dl comms workload sa muller giesa 2002

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Effects of Airborne Data LinkCommunication on Demands, Workloadand Situation Awareness

T. Muller and H.-G. Giesa

Department of Human-Machine Systems, Institute of Psychology and Ergonomics, Technical University of Berlin, Berlin,Germany

Abstract: An airborne air-to-ground data link communication interface was evaluated in a multi-sector-planning scenario using an Airbus A 340 full flightsimulator. In a close-to-reality experimental setting, eight professional crews performed a flight mission in a mixed voice/data link environment.Experimental factors were the medium (voice vs. data link), workload (low vs. high) and the role in the cockpit (pilot flying vs. pilot non-flying). Data linkcommunication and the usability of the newly developed communication interface were rated positively by the pilots, but there is a clear preference forusing a data link only in the phase of cruise. Cognitive demands were determined for selected sections of en-route flight. Demands are affected mainly byincreased communication needs. In the pilots view, although a data link has no effect on safety or the possibilities of intervention, it causes more problems.The subjective workload, as measured with the NASA Task Load Index, increased moderately under data link conditions. A data link has no general effecton pilots situation awareness although flight plan negotiations with a data link cause a distraction of attention from monitoring tasks. The use of a data linkhas an impact on air-to-ground as well as intra-crew communication. Under data link conditions the pilot non-flying plays a more active role in the cockpit.Before introducing data link communication, several aspects of crew resource management have to be reconsidered.

Keywords: Air-to-ground data link; Aviation; Cognitive demands; Communication; Crew resource management; Flight simulator; Multi-sector-planning;Situation awareness; Workload

1. INTRODUCTION

1.1. Background

A considerable increase in European and worldwide civilair traffic is expected in the next 20 years. Since the air

traffic management (ATM) system is currently operating atits limits, new models and concepts are already under

development. Any future increase in capacity should alsomean an increase or at least the maintenance of todays

safety standards and economic improvements (Eurocontrol

1998, 1999).The bottleneck in ATM is at least partially due to air

traffic control. Given specific safety standards and parti-

cular technical equipment, only a limited number ofaircraft can be controlled in one control sector at any

given time. In the past, this problem was solved by reducingthe size of sectors. However, this generally limits the

planning perspective of the air traffic controller and/or

makes more co-ordination necessary with neighbouringsectors.

1.2. Multi-Sector Planning and Data Link

A more promising approach introduces a new planningauthority called a multi-sector-planner (MSP) who oper-ates on a longer time perspective and aims at preventingrather than solving conflicts. Multi-sector planning is a keyfeature of an ATM concept developed at the TechnicalUniversity of Berlin (Fricke et al 2000). The MSP isresponsible for several radar sectors (multi-sector area).Aircraft flight plan data is used on the ground for acomputer simulation of planned traffic trajectories topredict conflicts in the multi-sector-area. Based on thesepredictions, conflict resolutions are calculated as modifiedflight plans and negotited via a data link with aircraft in themulti-sector area. Air traffic controllers (ATC) can thensupervise a greater number of aircraft when conflict density(conflicts per aircraft) is reduced.

The MSP requires permanently updated informationabout flight plans, current position of aircraft and short-term interventions by ATC. An efficient data link isneeded between the computers of the MSP and ATC onthe ground as well as to and from airborne flight manage-

Cognition, Technology & Work (2002) 4:211228Ownership and Copyright# 2002 Springer-Verlag London Limited

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ment systems (FMS). The VHF voice communication nowcommonly used between pilots and ATC will be replacedby digital data link communication. For a complexexchange of information (except flight plan negotiations)or in an emergency, voice communication will serve as abackup system.

In addition to the potential for traffic optimisation with

the MSP concept, further benefits over conventional voicecommunications are expected from the use of a data link forATC communications. Rehmann (1997) mentions areduction in miscommunication, a reduction in frequencycongestion, the potential for direct entry of data into anaircrafts autoflight systems and the permanent documenta-tion of clearance messages.

Several changes occur for airborne crews: negotiatingflight plan data is an additional task, the MSP is a furtherpartner in communication, and the data link interface is anew communication device. The ergonomic design of thedata link interface and the dependability of the entire

humanmachine system require special attention.

