observed during revenue service operations. For
example, the VNAV function will automatically level the aircraft off if there
is a conflict between the direction of the pilot entered MCP altitude and the
phase of the flightplan. Dialing the MCP altitude below the aircraft in the
climb phase of the flightplan results in an immediate level off. Other unusual
altitude targets include; an intermediate level-off at 10,000 feet during
descent to bleed off speed to satisfy the 10,000ft/250kt. restriction, an
intermediate level-off to intercept the glideslope, or when the aircraft has
descended below the Minimum Descent Altitude (MDA) on a non-precision approach.
3 keys to demystifying
VNAV selection of targets
First a deep understanding of the FMS flightplan and,
Second how the altitude and speed constraints are used to
determine targets is required.
This must
be coupled with knowledge of the dynamic relationship between the MCP and the
FMS flightplan for selecting targets.
Third, the "comer case" targets of the VNAV function
must be understood.
Automated selection of
VNAV pitch/thrust control modes
Automated
mode selection by the VNAV function of pitch/thrust control modes can be
confusing in two ways. The most common source of confusion is the autonomous
transition of the mode without pilot action. These "silent" mode
transitions are made when VNAV detects that certain criteria have been satisfied.
For example, when the aircraft speed exceeds a threshold (typically 20 knots)
above the FMS path speed, VNAV will autonomously switch control modes from
VNAV-PATH to VNAV SPEED. These thresholds are generally not annunciated on
cockpit displays.
The second source of confusion is the selection of control modes made
by VNAV given the circumstances of the aircraft. For example, several pilots
prefer to perform descents to crossing restrictions with a FLXed rate of
descent (i.e. vertical speed mode). By triangulating time (or distance) to the waypoint
and remaining altitude, pilots can ensure making the restriction. In certain circumstances
VNAV will choose speed-on pitch with idle thrust and request airbrakes to make
the restriction.
The key to understanding the choice of control
modes made by the VNAV
function is to understand the overall FMS philosophy on how descents are flown.
Automatic use of FMS
optimum path as a reference
One of the biggest contributors to pilot confusion with VNAV is the FMS computed optimum path. The path, computed by the FMS using models
of aircraft performance, takes into account the regulations and constraints of
standard arrival procedures (STARs) and published approaches. The nuances of
the path, such as how far way from waypoints deceleration as reinitiated, are
non-intuitive and worse not displayed in the cockpit.
When
the aircraft is capturing and maintaining the path, the aircraft altitude
control is earth-referenced with the goal of placing the aircraft 50-ft above
the runway threshold. This operates much like the glide slope except that the reference
be a misprovided by the FMS, not a ground-base transmitter. Unlike other
up-and- way control modes, the aircraft will maintain the path without drift in
the presence of wind.
When
the FMS optimum path is not constrained by crossing restrictions and appropriate
wind entries have been made, the aircraft will descend at the desired speed with
the throttles at idle. When the path is constrained or wind entries are sufficiently
inaccurate, speed must be maintained using throttles (for underspeed) and airbrakes (for overspeed).
This
"earth-referenced" control
of altitude has been observed to confuse pilots who, on request from ATC to
expedite the descent, add thrust or extend airbrakes. Because VNAV is
controlling to the path, these actions simply increase or decrease speed without
any effect on aircraft rate-of-descent.
The key
to understanding the VNAV behavior in descent is to have full knowledge of the
FMS optimum path.
Pilots
must understand the differences between airmass-referenced descents, such as
FLCH, and earth referenced descents on the path.
Pilots
primarily monitor the behavior of the VNAV function by monitoring the
trajectory of the aircraft.
Pilots are
"surprised" by the behavior of the VNAV function when the aircraft trajectory
or the thrust indicators do not match their expectations. For example, when the aircraft vertical speed fails to decrease as the
aircraft approaches an assigned altitude, pilots wonder whether the VNAV
function is commanding a capture to the altitude.
Secondary
sources of information on VNAV include the Flight Mode Annunciator (FMA),
targets on the Primary Flight Display (PFD) altitude tape and speed tape, and
various MCDU pages (e.g. RTE/LEGS (or
F-PLN), PROG page, CLB/CRZ/DES pages).
Research
Autopilot was demonstrated to be a source of pilot errors. This input device
resulted in two different autopilot behaviors depending on the situation when
it was selected. Selecting the vertical speed wheel:
l. when
the aircraft was outside the capture region, commanded the aircraft toffy to the
assigned altitude (and armed the capture).
2. when
the aircraft was inside the capture region, commanded the aircraft toffy away from
the assigned altitude (and disarmed the capture) Frequently pilots were unaware
of the dual nature of the vertical speed wheel, or could not distinguish
between the dual "modes" of the wheel. As a result pilots were
surprised by the behavior commanded by the autopilot. See also Palmer, Degani & Heymann, and NTSB.
