Date: 201510
Flight conditions: VMC
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Takeoff / Launch
Aircraft component: Indicating and Warning - Flight & Navigation Systems
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Human factors: Training / Qualification; Situational Awareness
Second reporter function: First Officer; Pilot Flying
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew Rejected Takeoff; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: After commencing the takeoff roll; at approximately 120 knots; the takeoff configuration warning horn sounded. My initial reaction was to question what was wrong; not whether we should continue the takeoff or reject. I would estimate that there was a second or two before the sound of the warning horn even registered that something may be wrong. Unlike a warning light that illuminates; giving some indication of where a problem may lie; the aural warning is more shocking: since we employ the 'silent cockpit' method in the high-speed regime; the aural warning seems that much more alarming; almost as if someone is shouting out; 'there's a problem here.' I looked down to see if I could identify any immediate problem; but was overcome by the thought that we were quickly accelerating and that there was no time to evaluate whether we had an actual configuration problem or whether the warning horn was malfunctioning; so I brought the thrust levers to idle. Of course; with the thrust levers closed the RTO was initiated and the commitment to reject the takeoff was no longer in question.  I performed the rest of the rejected takeoff procedure and cleared the runway.As soon as the aircraft was clear of the runway I took a look around the flight deck and concluded that the warning horn had likely malfunctioned and that the decision to reject the takeoff had not been the best course of action. In fact; I commented to the first officer that the reject was a 'mistake' and that I should have continued the takeoff.In addition to the pressure of the rapidity of the takeoff when making the 'go/no-go' decision; is the lingering thought that perhaps we; as a crew; had made an error during the pre-takeoff procedures. The new Before Takeoff Checklist is a rather lengthy and unwieldy checklist that completely occupies the first officer (not just the first officer on this flight; but all first officers) despite the Flight Manual insistence that he maintain 'situational awareness.' The suggestion to maintain situational awareness ignores the fact that the first officer can only do one thing at a time; either devote his attention to what he is doing with the FMC/paperwork or look out the window at what is going on outside. This new checklist also requires the captain to taxi the aircraft while monitoring the inputs of the first officer; as evidenced by the required responses the captain must make to checklist challenges. This new checklist allows for configuration changes depending on whether the 'final' takeoff data reflects the preliminary data and allows the first officer to make such changes as indicated on the final performance information. With the huge workload and potential for error by limiting the verbalize; verify and monitor capacity of both pilots during a critical time the potential for error is greatly increased. Certainly; the Before Takeoff Checklist includes a takeoff configuration check; which should warn of any incorrect configuration settings; but the potential for error is inherent in the design of this checklist.


Second narrative: Normal Taxi out to Runway; all checklist items accomplished in a timely manner. Normal takeoff thrust setting accomplished; no Takeoff Configuration Warning Horn after takeoff thrust was set. Intermittent takeoff warning horn at around 100 kts. Captain elected to Reject takeoff. Aircraft was stopped without incident and taxied clear of the runway. Emergency assistance was not required. Accomplished the Rejected takeoff checklist and appropriate brake cooling time was established. Returned to gate for inspection and consultation. Maintenance came on board and was informed of the situation. Mechanic did a Byte check and noted that a code was noted for a possible speed brake position problem. Mechanic cycled the speed brake lever and stated that this action would probably reset the system. After all maintenance checks were accomplished and the aircraft was released; a normal takeoff was accomplished.


Synopsis: B737 Flight Crew experiences a takeoff warning horn above 100 knots and rejects the takeoff; but believes that he made a mistake in doing so.