2. STATE OF THE ART

2.1. System Design and Acceptance

Numerous functional, procedural and design aspects of theimplementation of data link systems have been investigatedin empirical and analytical studies (SAE 1994; Rehmann1997). Particularly careful studies have been done on thelocation of data link interfaces within the cockpit: thedisplay should be located within the forward field of view

(Knox and Scanlon 1991; Kerns 1994; Rehmann et al 1995;Rehmann and Mogford 1996). For the input of information,the Multi Purpose Control and Display Unit (MCDU) isconsidered to be well suited (Rehmann et al 1995).Regardless of the flight phase in question, pilots shouldalways be in radio contact with an ATC facility (Rehmann1997). Van Gent (1995) has found that the ratedacceptance of a data link is highly dependent on thedisplays page layouts and operating procedures. Pilotsacceptance of air-to-ground data link communication alsostrongly depends on the flight phase; the highest accept-ability was found for ground operations and cruise.

2.2. Data Link Communication as a Part of theCrewAircraft System

The FAA Human Factors Team (1996) states that inmodern flight deck systems solutions that involve the entiresystem are required, not just focused solutions to individualproblems. The effect of the use of the new communicationsystem on the behaviour of the entire system has rarelybeen investigated explicitly in previous studies. In the caseof a data link a risk of interference between simultaneous

communications and flying actions is anticipated (Navarroand Sikorski 1999). Since the overall efficiency ofoperations is supposed to increase with the use of a datalink and at any rate not decrease (Kerns 1994), it isnecessary to evaluate the impact of a data link on the wholesituation. Problems experienced by the flight crew and adegraded perception of safety are indicators of bad system

integration.Since a lot of problems have occurred as a consequence

of too much automation, it is suggested that tasks should bedesigned in such a way that the operators are betterinvolved in the processes and enjoy a higher degree offreedom while operating the system. For flight guidance,such design characteristics have been discussed for twodecades as a measure to address the ironies of automationand to enhance safety (e.g. Wiener and Curry 1980;Bainbridge 1983; Amalberti 1993). Operators working inthe system are often confronted with situations whichcannot be foreseen by system developers (unknown

unknown situation; Sheridan 1983). It can be concludedas in the concept of human-centred automation (Billings1991, 1997; ICAO 1994) that new systems should bedesigned in a way that actively involves pilots and allowsthem the possibility of intervention (e.g. Boy andCacciabue 1997).

In highly automated systems such as modern civilaircraft, traditional performance measures, e.g. flight pathaccuracy, are hardly attributable to pilots. The focus has tobe directed on the pilots potential to maintain overallsystem performance on a tolerable (i.e. safe) level whenconfronted by system failures or the unexpected behaviourof technical systems. This potential is discussed in terms of

keeping the pilot in the loop (e.g. White 1991; Endsleyand Kiris 1995), situation awareness (e.g. Orasanu 1995;Endsley 1995, 1996) or attention allocation (e.g. Sarter1995).

2.3. Impact of Data Link Communication on PilotsWorkload

Studies do not agree on the effect of data link commu-nication on pilots workload. Kerns (1994) reports thatthere is little evidence that the use of a data link impactson the overall workload, but she emphasizes that there is ashift from an auditory and speech workload to a visual andmanual workload. Van Gent (1995) and Rehmann andMogford (1996) also found that pilots workload is notaffected by a data link. In a comparative study of three datalink interfaces (with voice as control condition), asignificant impact of the communication medium onworkload was identified for the pilot non-flying only,although both pilots tended to perceive workload asminimal under voice communication conditions (Reh-mann et al 1995). Lee et al (1997) investigated the usage

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of the flight management system in a way similar totypical data link applications; workload was significantlyincreased compared to the other experimental conditions.The authors point out that this is true for the approachphase only and report opposite findings for en-routeconditions.

Any change to the work environment will have

consequences for the pilots overall work. In aviation thedemands are analysed in many cases by investigating theireffect on the pilots using the concept of workload. Thefindings of workload modifications caused by data link aretypically based on pilots reports of perceived workload. Upto now there has been a lack of detailed analysis of theactual demands resulting from the use of a data link. Inaccordance with the general shift from sensory motor tocognitive tasks in modern cockpits (Billings 1997) anappropriate analysis should focus on cognitive demands.

2.4. Attention and Situation Awareness

The effects of data link communication on attention andsituation awareness are discussed in several terms: distrac-tion of attention from other tasks through the use of thedata link interface, reduction in attention demands becauseit is no longer necessary to monitor the VHF communica-tion of the surrounding traffic (party line), and reductionin situation awareness due to the loss of party lineinformation and the reduced transparency of communica-tion to the pilot flying.

