B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Human-Machine Interface
Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Became Reoriented; Flight Crew Overcame Equipment Problem
Human Factors; Procedure
Airspeed slowed to 190 KTS which was 20 KTS below the planned speed of 210 KTS during the configuration for approach about 15 NM final. There was wind sheer from nearby thunderstorm activity and the aircraft configuration was Flaps 1. The monitoring pilot called out 'watch airspeed' noting the downward trend and upon adding power I noticed the auto-throttles were disengaged. The auto-pilot and auto-throttles had been disengaged during the GILLL 1 Arrival due to the autoflight systems inability to comply with the restrictions on the arrival. The auto-pilot was subsequently re-engaged but neither pilot noticed that the auto-throttles had not been re-engaged and that the 'thrust' FMA was absent. We did not slow to below minimum maneuver speed for flaps 1; but were approaching an undesirable aircraft state before correcting. Contributing to the difficulty of compliance with the GILLL 1 Arrival was a change of the arrival transition from PEGLG to BBQUE and about three vectors off and then back on the arrival by ATC. Each time we were vectored off the arrival or told to hold an altitude and discontinue the profile descent after having been previously cleared to 'descend VIA.' We were given relief by ATC on some of the altitude and/or speed restrictions but were to comply with the subsequent restrictions. This required an increase in both flight and monitoring duties with multiple changes in flight modes; FMC programming and automation.
B737 Captain reports getting slow during arrival after multiple revisions to the arrival procedure by ATC; requiring autothrust and autopilot disengagement. The autopilot is re-engaged; but the attempt to reengage the autothrust system is not successful and not initially detected by the crew.