Date: 201411
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight phase: Takeoff / Launch
ATC / advisory: Tower ZZZ
Aircraft component: Autopilot
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Human-Machine Interface; Confusion
Second reporter function: Pilot Not Flying; Captain
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Person Flight Crew
When detected: Taxi
Result: Flight Crew Rejected Takeoff; Flight Crew Returned To Gate; General Maintenance Action
Contributing factors: Aircraft; Human Factors
Primary problem: Ambiguous

Narrative: We had an unscheduled aircraft swap and the arriving aircraft was about 35 minutes late; which is why our departing flight was delayed. All preflight items were accomplished normally including the Before Taxi flight control checks. We accomplished the Before Takeoff Checklist with all items completed satisfactorily and the Captain turned to line up the aircraft with the runway centerline. The Captain seemed to delay aircraft transfer a little longer than usual and after slightly advancing the thrust levers and told me to feel the stiffness in the controls because they did not feel correct. Also at this time I noticed that the Captain's flight director was slaved as the master flight director when I knew that I had selected my flight director as the master. I felt the very stiff controls and when I attempted to pull back on the elevator; the autopilot disconnect horn went off even though I did not see an autopilot A or B selected. We elected to reject the takeoff; as we had just started to roll and had very little if any indicated airspeed. We advised Tower; taxied clear; and after a short discussion about who; if anyone; had selected an autopilot on; elected to return to the gate to have Maintenance check out the control panel/autopilot. The only thing that I can think that happened was that at some time after the Before Taxi Checklist was completed with satisfactorily results and beginning the takeoff roll an autopilot (and probably the A as the Captain's flight director was designated as the master) was commanded on; even though I do not remember seeing any on; nor do I or the Captain remember or think that we selected it to on. Possibly a review of the control panel just before takeoff roll; but I honestly believe that we do enough checks currently.


Second narrative: We had an unscheduled aircraft swap on a through flight. The preflight; start and after-start procedures were normal; including a flight control check. We were cleared for takeoff. I am sure I conducted my normal routine of Flight Attendant notification; throttle burst and lowering the HGS (Heads-up Guidance System) and calling for the Before Takeoff Checklist. We took the runway and as I was about to transfer control to the First Officer; the controls felt very stiff. I delayed the control transfer and needed to convince myself there was truly something unusual with the flight controls. It felt as if the hydraulics were off or at a minimum at a lower than normal pressure. I elected to reject the takeoff and turned off at the first taxiway achieving no more than 30 knots on the abbreviated takeoff roll. I had the First Officer feel the controls and as he was cycling the controls; the autopilot disconnect warning sounded. We pulled onto the parallel taxiway and discussed what had just happened. Somehow between the Before Taxi checks and the runway; the autopilot had become engaged. Neither the First Officer nor I had engaged it purposely; but it had become engaged. We taxied back to the gate and had a Mechanic do a byte check. He came up with an 'autonomous fail' discrepancy. He had never seen this code and when Maintenance Control was notified; this aircraft had an autopilot history. I am still baffled as to how the autopilot became engaged.Sometimes I become robotic in the actions that I take and maybe I could possibly engage a system I had no intention of engaging without realizing it. I cannot imagine that this was what happened; but I can't explain why the autopilot was engaged either. My only suggestion is to be extremely purposeful in all that I do.


Synopsis: B737 flight crew experiences stiff flight controls as the takeoff roll is initiated and the takeoff is rejected at low speed. Upon performing a second flight control check the autopilot disconnect horn sounds and the crew returns to the gate for maintenance.