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.

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Original NASA ASRS Text

Title: 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.

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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.