B737-800
Flight Crew Air Transport Pilot (ATP)
Human-Machine Interface; Situational Awareness
Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Company Policy; Human Factors; Aircraft
We departed with the auto throttles MEL'd. I was the Pilot Monitoring on this leg.  In cruise flight at M.78; the Captain went back for a lavatory break.  In his absence; I inadvertently allowed the airspeed to decay to a few knots above the top of the yellow bar.   When he returned; he noticed our airspeed and immediately increased power to MAX CON and initiated a very shallow descent (approx 200-300 feet) to regain airspeed. He accelerated to M .76 and then climbed 200 feet back to cruise altitude. I'm not sure what caused my inattention.  I may have been setting up my iPad for the arrival or getting ready for the next position report.  I knew the Auto Throttles were inop; but because I hadn't been fly until the Captain went to the Lav; I guess it slipped my mind. I didn't touch the throttles while he was gone. The Captain was gone for probably less than 3 minutes. In future Auto Throttle inop situations; I think the pilot flying should brief the pilot monitoring what power setting was holding the airspeed before he/she leaves the seat; as a reminder that the Auto Throttles are off.
B737-800 First Officer reports being dispatched with inoperative autothrust with the Captain as pilot flying. In cruise the Captain takes a lavatory break and returns to find the airspeed dangerously low. The Captain uses maximum thrust and a shallow descent to regain the lost airspeed.