In contrast to listening to the voice radio, the handlingof incoming and outgoing messages via a data link requires

looking at the display and/or keyboard. This distracts thepilots attention from observing the airspace as well as frommonitoring other displays. Even if the principles of crewresource management dictate that the pilot non-flyinghandles communication, incoming data link messagesincrease the head down time of the pilot flying, too(Rehmann et al 1995; Jorna 1997; Lee et al 1997).

In todays air-to-ground communication, all messageswithin a control sector are transmitted on a single VHFfrequency (party line), i.e. pilots have to listen continu-ously to the call signs which identify the addressed aircraft.On the other hand, data link communication is point topoint, i.e. aircrews receive exclusively those messageswhich are intended for them. Therefore, data link leads to areduction in attention requirements regarding the necessityto continuously listen to the party line and identify callsigns. An analysis of incident reports (Rehmann 1995) aswell as simulator studies (Infield et al 1995) indicate thatthe confusion of similar sounding call signs is a severeproblem in todays air traffic. Finally, with voice transmis-sion problems occur with incorrect readbacks (short-termmemory overload) after longer ATC messages andambiguities due to procedural deviations, e.g. omission of

call signs (Morrow et al 1990). The avoidance of suchproblems requires increased attention. A data link typicallygives support by presenting the message until it isacknowledged, an additional message history functionality,and generally more standardized communication proce-dures (Navarro and Sikorski 1999).

On the other hand, the monitoring of the party line has

some advantages. The communication between ATC andcrews of other aircraft in the same geographical sector givesinformation about the traffic situation, weather conditionsetc. It is assumed that this information contributes to pilotssituation awareness. In the near future where only relativelyfew aircraft will be equipped with data link capabilities andreduced party line information is still available, no majorproblems with regard to pilots situation awareness areexpected (Infield et al 1995). In an exclusive data linkscenario, the lack of information is assumed to be moredramatic in a terminal environment and close-to-airportmanoeuvres than within the en-route environment

(Rehmann 1995).Voice communication with ATC, even if performed bythe pilot non-flying exclusively, is immediately accessibleto both pilots. Keyboard data entries and messages ondisplays outside the primary vision field are less transparentto the pilot flying. Therefore, the two pilots mentalrepresentations of the flight situation may differ to a largerextent than under voice conditions (Navarro and Sikorski1999). As a consequence, the pilot non-flying often readsincoming messages aloud to the pilot flying (e.g. Logsdon etal 1995) or synthetic speech (Rehmann and Mogford,1996) is used in data link scenarios.

2.5. Effects on Communication Behaviour

Many studies dealing with an air-to-ground data link focuson the transaction time as a central theme. Some of themreport an increase in duration (e.g. Kerns 1994; Logsdon etal 1995; Jorna 1997), while others find no significantdifferences (McGann et al 1998). These ambiguous resultsare due to different locations of the data link communica-tion interface in the cockpit, different ways to consider thetechnical transmission times (which can be of considerableduration in mode S radar transmission), additional tasksand finally the ergonomic design of the interface. Jorna(1997) comes to the conclusion that a data link in itself isnot critically slower as compared to voice, provided thatthe communication interface does not require extensivemanipulation to create an answer. Finally, increasedtransaction times can be an indicator that pilots performtasks with higher priority before answering to ATC(Navarro and Sikorski, 1999).

Even if a data link primarily influences air-to-groundcommunication, effects on the communication within thecockpit are observable as well. Hrebec et al (1995) report

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an increase in the number of communication activities andchanges in the structure of intra-crew communicationunder data link conditions. The first officers (who wereresponsible for air-to-ground communication in thisexperiment) became more active in communication, i.e.they performed more initiative speech acts than they didunder voice conditions. Crew communication patterns

were found to be good predictors of crew performance(Kanki et al 1989, 1991). Supplementary, intra-crewcommunication seems to be a coping mechanism thathelps to lessen and manage both the causes and effects ofstress (Sexton and Helmreich 1999).

