B737 Undifferentiated or Other Model
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 240; Flight Crew Total 12000; Flight Crew Type 5000
Situational Awareness
Aircraft Equipment Problem Less Severe; Inflight Event / Encounter CFTT / CFIT
Flight Crew Became Reoriented; Flight Crew Overcame Equipment Problem
Aircraft; Human Factors
After experiencing multiple changes to our estimated departure clearance times (EDCTs); and three re-releases though the Sabre system we were cleared into position on Rwy 28R. A Flaps 5; bleeds ON; reduced takeoff thrust (41C) was planned. The aircraft had a normal complement of crew members; no jumpseaters; 153 passengers (159 SOB; five children); ZFW 131;035 pounds; TOG 161;135; MAC 21.1; trim 5.75; OAT 28C; wind 266/10 (10 kts HW; 2 kts CW).  All checklists were accomplished correctly. As the Pilot Flying (PF) I advanced the throttles to 40% N1; allowed the engines to stabilize momentarily and then engaged TOGA and called for the Pilot Monitoring (PM) to 'check thrust.' I rested my hands lightly on top of the throttles. As the throttles advanced all appeared normal. The PM eventually began to make several adjustments to the throttle position advancing them to a higher power setting each time. As the takeoff roll continued I began to realize that the aircraft was accelerating slower than normal but not until after the '100 knots' callout. Eventually we achieved V1 and the 'rotate' callout was made by the PM. As I rotated the aircraft it became clear to both of us that we were much farther down the runway than experience suggested was normal. As the nose gear became airborne the PM said words to the effect that he was adding power beyond the reduced thrust setting of 96.5% N1. His comments exactly matched my instincts and without any comment or verbal statement I aggressively advanced the throttles to their maximum physical limit. We felt the sudden increase in power and cleared the approach light structures at the departure end of Rwy 28R. We climbed out normally. My recollections were that 1) I realized approximately one-half way down the runway that our acceleration was unusual but not in an alarming way and; 2) as I rotated I was concerned that the 737-900 tail may impact the runway with an overly aggressive rotation; and 3) the tail may impact the approach light structure if I did not rotate aggressively enough. I further stated that I tried to fly the aircraft smoothly off of the runway and then to aggressively rotate the nose after my instincts suggested I was clear of the pavement.  Once airborne the First Office (FO) and I compared our thoughts on what had happened. It was the FO's observation that 1) the throttles advanced slowly and; 2) they stabilized approximately 4% below the planned thrust setting.
After Pilot Flying (PF) called 'check thrust;' I noticed that auto thrust had failed to set proper thrust required for takeoff; indicated thrust settings were below those required in FMC; as I recall they were approximately 4% lower than required. I added power manually to match FMC required thrust setting for takeoff. As we were rolling down the runway; we were approaching the end of the runway and rotated approximately 1;000 feet before the runway threshold. At that time I realized that we may make contact with the approach lights and antennas at the end of the runway. I placed my left hand on thrust levers and started to advance them; I call 'increase power!;' I had my hand below PFs (on thrust levers) in case PF failed to respond to my call; and I was ready to advance power to max in case Captain hesitated; however Captain (PF) had immediately responded to my call and advanced power to full. By doing so; we gained extra performance and better rate of climb; in result we were able to avoid contact with obstacles. After takeoff we discussed this anomaly and determined that there might have been several aspects that have caused this anomaly. Either new Sabre flight planning has provided erroneous takeoff performance data; or Automatic Thrust control experienced problems; or both. We decided to contact Maintenance and enter automatic thrust anomaly into logbook for further investigation. We have also discussed the fact that; failure of automatic thrust to match required power setting; and manual intervention to increased power to desired setting; might have caused an additional runway roll and extended takeoff length; to achieve a required V1 and Vr speed. Just slight delay in achieving those settings might have caused an increased roll and delayed rotation positioning an aircraft in very close and potential dangerous proximity to runway end obstacles.
B737-900 flight crew reported slower than normal acceleration on takeoff roll; possibly resulting from autothrottle malfunction or error in weight and balance calculations. Captain added power at rotation to assure obstacle clearance.