On the Design of Flight-Deck Procedures - Intelligent Systems ...

During the actual approach the captain instructed the first officer to reconfigure the bleeds as the aircraftwas in a turn and descending through 1,700. The Safety Board stated that “reconfiguring the bleedswitches around 1,700 feet prevented a rapid pressurization change within the cabin, but was not reasonenough to risk pilot distraction at a critical point in the flight (NTSB, 1991 p. 32). The Safety Boardconcluded with a recommendation to the FAA to “revise the MarkAir flight checklist and trainingprogram to ensure that bleed switch deactivation for gravel runway landing is accomplished at sufficientaltitude so as not to be a distraction during critical phases of flight” (p. 36).The following is the MarkAir B-737 procedure for bleed switch configuration (NTSB, 1991):If landing on a gravel or contaminated runway, start APU, and when below 5,000 ft. MSL, configurepressurization system for engine bleeds off landing:Right pack switch - ONIsolation valve - CLOSEDLeft pack switch - ONNo. 1 engine bleed switch - OFFAPU bleed switch - ONNo. 2 engine bleed switch - OFFIn summary, a well managed crew schedules the required tasks within a window of opportunity in a waythat it will not be done too early or too late. For example, if one wishes to obtain the arrival ATIS, thereis no point in doing this task too early, as the information may change by the time the aircraft will startthe approach, particularly during rapidly changing weather. On the other hand, there may be penaltiesfor obtaining the ATIS information too late, since it is required for planning purposes.7.2.2 Sequencing in the WindowSome will argue that completing some tasks early within the window of opportunity is sometimesinefficient because there may be a change or a new constraint “down the road.” Here are some examples:(1) obtaining ATIS early -- there may be change in runway, weather, NOTAMs, etc.; (2) conducting theapproach briefing -- there may be a runway change; (3) positioning flaps or stabilizer setting for takeoff-- there may be a change in load manifest, runway assignment.Nevertheless, in many discrete cockpit tasks, such as performing checklist items, briefing, mandatorycompany calls, PA calls, configuration, etc., early completion within the window of opportunity mayhave advantages.1. The task (e.g., briefing), is done and removed from the queue early on. This reduces memoryload and cognitive scheduling effort from the pilot (“I have to find a good time between ATCcalls, configuration tasks, and prior to reaching 5,000 feet to do the approach briefing”).Decisions, options, and discussion can be formulated early, when the workload is still relativelylow.2. If an unanticipated event (e.g., generator failure) occurs, the diagnostic process and thereconfiguration process will not interfere with the task (e.g., approach briefing), as it was alreadydone early in the window of opportunity. Changes pertaining to the failure can be amendedduring the approach briefing later on.3. Conducting some aspect of the task, even if it may change later, guards against completelyforgetting it in unique cases where the procedure is vulnerable to human error. For example,positioning the flaps to a takeoff setting before starting to taxi (even though the crew expects afuture change in flap setting) may guard against totally forgetting to set flaps for takeoff (NTSB,1988; Degani and Wiener, 1990).An example of taking advantage of the early completion of tasks within the window of opportunity canbe seen in one company's policy of minimizing non-configuration tasks below 18,000 feet.Accordingly, we noticed that some flight crews turn the sterile cockpit light on prior to reaching 18,000(although the FAR mandates sterile cockpit below 10,000 feet). One may argue that this is sub-optimal,sterilizing the cockpit before it is required and thereby discourage necessary cabin-cockpitcommunications (Chute and Wiener, in preparation). On the other hand, flight crews that we observed45