3. OBJECTIVE OF THE STUDY

For a better understanding of the effects of a data link onthe working situation within the cockpit, an experimentalinvestigation was performed in which an experimental

prototype of a data link communication interface wascompared to conventional VHF voice communication. Theaim was to evaluate the use of a data link within a multi-sector planning scenario. It was necessary to re-examine theacceptability of the data link interface employed which wastested in several usability studies during the design processunder this aspect.

The experimental investigations were guided by theexpectation that data link communication with groundauthorities (ATC, MSP) has an influence on the cognitivedemands and as a consequence on pilots perception ofthe system state and workload. According to the findings of

earlier studies (see above), we expected effects on pilotssituation awareness and communication behaviour.

In contrast to typical usability studies, the performanceof a flight mission under close-to-reality conditions was theobject of the study, not the solving of isolated tasks.

4. METHODOLOGY

4.1. Equipment

4.1.1. Flight SimulatorFor reasons of safety, research strategy and economy, allexperimental investigations were carried out in a flightsimulator. For the experiments described in detail later, theAirbus A340 training and research full flight simulator ofthe ZFB (Zentrum fur Flugsimulation) Berlin was available.This flight simulator is equipped with full vision andmotion features as well as with a Scientific ResearchFacility that allows features to be adjusted in the FlightManagement System (FMS) and in several display andcontrol devices.

4.1.2. Data Link Communication InterfaceAn experimental prototype of a data link communication

interface was developed and implemented in the flightsimulator. It uses already existing displays: the NavigationDisplay (ND) and the Multi-purpose Control and Display

Unit (MCDU). The ND is located in the pilots primaryvision field. The MCDU is situated on the centre aisle

pedestal; therefore the use of the MCDU requires lookingdownward. Figure 1 (left) shows the location of the displaysin the cockpit. The ND was redesigned to provide space fortwo text lines and an icon related to the message, e.g.

symbolising a climb or descent. In the example in Fig. 1(upper right) the icon refers to the MCDU, where moredetails are presented. The MCDU, which is equipped with

an alphanumerical keyboard, is used for pilots entries andfor the presentation of more detailed messages like flightplans. Several new screen pages for data link communica-

tion with ATC and for flight plan negotiations weredeveloped (Muller et al 1999, 2001).

Some of the new MCDU pages were based on existingpages, e.g. the page for flight plan negotiation was derivedfrom the secondary flight plan index page. Other data link

pages (for ATIS, Pilots Requests, Message History) were

designed from scratch.

The system design process was a cooperation betweenflight engineers, human factors engineers and professional

pilots. Tasks, operational structures and technical systems

were designed simultaneously with a rapid prototypingtechnique. During this design process, professional pilotswere subjected to certain stages of development in usability

studies.

Two series of usability studies were carried out: oneconcentrated on flight plan negotiations, the other onshort-term (tactical) ATC messages.

In the first series, the new functionality and proceduresfor flight plan negotiations were considered by 13 pilots in

three phases corresponding to different stages of develop-ment. In the second series, data link functionality for short-

term communication between pilots and the controllerwere investigated in two phases with nine pilots (Muller et

al 1999).

With the integration of multi-sector planning, it is

possible to change the actual flight plan during flight formedium-term optimisations. A new feature allows the

pilots to initiate flight plan negotiations, e.g. due toweather conditions. In Fig. 2, the sequence of MCDU pagesfor flight plan negotiations between pilots and multi-sector

planner is given as an example of a data link communica-tion procedure. The sequence starts with the Flightplan

Negotiation Page (1). A copy of the active flight plan isloaded (COPY ACTV F-PLN) and displayed on the

MCDU screen (2). Now it can be modified. The

consequences of each modification on estimated time ofarrival, distance to destination and estimated fuel on board



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are displayed as delta values. The modified section of the

trajectory is highlighted to indicate the differences between

the active and the modified flight plan. After having

finished the modifications, the pilots proposal for the new

flight plan is sent to the multi-sector planner (3). Now it is

indicated that the negotiation is in progress (4). The multi-

sector planners decision regarding the requested flight plan

is displayed (5). In the example the flight plan is accepted

and can be activated (6).

As soon as a flight plan proposal is displayed on the

MCDU a graphical representation of the flight plan is also

available on the Navigation Display.

For the purpose of a factorial experimental design, a

simplified procedure for flight plan negotiations under voice

conditions was also developed. The core is an oral

transmission of a list of waypoints. Such a proceduremight also be of practical value in the case of a temporal

breakdown of data link transmission facilities and should be

available as a backup procedure.

