The use of the flight strip by the executive

The prim acy of flight strips and radar to the w ork of air traffic m anagem ent is a transparently clear observation of the field study. A lthough they have a reciprocal dependence, of the tw o devices it is the strip and not the radar w hich appears to occupy the central device role. The radar is

uninterpretable w ithout the flight strips, although the opposite is not true, and it is possible to m anage air traffic w ithout radar by using procedural control techniques of the sort studied by Bisseret and Leplat (1965). A m ost general observation was that com m ensurate w ith the m ovem ent of targets presented by the radar was the m ovem ent of strips into, across and out of the strips board. W ith the interactions of the controllers, the strips board w as a highly dynamic device, in spite of the fact that the only

transform ation which it could perform on itself was the alignm ent of adjacent strips under gravity.

An initial distinction can be m ade betw een pending and live strips. Strips w ere presented on the strip board w ithin designator racks corresponding w ith the beacons on the sector. As previously described, for each beacon along the route of an aircraft, there w as a corresponding strip in the appropriate strip board rack. Those strips derived from the flight plan subm itted by the aircraft before it began its journey, and arrived from a com puter printing station in the control room, to be placed at the control suite (see Protocol:(T01:17 01:17)). At this stage the strips were pending and were generally left above the rack designator, although at one point an assistant placed the strips directly into the rack "cocked out"

(Protocol:(T09:12 09:12)). The strips became live w hen they were moved into the rack below the designator; this occurred w hen an aircraft entering the sector first m ade contact w ith the controllers, at which point they took over responsibility for the aircraft. The controller issued the aircraft w ith a unique sqw ark code as given on the strip. The aircraft then tuned its air to ground communications system to this sqw ark code, and as a result, a datablock denoting callsign and altitude was added to the target on the controller's radar (see Protocol:(T17:20 17:24)). Note however, that the controller m ay already have detected the aircraft and come to consider it w ithin their planning horizon.

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The pending strips also indicated the source and extent of future dem ands on the controller, as already noted. Since sim ply the presence of pending strips indicated future dem ands on the controller, it follows that an absence of strips also conveyed an im portant message to the controller - that

dem ands w ould be reducing, and so this too could influence the controllers' current activities.

Of the live strips, a prim ary feature was their organisation w ithin the separate racks. This organisation was based on e.t.a at the beacon recorded on each strip. The e.t.a derived again from the flight plan filed by the aircraft and was based on intended speed and route and departure times. These times w ere commonly incorrect and a regular activity of the controller and the Crew Chief was the checking of these times, w ith a corresponding correction recorded on the strip. Protocol:(T13:58 14:00) refers to such a correction, following reference to current actual position of the aircraft and its speed. The significance for the controller of the chronological

organisation of the strips was indicated by the regular updating and

reorganisation activity, and also the instance w hen the controller described his protracted reasoning about the correctness of the time of one aircraft Protocol:(T14:12 14:28). Of the 145 strips used in this session, some 50 or 35% show ed a revision of e.t.a records: 4 (i.e., 3%) were less than 5 m inutes

different to the printed times; 19 (i.e., 13%) were betw een 5 and 15 m inutes different to the printed times; 27 (i.e., 19%) were more than 15 minutes different to the printed times. Six strips (i.e., 4%) show ed at least one subsequent correction.

The M anual of Air Traffic Services indicates a large vocabulary of symbols w hich can be used to annotate the strip, though only a subset of these was observed being used in practice. In addition to m aintaining an accurate account of etas, tw o major classes of record m ade by the controller on the strip described headings and heights: these, as suggested earlier, relate to the tw o major operators which the controller m ight apply to their problem space. Concerning heights, it has already been described how the Crew Chief, coordinating betw een sectors, defined the heights for aircraft at handover betw een sectors. The Chief wrote these heights on the strip for the executive controller. Because of the role of the TMA sector as

interm ediary betw een en-route and approach control, aircraft were

invariably required to change height and at the exclusive instruction of the

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Figure 4.1 Examples of strips collected from the Ringway Control Centre study (scanned image at 25% reduction).

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controller. Each time an instruction was m ade to change height, the new cleared height was entered on the strip, supplem ented by an arrow indicating climbing or descending (see Protocol:(T04:17 04:19).

In this w ay the strip provided a record of the executive controller's executed decision. H ow ever the aircraft m ay not have acted on that instruction at that time: only w hen it reported beginning the m anoeuvre and vacating its previous height did the controller cross o u t the height previously recorded on the strip. This procedure was an im portant p art of m aintaining an

accurate record, since the delay betw een instruction and execution was often actively used as part of controller's m anagem ent technique: Protocol:(T06:33 06:35) refers to a situation w here the controller left the decision of w hen to execute an instructed clearance for descent to the initiative of the pilot and did not cross out the previous level on the strip. This procedure enabled the controller, selectively attending to m any separate parts of the traffic

m anagem ent problem , to precisely interpret the current state of any aircraft at any time, in a way that w ould not be possible w ith the radar alone:

although the radar described the height w ithin 100ft of an aircraft at each instant, only the strips could tell w hich aircraft m ight initiate height changes, and the levels at w hich aircraft already in climb or descent w ould rem ain. At all times then, a record was available of the height decisions m ade and instructed.

There is an im portant feature of the height records m ade on the strip which m ight now be recognised and is in the difference betw een the heights which the Crew Chief and the Executive Controller recorded on the strip. The Crew Chief recorded his planned coordination heights at the sector boundary. These were planned future heights w hich he advised the executive controller to realise w ithin his traffic m anagem ent. Yet the records m ade by the executive controller described only executed plans: even if the controller had a scheme for subsequent height changes, he only w rote the executed decisions on the strip. The difference betw een future planned b u t unexecuted, and executed planned heights is im portant. The tw o different kinds of height entry are distinguishable by the different colour pens used by the executive controller and Crew Chief, by the Chief w riting the coordinated height in a different box, and by w riting a C (for coordinated) character over a descending arrow. It was not possible from observation to determ ine w hether the Crew Chief's height entries on the

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strip always preceded those of the Executive Controller, though Protocol:(T00:37 0038) points to such a sequence.

