Aviation Troubleshooting: Analysis of the Vertical Navigation (VNAV) Function

SOURCES:

Lance Sherry
RAND Honeywell International, Inc.
Phoenix, Arizona

Michael Feary
NASA Ames Research Center
Moffett Field, California

Peter Polson
Department of Psychology
University of Colorado, Boulder, Colorado

Randall Mumaw
Boeing - Commercial Airplane Group
Seattle, Washington

Everett Palmer
NASA Ames Research Center
Moffett Field, California

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.