Narrative:

On takeoff; when thrust was advanced from stabilized 40% N1 to takeoff power; the #2 engine lagged three to five seconds slower than #1 engine. We did not think too much of it as the takeoff power setting was attained prior to 80 knots. The takeoff and climb were uneventful. A few seconds after we leveled off at the cruise altitude of FL290 and 320 KIAS; we began feeling a small vibration that continued to get more intense; to a severe intensity. My first thought was the aircraft's assigned MEL for a missing vortex generator which stated 'during cruise flight with one or more vortex generators missing; occasional vertical motions may be felt which appear to be light turbulence.' at the same time as this thought; the flight attendants began ringing the flight deck. The B flight attendant stated flames were coming from the #2 engine tail. Looking at the #2 N1 indications; we noticed it had rolled back and surging between 35-40% while the thrust levers were matched at approximately the 85% position. We did not notice any other abnormal indications or evidence of fire. I disengaged the auto throttles and retarded the #2 thrust lever halfway; but it did not stop the engine surging or the severe aircraft vibrations. I retarded the throttle all the way to idle. Within 10 seconds; the aircraft vibrations ceased and the #2 engine surges ceased. The flight attendants reported no more flames from the #2 engine tail. We discussed the possibility of a compressor stall situation; but with severe intensity of the aircraft vibration unlike any compressor stalls we had ever previously experienced in our flying careers (and the reported presence of flames); we decided to land at the nearest suitable airport. I requested to divert [to a nearby alternate]. As we descended; we considered running the qrc/QRH engine fire or severe damage or separation checklist. The #2 was stable at idle with no abnormal indication on the flight deck and the flight attendants confirmed there was no further evidence of fire. We concluded we no longer had 'fire or airframe vibrations with abnormal engine indications' so we did not run that checklist. We elected to run the QRH engine high vibration checklist and plan a flaps 15 landing with #2 engine running at normal idle. We also informed the crew and passengers of the divert plan. The approach and landing was uneventful. We cleared the runway and stopped for the fire crews to check the aircraft. They reported no abnormal indications; so we shut down #2 engine normally and taxied to the gate. The aircraft parking; shutdown and deplaning of the passengers was uneventful. Local maintenance; maintenance control; dispatch; the supervisor on duty and chief pilot on call were all briefed on the incident.

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Original NASA ASRS Text

Title: B737-700 flight crew reported diverting to an alternate airport after experiencing severe vibration and compressor stalls from the #2 engine at FL290.

Narrative: On takeoff; when thrust was advanced from stabilized 40% N1 to takeoff power; the #2 engine lagged three to five seconds slower than #1 engine. We did not think too much of it as the takeoff power setting was attained prior to 80 knots. The takeoff and climb were uneventful. A few seconds after we leveled off at the cruise altitude of FL290 and 320 KIAS; we began feeling a small vibration that continued to get more intense; to a severe intensity. My first thought was the aircraft's assigned MEL for a missing vortex generator which stated 'During cruise flight with one or more vortex generators missing; occasional vertical motions may be felt which appear to be light turbulence.' At the same time as this thought; the flight attendants began ringing the flight deck. The B Flight Attendant stated flames were coming from the #2 engine tail. Looking at the #2 N1 indications; we noticed it had rolled back and surging between 35-40% while the thrust levers were matched at approximately the 85% position. We did not notice any other abnormal indications or evidence of fire. I disengaged the auto throttles and retarded the #2 thrust lever halfway; but it did not stop the engine surging or the severe aircraft vibrations. I retarded the throttle all the way to idle. Within 10 seconds; the aircraft vibrations ceased and the #2 engine surges ceased. The flight attendants reported no more flames from the #2 engine tail. We discussed the possibility of a compressor stall situation; but with severe intensity of the aircraft vibration unlike any compressor stalls we had ever previously experienced in our flying careers (and the reported presence of flames); we decided to land at the nearest suitable airport. I requested to divert [to a nearby alternate]. As we descended; we considered running the QRC/QRH Engine Fire or Severe Damage or Separation Checklist. The #2 was stable at idle with no abnormal indication on the flight deck and the flight attendants confirmed there was no further evidence of fire. We concluded we no longer had 'Fire or Airframe vibrations with abnormal engine indications' so we did not run that checklist. We elected to run the QRH Engine High Vibration Checklist and plan a flaps 15 landing with #2 engine running at normal idle. We also informed the crew and passengers of the divert plan. The approach and landing was uneventful. We cleared the runway and stopped for the Fire Crews to check the aircraft. They reported no abnormal indications; so we shut down #2 engine normally and taxied to the gate. The aircraft parking; shutdown and deplaning of the passengers was uneventful. Local Maintenance; Maintenance Control; Dispatch; the Supervisor on Duty and Chief Pilot on Call were all briefed on the incident.

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.