There are six behaviors commanded by the VNAV function during the descent
and approach phases of the flightplan, when the goal of VNAV is to DESCEND
TO THE FINAL APPROACH FIX (FAF):
1.
Descend on FMS Optimum Path
2.
Descend Return to Optimum Path from Long (Late)
3.
Descend Converge on Optimum Path from Short (Early)
4.
Maintain VNAV Altitude (i.e. altitude constraints, MCP altitude, or other VNAV
altitudes)
5. Descend
Open to VNAV Altitude to Protect Speed
6. Descend
to VNAV Altitude, Hold to Manual Termination
The
basic underlying concept of the VNAV function is that the VNAV function constructs
and strives to fly an optimum path to the FAF. This path is a geographically-fixed
pathway from the cruise flight level to the runway that is designed to optimize
fuel bum and time, and takes into account the altitude crossing restrictions, and
speed and time constraints. It is flown in much the same way as the aircraft
flies a glideslope beam.
To stabilize the aircraft at the FAF the VNAV function commands trajectories
to capture and maintain the path. The appropriate trajectories are determined by
decision-making rules embedded in the software that take into account the
position and speed of the aircraft relative to the path and other parameters.
The VNAV function will automatically transition between commanded behaviors
based on the situation perceived by the automation based on sensor data.
For example, when the aircraft is commanded to initiate
the descent before the optimal FMS computed Top-of-Descent, the VNAV function
automatically commands a VNAV Behavior to Descend and Converge on the Optimum
Path, usually with a fixed rate-of-descent. The rate-of-descent is selected
such that the aircraft converges on the optimum path (Figure 2).
Alternatively,
when the aircraft initiates the descent beyond the Top-of-Descent, the VNAV
function automatically commands a VNAV Behavior to Descend and Return to Optimum
Path. This VNAV behavior commands a descent at idle-thrust. Some VNAV functions
increase the speed target to ensure convergence of the path (Figure 2).
Frequently
the VNAV function determines that additional drag is required to converge on
the optimum path and requests extension of the air-brakes via an ND and MCDU message.
Attitudes-Toward-Automation
Questionnaire
Please
indicate your agreement or disagreement with the following statements by
circling the words that best describe
your feelings:
1. I am
concerned about a possible loss of my flying skills with too much automation.
☐Strongly Agree ☐Agree ☐Neutral
☐Disagree ☐Strongly
Disagree
2. The
automation in my current aircraft works great in today's ATC environment.
☐Strongly Agree ☐Agree ☐Neutral ☐Disagree
☐Strongly Disagree
3. I
always know what mode the autopilot/flight director is in.
☐Strongly Agree ☐Agree ☐Neutral ☐Disagree
☐Strongly Disagree
4. I
use the automation mainly because my company wants me to.
☐Strongly Agree ☐Agree ☐Neutral ☐Disagree
☐Strongly Disagree
5.
Automation frees me of much of the routine, mechanical parts of flying so I can
concentrate on "managing" the flight.
☐Strongly Agree ☐Agree ☐Neutral ☐Disagree
☐Strongly Disagree
6. In
the automation of my current aircraft, there are still things that happen that
surprise me.
☐Strongly
Agree ☐Agree
☐Neutral ☐Disagree ☐Strongly Disagree
7. I
make fewer errors in the automated airplanes than I did in the older models.
☐Strongly Agree ☐Agree ☐Neutral ☐Disagree ☐Strongly Disagree
8.
Automation helps me stay "ahead of the airplane".
☐Strongly Agree ☐Agree ☐Neutral ☐Disagree
☐Strongly Disagree
9. I
spend more time setting up and managing the automation (CDU, FMS) than I would
hand-flying or using a plain autopilot.
☐Strongly Agree ☐Agree ☐Neutral ☐Disagree
☐Strongly Disagree
10.
Automation does not reduce total workload, because there is more to monitor
now.
☐Strongly Agree ☐Agree ☐Neutral
☐Disagree ☐Strongly
Disagree
11. I
always consult the flight mode annunciator to determine which mode the autopilot/
flight director is in.
☐Strongly Agree ☐Agree ☐Neutral ☐Disagree
☐Strongly Disagree
12.
Training for my current aircraft was as adequate as any training I have had.
☐Strongly Agree ☐Agree ☐Neutral ☐Disagree
☐Strongly Disagree
13. I
use automation mainly because it helps me get the job done.
☐Strongly Agree ☐Agree ☐Neutral ☐Disagree
☐Strongly Disagree
14. It
is easier to bust an altitude in an automated airplane than in other planes.
☐Strongly Agree ☐Agree ☐Neutral ☐Disagree
☐Strongly Disagree
15.
Sometimes I feel more like a "button pusher" than a pilot.
☐Strongly Agree ☐Agree ☐Neutral ☐Disagree ☐Strongly Disagree
16.
There are still modes and features of the autoflight system that I don't
understand.
☐Strongly Agree ☐Agree ☐Neutral ☐Disagree
☐Strongly Disagre