B737-700
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 223
Troubleshooting; Training / Qualification; Situational Awareness; Human-Machine Interface; Distraction; Confusion
Aircraft Equipment Problem Critical
Flight Crew Rejected Takeoff; Flight Crew Returned To Gate
Aircraft; Human Factors
Starting the takeoff roll the First Officer pushed up the thrust levers to about 40% N1. The engines looked good so he pushed the thrust levers further up; selected TO/GA (engaging the autothrottles); and said: 'Set takeoff thrust.' I monitored the thrust levers advancing to only about 85% N1 and computed takeoff thrust was 87% N1. So; I pushed the thrust levers up just a little to get the needed 87%; but the N1s did not increase. I then pushed the thrust levers even further up (N1 gauge showed requested power in the low to mid 90% N1 range); but actual N1 was still at about 85% on both engines. I decided to reject the takeoff at this point; as I didn't know why the N1s would not increase past 85% or if I even had enough thrust/runway to takeoff; as we were not very far down the runway and still below V1. But; I did know that the computed numbers were now invalid with not achieving required N1s.Although we were not very far down the runway when I decided to reject the takeoff; we were already above 90 knots because the autobrakes did engage. We estimated our speed at the abort to be about 100 knots. By the time I looked at the airspeed during the reject; we were already below 80 knots and decreasing. When the autobrakes engaged during the reject; they go to max braking and I know that got everyone's attention. It was amazing how quickly they stopped us. By the time the thrust reversers were spooling up; we were just about stopped. I stopped the aircraft straight ahead on the runway; told the passengers to remain seated; and that we had just aborted our takeoff due to engine indications. We then cleared the runway and checked the brake energy tables before going back to a gate. This was an insidious malfunction; as everything was looking normal and we never got any indications or lights of an engine malfunction; other than not being able to achieve a thrust setting above about 85% N1. Because it was not a classic reject condition; like what we practice for in the simulator; after it was all over; back at the gate; flows and checklists completed; I started thinking about the event and I honestly can't remember if I raised the speedbrake lever during the reject. My First Officer was obviously doing the same thing because he asked me later if I raised it and I told him; 'Not sure.' I hope I did; but I'm sure someone will let us know if I didn't. I definitely need to do a better personal job of running through reject procedures before every takeoff to make sure it is second nature. It was not a factor this time as we were not that fast yet and we had lots of runway left; but on a true high speed reject it is critical.Since I don't know what caused the engines to not accelerate past 85% N1; I don't have any input for that. But what would have been nice; if one could from the cockpit actually look up someway in the FMC what the max airspeed you achieved during a rejected takeoff to make sure we have accurate brake energy data. I know the aircraft recorders have it; but that doesn't help us in the cockpit when we are trying to figure it brake energy and brake cooling requirements following a reject.'
Both N1s were below the calculated RED takeoff N1 of 87.0 and both engines were steady; not increasing despite the Captain continuing to push both thrust levers forward and the white N1 predictor arcs showing above 85.4% N1 on both N1 dials. It seemed like the autobrake and anti-skid began to cycle. Captain applied his wheel brakes to attempt to alleviate and he finally overrode the autobrake and brought the aircraft to a stop. We coordinated with Ground and Operations to hold out for approximately 20 minutes to allow for brake cooling and then proceeded back to original gate.
A B737-700 flight crew rejected their takeoff when neither engine N1 reached the calculated takeoff thrust either via autothrottle or manual input.