Narrative:

We were turning right base to final in heading select mode; cleared for a visual approach. We were at about 8.5 DME with the aircraft configured at flaps 5; about 180 KIAS; with the autopilot/autothrottles engaged and approach mode armed. We were given a descent from 4000 ft to 2500 ft AGL; which I executed in level change. We were advised to follow traffic and I was looking primarily outside for the traffic and runway alignment with minimal instrument crosscheck since I thought the autopilot and autothrottles were engaged. Although my hands were on the controls; somewhere in this sequence the autothrottles apparently became disengaged and I failed to see a flashing red autothrottle warning light or notice the missing autothrottle FMA. I may have bumped the autothrottle disconnect switch on the throttles. I called for 'gear down' as we rolled out on final. Shortly afterwards I noticed that the aircraft was below the bugged flap 5 speed and the captain prompted 'airspeed; airspeed' and directed me to lower the aircraft pitch attitude. The aircraft was at approximately minimum maneuver speed; about 130 KIAS. I disengaged the autopilot and I think the autothrottles were already disengaged with the throttles in idle (from the level change descent). I lowered the aircraft pitch attitude slightly and we advanced the throttles while I called for flaps 15. We re-acquired flaps 15 airspeed at about 2300 ft AGL; then continued to configure the aircraft to flaps 40 and slowed to final approach speed. We completed the landing checklist and completed a normal landing. The captain and I identified the slow airspeed problem at about the same time as we referenced the airspeed indication on the pfd. I was concentrating my attention outside the aircraft thinking that both the autopilot and autothrottle were engaged and I failed to maintain an adequate instrument crosscheck. Somewhere in the sequence of events I think the autothrottles became disengaged and I did not detect the decrease in airspeed until we were approximately 25 KIAS slow. I lowered the aircraft pitch attitude; the captain and I advanced the throttles and I called for flaps 15. The combination of these actions brought us back to flaps 15 airspeed. An audible warning of the autothrottle disconnect might have prevented this event. I need to maintain a better instrument crosscheck even in VMC conditions with the autopilot and autothrottle engaged.

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Original NASA ASRS Text

Title: B737-700 First Officer reports Auto Throttle becoming disengaged inadvertently; without his knowledge; during a visual approach. Airspeed is 25 knots below bug before it is detected and corrected at 2300 feet AGL.

Narrative: We were turning right base to final in heading select mode; cleared for a visual approach. We were at about 8.5 DME with the aircraft configured at flaps 5; about 180 KIAS; with the autopilot/autothrottles engaged and approach mode armed. We were given a descent from 4000 FT to 2500 FT AGL; which I executed in level change. We were advised to follow traffic and I was looking primarily outside for the traffic and runway alignment with minimal instrument crosscheck since I thought the autopilot and autothrottles were engaged. Although my hands were on the controls; somewhere in this sequence the autothrottles apparently became disengaged and I failed to see a flashing red autothrottle warning light or notice the missing autothrottle FMA. I may have bumped the autothrottle disconnect switch on the throttles. I called for 'Gear Down' as we rolled out on final. Shortly afterwards I noticed that the aircraft was below the bugged flap 5 speed and the Captain prompted 'Airspeed; Airspeed' and directed me to lower the aircraft pitch attitude. The aircraft was at approximately minimum maneuver speed; about 130 KIAS. I disengaged the autopilot and I think the autothrottles were already disengaged with the throttles in idle (from the level change descent). I lowered the aircraft pitch attitude slightly and we advanced the throttles while I called for flaps 15. We re-acquired flaps 15 airspeed at about 2300 FT AGL; then continued to configure the aircraft to flaps 40 and slowed to final approach speed. We completed the landing checklist and completed a normal landing. The Captain and I identified the slow airspeed problem at about the same time as we referenced the airspeed indication on the PFD. I was concentrating my attention outside the aircraft thinking that both the autopilot and autothrottle were engaged and I failed to maintain an adequate instrument crosscheck. Somewhere in the sequence of events I think the autothrottles became disengaged and I did not detect the decrease in airspeed until we were approximately 25 KIAS slow. I lowered the aircraft pitch attitude; the Captain and I advanced the throttles and I called for flaps 15. The combination of these actions brought us back to flaps 15 airspeed. An audible warning of the autothrottle disconnect might have prevented this event. I need to maintain a better instrument crosscheck even in VMC conditions with the autopilot and autothrottle engaged.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.