Usability Inspection of the MD-11 Aircraft Multifunctional Control Display Unit

Kheng-wooi Tan and Jennifer M. RileyMississippi State University

Human-Centered Design of Automation• Human factors issues associated with design and

implementation of automation of interest:– Scerbo, 1996– Hilburn et al., 1997

– Endsley, 1996– Parasuraman et al., 1998

• Research driven by need for human-centered design of automated systems:– need to consider human cognition and behavioral effects

associated with use of automation (static or flexible)• Lack of focus, however, in automated system design

on human-computer interaction (HCI) :– communication (input / output) between human user and

computer - direct or indirect - influenced by style of interface (Dix et al., 1998)

– usefulness and usability important to effective HCI in interactive systems

HCI in the Aircraft Cockpit• HCI important in cockpit automation:

– pilot uses Multifunctional Control Display Unit (MCDU), flight deck computer, to interact with automated flight management system

– good pilot-MCDU interaction critical to aircraft safety:• aviation accidents attributed to problems with pilot-MCDU

interaction (e.g., the American Airlines flight 965 CALI incident (1995))

Input

Output

Multifunctional Control Display Unit

Video display terminal

Numeric keypad

Alpha keypad

Line Select Keys

(LSK)

Mode keys

MCDU used to preprogram flight from take-off through

en-route to landing

• Flight path planning:– identifying flight plan

waypoints– waypoint data entry

• headings• altitude• airspeed

Study Objectives• Assess usability of MCDU based on general usability

principles (Dix et al., 1998)• Identify usability violations of MCDU design and

functionality:– three major principles of interest, including supporting

heuristics Learnability

•Predictability

•Synthesizability

•Familiarity

•Generalizability

•Consistency

Flexibility

•Dialog Initiative

•Multi-threading

•Task Migratability

•Substitutivity

•Customizability

Robustness

•Observability

•Recoverability

•Responsiveness

•Task Conformance

Procedure• Conducted usability inspection with high-fidelity,

simulated MCDU using typical flight planning task:– usability inspection methods:

• non empirical techniques relying on judges to predict user problems with interfaces

• demonstrated to be cost-effective and reliable for evaluating interactive systems, like MCDU (Virzi, 1997)

– group expert review on MCDU:• human factors specialists acted as

surrogate users of MCDU • six (6) experts critique design and

functionality– experts had background in industrial

engineering and specialization in ergonomics

Flight Path Planning Task• Programming 3-waypoint flight plan using MCDU:

– required experts to:• navigate through “pages” of MCDU: Active Flight Plan (AFP)

Pages 1 and 2, Duplicate Waypoint (DUP) Page, Lateral Revision (LR) Page, Vertical Revision (VR) Pages 1 and 2

• use numeric and alpha keypads, LSK and mode keys

– task steps:1. Start at AFP Page 12. Enter first waypoint using keypads3. Verify waypoint is correct and insert into flight plan4. Repeat Steps 2 and 3 for 2nd and 3rd waypoint5. Go to LR Page using LSK keys6. Enter information on airway and STAR using keypads 7. Return AFP Page 1 using LSK keys 8. Go to VR Page 1 using LSK keys

More Task Steps9. Enter information on altitude, speed, etc. using keypads10. Go to AFP Page 1 using LSK keys11. Go to AFP Page 2 using mode keys12. Go to VR Page 2 using LSK keys13. Enter information on altitude, wind speed, etc. using keypad 14. Return to AFP Page 2 using LSK keys15. Repeat Steps 5 - 14 for 2nd and 3rd waypoints

• Usability violations identified by experts for each principle:– learnability - ease with which new user can

begin effective interaction with system– flexibility - multiplicity of ways user and

system exchange information– robustness - level of support provided to user

in assessing and achieving goals

Definitions

Violations of System Learnability• High level of system complexity:

– several different methods to accomplish same task – many buttons have multiple functions (e.g., LSK keys used

to navigate between pages and execute commands)• Design fails to facilitate guessability• Poor synthesizability/feedback on pilot actions at

interface:– feedback to pilot not displayed on MCDU – pilot must view other cockpit displays (e.g. navigational

display) to determine outcome of action• MCDU design fails to provide global assessment of

flight parameters and aircraft status:– requires extensive pilot interpretation and mental

transformation and aggregation of data

Violations of System Flexibility• System does not allow for multi-threading:

– pilot may not interact with more than one task/page at same time

– pilot must enter data for one waypoint on several pages at different times

• Lack of modifiability:– system only allows for text entry (no direct-

manipulation interface)– cannot customize interactions on basis of pilot

preference/skill level• System pre-emptive design:

– pilot not free to initiate actions – system initiates dialog and user responds to requests for

information

Violations of System Robustness• MCDU design fails to provide default mode/state to

assist pilot with passive recall of information and appropriate actions at interface

• Poor reachability, navigability, and observability:– difficult to navigate through pages in sequence– pilot may not go directly from VR Page 1 to VR Page 2

• System design suffers from lack of recoverability:– Only allows for backward recovery– difficult to apply changes to flight plan or correct mistakes

• Poor system transparency:– lack of instantaneous response/feedback through MCDU

display terminal

Potential Design Improvements• Provide graphical user interface (GUI) to

incorporate direct manipulation (DM) in HCI:– provide GUI with icons and

menus (like Window- based systems):

• decrease complexity associated with command-line interface text entry

• increase familiarity and learnability of MCDU

– provide graphical representation of flight pages and flight paths:

• increase observability and ability to globally assess entire flight path

Potential Design Improvements (Continued)• Provide hierarchical representation of flight pages

for selection as needed:– facilitate user pre-emptive interaction– improve overall pilot-MCDU communication– improve system browsability

AFP 1

LR VR 1AFP 2 DUP

VR 2

Potential Design Improvements (Continued)• Provide capability for interleaved multi-threading of

activities:– permit temporary overlap between separate pages of

MCDU for waypoint data entry:• decrease pilot working memory load associated with remembering

what data has/has not been entered for particular waypoint on particular page

• facilitate user pre-emptive interaction• improve overall pilot-MCDU communication

Active Flight Plan P1

Active Flight Plan P2

Vertical Revision p2BAQBIVOS OTU

Vertical Revision P2

Vertical Revision P1

BAQ N10 53.6BIVOS N09 17.4OUT N07

BAQ BIVOS OUT

Potential Design Improvements (Continued)

• Minimize number of functions associated with each LSK key - increase consistency of pilot interaction with MCDU

• Provide mode indicators - increase system transparency and pilot awareness of current functions of buttons and controls

• Change physical layout of control buttons on MCDU:– use QWERTY layout for alpha keypad – relate design of 12 mode keys to 12 function keys on

conventional keyboard • increase familiarity of system • decrease time for text data entry

Operational Benefits• Performance improvements associated with lower

working memory load• Enhanced SA associated with increased system

transparency• Decreased time to achieve effective interaction

with MCDU for novice pilots