Figure 3 gives an overview of the handling of ATC

short-term messages. It starts with the incoming messages

TURN RIGHT HEADING 230. (Note: Same message is

displayed on the Navigation Display). Pilots have the

choice to accept (WILCO) or to reject (UNABLE). In

the example, the pilots accept by pressing WILCO. The

transmission to ground requires activation of the SENDkey afterwards.

4.2. Experimental Design

4.2.1. ParticipantsThe participants in the experiments were 16 professionalpilots (eight crews) from various airlines with Airbus typerating. Half of them were captains and half were firstofficers. The mean professional experience was 13.5 years asa commercial pilot. Table 1 gives further information.

4.2.2. ProcedureThe participating pilots received a short manual attachedto the invitation to the experiment in which the new datalink communication interface and the modified commu-nication procedures were described. Before starting withthe experiment, the pilots first had to handle typicalcommunication tasks (e.g. ATC requests, flight plannegotiations) on a PC-based training version of the datalink communication interface to ensure that each partici-pant had basic experience in using the system. After thistraining session and the preparation of the physiologicalmeasurement equipment, the pilots received a briefingwhich included an introduction to the underlying air traffic

Fig. 1. Location of Navigation Display (ND) and Multi-purpose Control and Display Unit (MCDU) in the cockpit of the Airbus A 340 (captains side;another set of displays is located on first officers side).



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Fig. 2. Example of the sequence of MCDU pages for flight plan negotiations initiated by the pilots.

Fig. 3. Example of the reception of a short term ATC message and compliance by aircrew.



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management system with the MSP, the distribution of thedata link areas and the flight mission. Required documents

like flight plans, information about technical parametersand maps were handed over to the pilots. Although the

aircraft was already positioned at the runway when the

pilots entered the flight simulator, the simulator sessionstarted with the manual input of the flight plan data into

the flight management system and preparations for theflight (which normally takes place at the gate) to bring the

pilots in the loop. The properties of the simulated flightand the methods used will be described later.

4.2.3. Flight ScenariosTo evaluate the ATM conception under controlled as wellas close-to-reality conditions, flight mission scenarios were

worked out. The scenarios were detailed scripts coveringmessages from ATC and MSP, certain events which

occurred during flight at scheduled time or location andgeneral flight conditions (e.g. daytime, weather). The

pilots mission was a flight from Copenhagen to Frankfurt

and vice versa. During this flight, the aircraft passedthrough four radar sectors. In different versions of the

scenario, a data link was provided in the two northern

sectors only or in the southern sectors respectively. Underboth conditions (voice and data link) the initial call after

entering a new sector was performed via voice. During the

round trip of a duration of approximately 2 hours, eight test

sections of special interest (approximately 3 minutes each)

were defined where scheduled events (e.g. technical failres,

weather fronts) produce different degrees of workload (Fig.

4). The pilots were not informed about the beginning and

end of the test sections before or during the flight.

Standard ATC messages are typically allocations of flight

levels or headings and identification procedures when

entering a new radar sector. There were two to four of these

standard messages per test section. In four sections, flightplan negotiations took place. Flight plan negotiations are not

a standard procedure for pilots yet. Under voice conditions,

a simple procedure was used which started with an ATC

message like There is a flight plan change report when

ready to copy. Under data link conditions, the respective

messages were displayed on the ND/MCDU with the newly

designed data link pages beginning with ATC constraint

list received. In three sections, flight plan negotiations

where initiated by ground authorities. Since pilots were

briefed to re-route by means of flight plan negotiations if

Table 1. Test subjects: age and professional experience (mean and standard deviation)

All pilots (n = 16) Captains (n = 8) First officers (n = 8)

Age/years 38.7 (SD = 7.8) 42.5 (SD = 7.9) 34.7 (SD = 5.9)Experience as pilot/years 13.5 (SD = 9.1) 19.9 (SD = 8.9) 7.1 (SD = 2.4)Total flight time/hours 7706 (SD = 5140) 11,288 (SD = 5115) 4125 (SD = 1067)

Fig. 4. Definition of test sections.



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the weather conditions worsened, pilot-initiated negotia-tions (with the pilots own flight plan proposal) took placein sector D. Technical faults were in one case theoccurrence of engine vibrations which required activityby the pilots, including shutting down the affected enginefor a while. In another case, a failure in the heating of astandby pitot probe tube was displayed. The fault had to be

recognised, but did not require any further action. In twosections, faulty ATC commands were sent, e.g. maintain onflight level 220 when the aircraft was actually on flightlevel 250.

