Date: 201502
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Takeoff / Launch
Aircraft component: Autothrottle/Speed Control
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Second reporter function: Captain
Anomaly: Aircraft Equipment Problem Less Severe; Ground Event / Encounter Other / Unknown
Detector: Person Flight Crew
When detected: Other Takeoff roll
Result: Flight Crew Rejected Takeoff; General Maintenance Action
Contributing factors: Weather; Human Factors; Aircraft
Primary problem: Aircraft

Narrative: During taxi out to Runway (short taxi; no line) with temperature around minus 12 degrees C; as we were cleared to line up and wait; the Captain called for the Before Takeoff Checklist. When the checklist was complete; he stated that the autothrottle would not arm. We thought this might be due to the very cold temperature; and elected to do a manual throttle takeoff (same as in the older aircraft) which is smoother anyway; due to the 3;000 feet level off. During takeoff roll; after I called for setting takeoff thrust; the Captain quickly elected to reject. When I noted the airspeed; we were decelerating through 67 knots. We then stopped; checked brake cooling requirements in the OPC (there were none); and taxied back to the gate where Maintenance met us and noted our TAT probe was erratically changing and wanted to further investigate.If the autothrottle will not arm in an new model aircraft; Pilots should investigate further instead of continuing takeoff without them like we do in the older model aircraft.


Second narrative: When I tried to arm the autothrottle; something like 'A/T Temp Limit' annunciated on the Upper Center MFD; in place of the TAT reading. We thought this might be due to the very cold temperature; and elected to do a manual throttle takeoff (same as in the older aircraft) and try to engage the autothrottle again once airborne.We also noticed the TAT would occasionally fluctuate from plus 99 degrees C to approximately plus 40 degrees C; but recalled that the TAT is unreliable on the ground; without airflow; due to the probe heater. So; this fluctuation did not cause any concern.During takeoff roll; with thrust levers full forward (Emergency Thrust Position) both N1s indicated less than required max thrust. I also noted what I thought was less acceleration than I expected during a maximum thrust takeoff. So; we rejected the takeoff at approximately 70 KIAS; and exited the runway.


Synopsis: The Captain of a B737-700 had been unable to arm the auto-throttle during taxi-out; but attributed the fault to the very cold ambient air and elected to depart using manually set max thrust. The decision to do so was predicated on the incorrect assumption that a previously noted broadly fluctuating TAT reading was merely the result of the cold OAT and the fact the probe was heated thus producing unreliable readings. They rejected their takeoff at ~70 Knots when their max takeoff thrust setting failed to produce calculated N1 levels.