A B737-800's engine surged from about 40% to 105% during climb at FL210 IMC with engine anti-ice on. An emergency was declared and the flight returned to the departure airport with the engine at idle.
Synopsis
A B737-800's engine surged from about 40% to 105% during climb at FL210 IMC with engine anti-ice on. An emergency was declared and the flight returned to the departure airport with the engine at idle.
Narrative
We made max thrust takeoff due to wind shear. Normal climbout through FL210. Through approximately FL220 felt yaw as if someone had tapped on a rudder pedal. After second such yaw; Captain identified engine fluctuations and disconnected autothrottle. Simultaneously I disconnected autopilot and maintained aircraft control; which involved leveling off and adding rudder. Number 1 engine indications showed N1 fluctuating a large amount. Captain had me confirm that number 1 engine was the one fluctuating and that he was holding the number 1 thrust lever. I confirmed and he attempted to regain control of the engine by retarding the thrust lever. Soon thereafter we declared an emergency with Center and told them that we would need to level off. We continued discussing the number 1 engine indications and what might be happening. We decided to return to the departure airport and asked ATC for a turn direct and a lower altitude. ATC immediately gave us a right turn direct the airport and a decent to FL200. I took ATC communication shortly thereafter so Captain could run checklists; contact company; etc. We agreed that engine surge/limit/stall was the appropriate checklist. Engine responded more or less normally after our turn and descending below FL200 on the way back to the departure airport. Captain ran QRH but we decided that there was no reason to shut number 1 engine down since we had control of it at this point. ATC vectored us on an extended downwind for the runway. Captain contacted Operations/Maintenance/Dispatch/flight attendants while I told ATC we would need a bit of time to get caught up with all of that. When he was done he took control of the aircraft so I could brief the approach. When this was complete we flew a normal 2-engine ILS to the runway. Other than adding a few knots to target speed for gusts; the approach; landing; and rollout was completely normal. The yaw 'pulse' was my first indication that we had engine problems. The yaw did not seem large so I initially thought that the Captain might have tapped the rudder pedal by accident when he was moving his seat. The second yaw pulse dispelled that notion; and right after that; the Captain asked out loud if we were having an engine failure. After that I first went to my PFD to ensure that I was 'flying the aircraft' within parameters and after that was accomplished I saw the fluctuations on the engine instruments. No idea what the cause was at this point and why the engine behaved the way it did.
Second reporter narrative
The surge caused N1/N2 fluctuations on the number 1 engine between 40% and well over 105%. The auto throttles were disconnected; and the number 1 thrust lever was brought back to regain control of said engine. An emergency was declared and a new clearance direct to our departure airport was obtained. The QRH for engine stall surge was completed through step number 6. (Note; bringing the number 1 thrust lever back did not appear to correct the condition) this led us to believe 1 of 2 things: 1. Loss of engine control; or 2 engine stall surge. Aircraft was under control with no other abnormal indications and N1 being kept under 55% for the remainder of the flight.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.