4.2.4. Experimental FactorsTwo levels of workload were implemented with thedefinition of the test sections. Since a data link is providedin the northern or southern sectors only, two levels of thecommunication medium were available. Furthermore,pilots were briefed to take their roles as pilot flying orpilot non-flying according to a schedule and to change roles

only one time at the stopover. Finally, a balanced factorialdesign of the type 2 6 2 6 2 with the (within-subect)factors communication medium (voice communication vs.data link), workload (low vs. high) and role (pilot flying vs.pilot non-flying) was obtained (Fig. 5).

4.3. Methods of Measurement

4.3.1. QuestioningAfter finishing the simulator flight, the pilots wereconfronted with video recordings from the cockpit takenduring the test sections. To support the recognition of thesituation, each section began and ended with an ATCmessage. After watching the video sequence of a flightsection, pilots answered a questionnaire. The questionnairefor each section was split into two parts: a Germantranslation of the NASA Task Load Index (TLX; Hart andStaveland 1988) and a questionnaire for situation judg-ment.

The NASA TLX contains six items to measure subjectsperceived workload on scales ranging from 0 to 100. Theseitems are Mental demand, Physical demand, Temporaldemand, Performance, Effort and Frustration. In addi-tion, an overall workload score is computed from aweighted combination of the six scores. The weights weredetermined from a paired comparison of relevance by theparticipants according to Hart and Staveland (1988). The

NASA TLX has been used in numerous studies in theaviation field and has turned out to be a valid and reliableinstrument for workload measurement (Gawron 2000).High correlations between the total workload measuredwith the NASA TLX and task difficulty have been found

(Hancock et al 1995). There are recommendations aboutinterpreting the scores on the individual scales of the

NASA TLX as well as about the use of the weighedsummary score (Hart and Staveland 1988; Byers et al 1989;Moroney et al 1992). Moderate delays of 15 minutesbetween task performance and filling out the questionnaireare reported to have no impact on the results (Moroney etal 1995).

In a second questionnaire, pilots rated the situation in

different dimensions. For the analysis reported here, ratingson the following scales were used: Safety, Sufficientpossibilities of intervention, Problems perceived and threescales on the knowledge of flight parameters (primary flightparameters, engine parameters and navigation parameters).

All situational dimensions were rated with a five-pointscale.

The reported knowledge of flight parameters is perceivedto give information about pilots situation awareness. Thetechnique of measurement is similar to the China LakeSituation Awareness Rating Scale (described in Gawron2000). Since subjective measurements of situation aware-ness are of limited reliability, e.g. because they can reflectonly such deficiencies in situation awareness which areconscious to the rater, they are supplemented by relatedperformance aspects according to the recommendations of

Jones (2000).After the assessment of the test sections, pilots rated the

system usability with a usability scale adapted from Brooke(1996). Finally pilots attitude towards data link communica-tion was measured with another seven-item scale. For bothratings, a five-point scale of agreement (like above) wasused. All questionnaires used were in German.

4.3.2. Video AnalysisThe same video recordings as mentioned above wereanalysed under three aspects: cognitive demands, commu-nication activities and attention-related performanceindicators.

The assessment of cognitive demands employed a methodclosely related to a part of the Cognitive Reliability andError Analysis Method (CREAM) introduced by Hollnagel(1998). The analysis was oriented on the ExtendedMethod for performance prediction, which starts with atask analysis following a given list of 15 cognitiveactivities. From these cognitive activities, four cognitivefunctions or cognitive demands have been derivedaccording to Hollnagel (1998): observation, interpreta-tion, planning and execution. The data used in theanalysis reflects the ideal course of actions. It is based on

Fig. 5. Experimental factors (VC = voice communication; DL = data link;WL = workload).



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the analysis of the test sections of the flights of two out ofthe eight crews and is corrected with perfect performance asa control.

