Date: 201402
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Night
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Takeoff / Launch
Airspace: Class C ZZZ
ATC / advisory: Tower ZZZ
Aircraft component: Turbine Engine
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Flying; First Officer
Human factors: Human-Machine Interface
Second reporter function: Captain; Pilot Not Flying
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed in Emergency Condition; Flight Crew Returned To Departure Airport
Contributing factors: Aircraft; Human Factors; Weather
Primary problem: Aircraft

Narrative: Initial takeoff with planned reduced thrust setting (engine anti-ice ON) in FMC (light load/approximately 50 passengers). Ground operations to include pre-checks; push-back; engine start and taxi were all normal. Weather cold (-18 C) and dry (no new precipitation); pre-dawn time frame. Taxiways and runway were mostly clean/clear and dry (no packed snow or ice). Small patches/amounts of hard packed snow/ice in initial jetway/J-line/gate/ramp area. I was pilot flying. Aligned with runway; manually advanced thrust levers to slightly above 40% N1 and allowed engines to stabilize momentarily; then continued to manually advance thrust levers towards planned reduced takeoff N1 setting. As engines appeared to be approaching (within 15-10%) planned reduced takeoff N1 setting; depressed the TOGA button (autothrottles armed) and called for pilot not flying to 'set takeoff thrust.' As aircraft continued to accelerate down runway; it required more left rudder input than normal/expected to maintain the runway centerline. Verbalized to pilot not flying '...the aircraft feels funny/unusual...' and began to further describe how the flight controls felt (primarily the left rudder input). Pilot not flying had already made the '80 knot' call. Pilot not flying followed shortly with 'V1' call. It required an unusually large amount of left rudder to maintain centerline track and thought there was a strong crosswind component I was unaware of or had missed. Pilot not flying called 'Rotate;' and I began to smoothly pull yoke back. Control pressure on the yoke felt unusually 'light;' but I maintained a smooth and consistent application of back pressure. As aircraft rotated; it took more left rudder input to maintain runway alignment. As the aircraft rotated; pilot not flying noted Number 2 Engine was not producing planned reduced power takeoff N1 (N1 was around 40-45%) or thrust. Continued takeoff; basically as an Engine Failure During Takeoff Profile (or 'V1 Cut'); Non-Normal Profile/Procedure. No Special Procedure for runway so continued on the assigned heading of 170 and climbed according to the Engine Failure During Takeoff Profile. As climb continued; noted pilot not flying had disconnected autothrottles and Number 2 Engine was only at approximately 42% N1; despite both thrust levers both being at a normal takeoff thrust setting position/range. White arc on Number 2 N1 was up to a normal 90%-ish N1; but actual N1 was only ~42%. We assessed a possible 'NG Aircraft Takeoff Thrust Shortfall.' We determined there was no QRH guidance for 'NG Aircraft Takeoff Thrust Shortfall.' Informed ATC we needed to return due to a mechanical issue. Vectored back around for the visual approach (backed up with the ILS). [We] decided to treat this as a 'One Engine Inoperative Landing' with Captain accomplishing landing. [We] referenced One Engine Inoperative Landing QRH procedure/checklist for Flaps 15 One Engine Inoperative landing. [We] transferred aircraft control on base/dogleg to final. Approach; landing and taxi-back were normal and uneventful. Fix NG EEC software to eliminate 'NG Takeoff Thrust Shortfall' events. Until NG EEC software is 'fixed;' Pilots (both pilot not flying and pilot flying) need to be extremely vigilant for the earliest indications of a 'NG Takeoff Thrust Shortfall' under all conditions. Our 'cold' weather and 'light weight' condition allowed the aircraft to accelerate very quickly to 80 KTS; V1 and Vrotation; even with the 'NG Takeoff Thrust Shortfall;' the large rudder inputs and unusual flight control feel were clues to both pilot flying and pilot not flying. Provide flight crew/pilots with additional 'guidance' on exactly how (the Company would like) flight crew/pilots to handle a 'NG Takeoff Thrust Shortfall' event. Current bulletin provides limited guidance/pilot action. Does not address: 'NG Takeoff Thrust Shortfall' event in relation to the low/high speed '80 knot' decision point to V1 [treat it as a Rejected Takeoff Non-Normal Procedure/Profile (engine failure)?] between 80 knots and V1...or... if takeoff is continued between 80 KTS and V1 [treat it like an Engine Failure during Takeoff Non-Normal Procedure/Profile] and once airborne with a 'NG Takeoff Thrust Event' treat it with the One Engine Inoperative Landing QRH and the One Engine Inoperative Non-Normal Approach procedures/profiles.


Second narrative: On receiving takeoff clearance; I told the First Officer that he had the throttles. He advanced them to 40 percent N1 and clicked TOGA. The engines spooled up to takeoff thrust and [First Officer] stated takeoff thrust and I replied Set. I diverted my attention down the runway and to our right as another aircraft was on the diagonal taxiway had turned on his lights to cross another runway. Once assured of his location; I focused on the runway. Being that it was very cold and a fairly light flight; acceleration appeared normal. Passing through 110 KTS; the First Officer stated that the aircraft steering seemed sluggish and wondered if the hydraulics were normal. I glanced at the glare shield and the overhead panel and no lights had illuminated. By then we were at V1. I called V1 rotate. As the First Officer called gear up and while retracting the gear; I looked and noticed N1 on the Number 2 Engine was at 60 percent. We climbed out and cleaned up the airplane on profile. Any attempt to increase thrust had no effect. The command sector would respond; but no thrust change. We advised ATC we needed to return and got an extended downwind. While on downwind; the Number 2 Engine would only produce 48 percent N1 - no more. The weather was good and would allow us a visual approach. I decided we would do a flaps 15 landing and treat it as a single engine landing. We checked the performance and it was fine. I decided not to shut down the engine due the fact that it was operating normally for the most part and was still producing thrust. We transferred control of the airplane so I could make the landing. We landed uneventfully and taxied to the gate.


Synopsis: B737-700 experiences a partial loss of thrust during takeoff that is not detected by either pilot until airborne. Number Two Engine was only producing 60% N1 at lift off and less than 50% on downwind. Flight returns to the departure airport.