Date: 202006
Local time of day: 0601-1200
Location: ZZZ.ARTCC, US
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Reporter organization: Air Carrier
Reporter function: Captain
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Second reporter function: First Officer
Anomaly: Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Overcame Equipment Problem
Contributing factors: Environment - Non Weather Related; Human Factors; Procedure
Primary problem: Human Factors

Narrative: After departing ZZZ in cruise; I disengaged the autothrottles and autopilot to trim the aircraft. Autopilot was reconnected; but I inadvertently forgot to reconnect the autothrottles. We then encountered continuous light turbulence; I selected .76 in the speed window and manually pulled the throttles back; not realizing that the autothrottles were not on.The speed dropped into the upper yellow band; we got a 'buffet alert' in the FMC. Throttles were manually advanced to continue; but speed recovery was slow so we asked for a descent from FL340 to FL320. Speed recovery was rapid after we started down. In cruise we got a low speed buffet alert. Airspeed was in the lower yellow band. Power levers were manually advanced to continue power; but recovery was not adequate. We asked for a descent from FL340 to FL320. Speed recovered.This was the second leg of my 1st trip back after X months off. I thought after an uneventful 1st leg I thought I was through the danger of being rusty; boy was I wrong. The major cause was my complacency after an uneventful first leg coming back from X months off. I wrongly figured I had knocked the rust off on when the first leg went so well. We don't disconnect the autopilot/ autothrottles in cruise very often; this requires extra vigilance to make sure the aircraft is in the correct configuration.


Second narrative: Departed ZZZ; aircraft needed constant retrimming; which it did on flight ZZZ1- ZZZ day before (same aircraft).  Leveled off at FL340.  Smooth air; needed retrimming after level off.  CA disconnected A/P and A/T to retrim ailerons; then reengaged A/P.  I did not notice that A/T [had] not reengaged.  Shortly after; entered area of constant light turbulence occasional moderate bump; slowed speed to .76 M.  Speed not coming back; CA pulled throttles back to Maintain .76.  Shortly after; got BUFFET ALERT; added power and started descent to FL 320.  At that point; CA realized A/T not engaged.  Engaged A/T;  Normal after that. Always be extra vigilant; especially when disengaging A/P and A/T at altitude.  Since retrimming not usually necessary; disengaging at altitude not done that much; and due to not flying as much; and a lot of deadheading; did not catch the A/T.


Synopsis: Air carrier flight crew reported experiencing a low speed buffet alert during cruise due to having the aircraft in the incorrect configuration with autothrottles not engaged. Lack of flying was cited as a contributing factor.