A communication analysis was carried out because a largeportion of all cognitive activities are related to commu-nication. Units of analysis are so-called communicationacts. A communication act is the flow of communication

until there is a change in speaker/sender, a change in topicor a break of 5 seconds or more. The classification ofcommunication acts into initiative and response commu-nication acts follows a scheme introduced by Kanki et al(1989). In addition, communication acts were classified bytopic. Topics are combined into nine groups: (1) proce-dural communication (new procedures designed for thesimulator flights), (2) data link system (statements,questions, answers or self-instructions related to thefeatures and the use of the date link interface), (3)event-related elements (related to the scheduled eventsweather and technical faults, see Fig. 4), (4) other cockpit

systems (e.g. autopilot), (5) about air-to-ground commu-nication (e.g. speculations about ATCs intentions), (6)flight parameters (e.g. speed, altitude, heading), (7)flightplan, (8) flight (general) and (9) others.

Some attention-related performance indicators were ex-tracted from video recordings. According to the CrewCo-ordination Concept, pilots are obliged to inform theother crew member(s) about any deviations from theregular process. Video recordings were analysed as towhether, after which response time, and by which of thetwo pilots critical events were detected. Critical eventswere faulty ATC messages and technical failures (seeabove). An indicator for detection was the first statement

related to the critical event.

5. RESULTS

5.1. Acceptance of Data Link Communication

The acceptance of data link communication comprises twoaspects: the acceptance of a data link in general and of thecommunication interface used in the experiment inparticular. The general attitude towards data link commu-nication was measured as the degree of agreement withseven statements listed in Table 2. The right column of thetable shows the mean scores of a five-point scale where 0corresponds to total disagreement and 4 to total agreement.Three of the scales are inverted, so higher scores correspondto a positive attitude towards a data link. The resultsindicate that pilots do not expect general problemsregarding safety and errors (items 1 and 5), but they donot want to do without voice communication (items 2, 6and 7). Furthermore, they do not expect that their job willbecome either less interesting (item 4) or easier (item 3)when using a data link.

The pilots ratings on the usability scale (Table 3) for

the concrete system are more homogeneous than the

ratings for a data link in general. Mean ratings for the ten

sub-scales as well as the total usability score tend to be

about 3 on the scale ranging from 0 to 4. A total of 96.9%

of all 160 (10 items 6 16 pilots) ratings are non-negative

(a rating of 2 or higher) and 79.4 % are clearly positive

(rating of 3 or 4), but only 14.4 % are extremely positive

(rating of 4). Even if there is a clear tendency towards a

positive rating of the system, it is not ideal yet.

None of the two scales makes any restrictions on the

scope of application. Table 4 shows that the data link

interface is rated worse for take off and approach, mainly

positive for climb and descent, and clearly positive for thecruise (100 % of all ratings are 3 or 4).

Table 2. Mean (M) and standard deviations (SD) of agreement withstatements regarding data link communication (n = 16 pilots); scalesranging from 0 (I totally disagree) to 4 (I totally agree); some scalesinverted, so higher values indicate positive attitude towards a data link

Item M (SD)

1. Data link contributes to flight safety 2.62 (.89)

2. Cutting down the voice contact with ground is a

disadvantage [score inverted]

2.06 (.68)

3. Data link facilitates the work in the cockpit 2.06 (.77)

4. With data link the work in the cockpit becomes lessinteresting [score inverted]

3.06 (1.06)

5. Data link is less error tolerant than VHF com-munication [score inverted]

3.00 (.63)

6. I dont really care whether the communicationbetween controller and pilot is done via data link ornot

1.81 (.83)

7. Data link will completely replace VHF communica-tion in the future

1.19 (.98)

Table 3. Mean (M) and standard deviations (SD) of agreement withusability items and overall usability score (n = 16 pilots); scales rangingfrom 0 (I totally disagree) to 4 (I totally agree); some scales inverted, sohigher values indicate higher usability

Item M (SD)

I think that I would like to use this system frequently 3.00 (.63)

The system was unnecessarily complex [score inverted] 3.13 (.34)

The system was easy to use 2.94 (.57)

The various functions of the system were integrated well 3.00 (.63)

There was to much inconsistency in this system [scoreinverted]

2.88 (.72)

Most pilots would learn to use the system very quickly 3.13 (.81)

The system was very cumbersome to use [score inverted] 3.00 (.63)

I felt very confident using the system 2.62 (.62)

I had to get used to a lot of things before I could get goingwith this system [score inverted]

2.44 (.81)

The technical changes (ND, MCDU pages) and its use fitwell into the overall cockpit design (Airbus philosophy)

2.94 (.57)

Usability score 2.91 (.35)