The procedures for recording heading instructions on the strip appeared more am biguous. As previously noted, som e aircraft were p u t on headings w hilst others were instructed to navigate for specified beacons and report again at those beacons. The use of these tw o control techniques w as not explicitly recorded on the strips. If an aircraft w as p u t on its ow n navigation, this instruction was not recorded on the strip; if an aircraft was p u t on a heading, a record of the heading was ad d ed to the strip. This was apparently logical since a given strip indicated the next beacon to w hich the aircraft w ould travel, and a direct navigation to th at beacon could be assum ed unless a record was m ade to the contrary. A record of a heading instruction was of this sort. W hen headings were instructed, these were recorded on the strip, as indicated in Protocol:(T04:57 05:03). H ow ever a record of a

continuous sequence of headings, or a crossing out of previous headings, was not apparent. The strip referred to in the above section of the protocol only records '050'. The controller's com m ent about the use of the radar for decisions about heading, and flight strips for decisions about levels

(Protocol:(T00:26 00:28), w ould explain the noted differences in strips use for these kinds of information.

Com m unication betw een members of the sector team has already been discussed, and the observation was m ade that the majority of

com m unication was not spoken, including verbal com m unication. In this the strip played a vital role in the record it offered to other m em bers of the team of decisions m ade by the executive controller. The sim ultaneity of these com m unications was indicated by a situation w here the executive controller is unable to view a strip because the Crew Chief w as w riting on an adjacent strip (see Protocol:T01:51 01:53); the rapidity of communications over the strips was indicated by the collisions betw een hands over the strips, extending to the accidental pen m arking on the back of another's hand (Protocol:(T04:53 05:03). Strips com m unication was also facilitated in the 'cocking out' of strips from the rack. A cocked out strip indicated an aircraft w ith w hich some problem has been identified, b u t not yet resolved, or an aircraft for which responsibility was being transferred betw een members of the sector team, as referred to in Protocol:(T13:58 14:00).

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W hilst recording instructions issued to aircraft, and enabling

com m unication w ithin the team, it m ight be ventured that the strips principal role was as a planning tool. The strips enabled the controller to construct goals for the traffic, since they alone inform ed the controller of the desired route, speed, height, and etas of aircraft. U sing this information, the controller constructed a plan for the passage of aircraft through the sector w ith in the constraints of the projected traffic situation, including

m aintaining separations. Because the strips were organised in term s of e.t.a at selected geographical locations (the beacons), they enabled a m onitoring of the future situation, including the prediction of separation conflicts. The controller could look dow n a rack of strips and identify aircraft which w ould be at the same height at proximal future times.

The m aintenance of the flight strips board was a constant feature of

controller activity. As aircraft progressed through the sector, new strips for that aircraft became relevant and previous strips became obsolete. Both controller and Crew Chief w ould update the strips board in a m ethodical sequence culm inating in the 'dead strips' being tossed into a box.

Protocol:(T00:37 00:38) describes such a sequence. Looking near the bottom of a strip rack, the controller identified aircraft that m ight have passed the beacon reporting point by consideration of the time m arked on the strip and the current (clock) time. He noted the Callsign on the strip and found the relevant target on the radar, confirming that the aircraft had passed the beacon; he then rem oved the dead strip. This m aintenance activity was conducted spontaneously during quiet periods (Protocol:(T00:37 00:38)) and so appeared as p art of the controller's planning of his ow n activity, referred to earlier. More than sim ply triggering a 'tidying up' activity, the controller appeared to plan his future inform ation needs and state of the strips. For example, the controller in one instance w rote inform ation not on the current live strip for the aircraft, b u t on the subsequent strip, because the current live strip w ould soon be rem oved (Protocol:(T02:22 02:23)). Finally, the im portance of correctly m aintaining the strip board was indicated by an instance w here the controller threw away the last strip for an aircraft

leaving the sector, b u t the Crew Chief subsequently needed to instruct the aircraft for a m odified coordination. (Protocol:(T00:13 00:320)). The

executive controller was initially unable to recall the height clearance instruction given to the aircraft, and instead recalled the clearance for another aircraft. The aircraft was still displaying its actual, but changing height on the radar (it was still sqw ark transm itting on the controller's

Chapter 4 94

frequency); by watching the target on the radar, the controller was eventually able to make a correct recollection.

This discussion of the use of the flight strip by the executive controller observed at the Ringway Control Centre completes the account of the operational ATM system. Taken as a whole, the account reveals the inform al understanding of the operational system acquired through the field study.

The study confirmed the m uch quoted complexity of the system in its social, cognitive and technical dim ensions, and suggested the intractability of perform ing a controlled, complete and detailed analysis. Further, the need for data collection to be inobtrusive lim ited the research that could be perform ed, and data on major aspects of the system, specifically the traffic state, could not be obtained. It was apparent that an analysis that w as both more complete and more detailed, w ould need to employ a simplified sim ulation of the system. The following chapter describes a laboratory reconstruction of an ATM system (an rATM system), including the re­ created operational use of the flight strip. The study of the operational system was informally used to design the reconstruction. A lthough the reconstruction was inevitably a crude sim plification of the operational system, the com parison can, and m ust be made. The account of the operational system given in this chapter is used as a basis for that com parison.

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Chapter 5 Reconstructing the ATM system

In document Cognitive engineering and the rationalisation of the flight strip (Page 90-97)