B747 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew FLC complied w / Automation / Advisory; Flight Crew Overcame Equipment Problem
Aircraft; Human Factors
Approaching [destination] at FL340 we were assigned a lower altitude by ATC. The auto throttles were a Deferred Maintenance Item (DMI) and as the FO reduced the throttles towards idle the airspeed began to decrease prior to the airplane pitching down to initiate the descent. As the airspeed approached the yellow caution ribbon I brought this to the FO's attention. Thrust was increased; however; before this could take effect the airspeed decreased into the yellow range and we received the AIRSPEED LOW annunciation. It took approximately 5-10 seconds before the additional thrust and descent restored the airspeed to the proper descent speed of 268 KTS. We were VMC in smooth air and level flight at the time so no additional 'g' load was being imposed on the aircraft.
The aircraft was operated without the auto throttle due to a MEL. When the flying pilot began descent he reduced power but forgot he was in VNAV and not at top of descent. The aircraft began slowing down; but not descending. The airspeed low EICAS message displayed. The flying pilot added power and turned off the Autopilot and hand flew the aircraft back to speed while descending. Then requested flight level change and continued the descent; approach and landing uneventfully.
The flight crew of a cargo aircraft reported a significant decrement in airspeed at the top of descent. This was attributed to an inoperative auto throttle system; auto flight mismanagement; and fatigue.