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5.2. Demands

The demands produced by the tasks within the flightscenario were compared according to the eight cells of theexperimental schedule. Each cell contains data from fourtest sections of the flight (according to Fig. 4 high workloadconditions exist in sections A, C, F and H, low workloadconditions in sections B, D, E and G). The cognitivefunctions profiles are shown in Fig. 6. The figure indicatesthat the overall load (count of coded cognitive functions

during one test section) increases for both of the pilots withhigher workload as well as with the switch from voicecommunication to data link. The overall amount ofemployed cognitive functions tends to be slightly higherfor the pilot non-flying. The pilot flyings task employs thefunctions of interpretation and observation more oftenthan the pilot non-flyings task. On the other hand, for thepilot non-flying there are more codings of execution. Notethat a large portion (approximately half) of the executionencodings are due to the activity of communication. Thefunction planning is found only under high workloadconditions and is closely related to flight plan negotiations.With the exception of planning, the counts for the

cognitive functions correlate positively with each other

(Spearman r = 0.33 . . . 0.71).

Since the duration of the test sections was not fixed, but

depended on reaching certain waypoints or solving tasks,

duration is, among other effects, a result of the crews

performance. Therefore it is treated as an dependent

variable, too (Fig. 7). Mean time consumption correlates

positively with the counts of all four cognitive functions

(r = 0.39 . . . 0.50).

For an estimation of workload derived from the pilots

cognitive functions, employed during flight, the temporal

density of the employment of these functions is of special

interest. Figure 8 shows the employment of the four

cognitive functions per time. As opposed to the absolute

Table 4. Ratings of the suitability of the datalink interface by flight phase

Suitability for . . . M (SD)

Take-off 1.50 (1.21)Climb 2.37 (1.09)Cruise 3.50 (0.52)Descent 2.69 (0.79)Approach 1.50 (1.03)

Fig. 6. Cognitive function profiles for the eight factor combinations.

Fig. 7. Mean duration of flight sections (in seconds) for the eight factorcombinations.



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values in Fig. 6, the count of cognitive functions per timeshows an inverted tendency: a decrease with higher work-load and the switch to the new communication medium.

5.3. Pilots Subjective Workload

The perceived workload was surveyed using the NASATask Load Index (TLX). As depicted in Fig. 9, the groupmeans of ratings were rather similar for all the six scales.The ratings on all the scales correlated positively with each

other (r = 0.41 . . . 0.81). As expected, mental and tem-

poral demands were rated higher than physical demands.

Typically there was an increase with higher workload (WL)

and with the switch from voice communication (VC) to a

data link (DL). Ratings were almost identical for both of

the pilots. All the scales range from 0 to 100 (good to

poor for the performance scale, low to high for all other

scales). Since all group means were within a range from 10

to 50, the perceived workload might be characterised as low

to moderate for all experimental conditions.

Fig. 8. Cognitive function per time for the eight factor combinations.

Fig. 9. Results from subjective workload ratings on the six scales of the NASA Task Load Index for the eight experimental conditions (group means).



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The combined score of the six scales of the NASA TLX(Fig. 10) shows the tendencies more clearly. It shows an

additional impact of workload and communication medium

on perceived workload where both of the experimental

factors have effects of approximately the same size.

Differences between the pilot flying and pilot non-flying

are negligible.

The TLX ratings (Fig. 10) reflect the overall demands

(Fig. 6) and duration of the test sections (Fig. 7) to a higher

extent than the temporal density of the demands (Fig. 8).

Mean ratings correlate positively with the duration

(r = 0.58) as well as with the counts of cognitive functions

(r = 0.25 . . . 0.66). There is an even stronger correlation(r = 0.62) between the additive count of all cognitive

functions (as indicated by the total height of the columns

in Fig. 6), but no correlation with the cognitive functions

per time (r = 0.02).

In Table 5, the results of an analysis of variance of the

six TLX scores and the combined score are shown. The

effects of the medium on the individual scales are not as

evident as they are in the combined scale. The analysis of

variance confirms that both of the pilots perceive demands,

effort and even frustration in an equal way: there is nosignificant difference in any of the scales between pilotflying and pilot non-flying.

In contrast to all other scales of the TLX, there are nosignificant main effects of the three experimental factors onthe performance scale. However, there is a significantinteraction effect of medium and workload (F = 4.99; p

dl comms workload sa muller giesa 2002

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