Narrative:

At approximately 3;500 ft and idle power; day VMC conditions; visual approach to runway X; the first officer called for flaps 15. I selected flaps 15 and; subsequently; the aircraft began to shake as though we had thrown a tread off the front tire of a pickup truck traveling 30 mph. The first officer discontinued the descent and I immediately declared an emergency with tower indicating we would be flying straight ahead. We were completely unsure of the cause of the rhythmic shaking of the aircraft. The first officer asked if I wanted to fly the plane as we both had our hands on the controls; and I said; 'no; you fly the aircraft.' I began to scan instruments and found both the N1 and egt gauges for the number one engine completely shaded red indicating that limits had been or were being exceeded. The first officer called for the engine failure emergency action items and we shut down the number one engine. When the start lever reached the cutoff position; we heard a loud bang; but the shaking subsided. The tower asked if we wanted to land and that we could use any runway. I considered 'engine failure in the approach configuration' procedure; but we were probably too high to land without additional flaps and elected to fly around the pattern. Tower asked if we could climb and I asked the first officer if he wanted to climb and he did so we accepted a clearance to 4;000 ft. I asked the first officer if he wanted to reconfigure and he said he would rather leave the flaps and gear as they were. I set up flight director guidance for him and proceeded to accomplish items on the engine failure checklist as we flew downwind for our second approach including bringing APU power to the number one bus. I tested the fire system and it was normal; and did not want to fly any farther than necessary from the airport. We were cleared to land and the first officer continued flying until we were approximately 1;500 ft AGL. I indicated to the first officer that I would have to make the landing and inquired whether the aircraft was in trim before transferring control. We were slightly fast; and I was distracted by not being able to reset my flight director as it would not capture the approach mode. We were on speed; but I flew slightly below the glideslope probably because of the flaps 15 attitude. Landing was normal and; as we slowed; I asked the passengers to remain seated and told them we would be having the fire crew inspect the left engine. I asked the flight attendants about the cabin; it was normal. I asked the 'B' flight attendant to open the aft entry door to observe the number one engine. He indicated that all appeared normal to him. The fire crew also indicated that the engine appeared normal and I opened the cockpit window and saw nothing unusual. We taxied to the gate and deplaned the passengers after I apologized for the unusual events. I told them that I was uncertain what exactly had happened. A deadheading flight attendant and several passengers told me that they observed shaking and flames coming from the number one engine in-flight. I inspected the engine and found nothing unusual other than the sooty tailpipe. Maintenance control indicated this morning to me that the engine would be changed and that perhaps one of the shafts had separated and thrown the engine out of balance causing the shaking we experienced. If possible; I would like to know what is found during the tear-down. I am quite surprised that the engine failed while idling; we normally expect to see this during acceleration and high output. The event began with the selection of flaps 15; which made our initial reaction to the event seem to be related to the airframe rather than the engine. We obviously had several checklists to choose from.

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Original NASA ASRS Text

Title: A B737-700 Left Engine failed at idle during descent for landing but because the aircraft was high; a go-around was begun; the engine shutdown; an emergency was declared; the QRH completed; and an uneventful landing followed.

Narrative: At approximately 3;500 FT and idle power; day VMC conditions; visual approach to Runway X; the First Officer called for Flaps 15. I selected Flaps 15 and; subsequently; the aircraft began to shake as though we had thrown a tread off the front tire of a pickup truck traveling 30 MPH. The First Officer discontinued the descent and I immediately declared an emergency with Tower indicating we would be flying straight ahead. We were completely unsure of the cause of the rhythmic shaking of the aircraft. The First Officer asked if I wanted to fly the plane as we both had our hands on the controls; and I said; 'No; you fly the aircraft.' I began to scan instruments and found both the N1 and EGT gauges for the number one engine completely shaded red indicating that limits had been or were being exceeded. The First Officer called for the engine failure emergency action items and we shut down the number one engine. When the start lever reached the cutoff position; we heard a loud bang; but the shaking subsided. The Tower asked if we wanted to land and that we could use any runway. I considered 'engine failure in the approach configuration' procedure; but we were probably too high to land without additional flaps and elected to fly around the pattern. Tower asked if we could climb and I asked the First Officer if he wanted to climb and he did so we accepted a clearance to 4;000 FT. I asked the First Officer if he wanted to reconfigure and he said he would rather leave the flaps and gear as they were. I set up flight director guidance for him and proceeded to accomplish items on the Engine Failure Checklist as we flew downwind for our second approach including bringing APU power to the number one bus. I tested the fire system and it was normal; and did not want to fly any farther than necessary from the airport. We were cleared to land and the First Officer continued flying until we were approximately 1;500 FT AGL. I indicated to the First Officer that I would have to make the landing and inquired whether the aircraft was in trim before transferring control. We were slightly fast; and I was distracted by not being able to reset my flight director as it would not capture the approach mode. We were on speed; but I flew slightly below the glideslope probably because of the Flaps 15 attitude. Landing was normal and; as we slowed; I asked the passengers to remain seated and told them we would be having the fire crew inspect the left engine. I asked the flight attendants about the cabin; it was normal. I asked the 'B' Flight Attendant to open the aft entry door to observe the number one engine. He indicated that all appeared normal to him. The fire crew also indicated that the engine appeared normal and I opened the cockpit window and saw nothing unusual. We taxied to the gate and deplaned the passengers after I apologized for the unusual events. I told them that I was uncertain what exactly had happened. A deadheading Flight Attendant and several passengers told me that they observed shaking and flames coming from the number one engine in-flight. I inspected the engine and found nothing unusual other than the sooty tailpipe. Maintenance Control indicated this morning to me that the engine would be changed and that perhaps one of the shafts had separated and thrown the engine out of balance causing the shaking we experienced. If possible; I would like to know what is found during the tear-down. I am quite surprised that the engine failed while idling; we normally expect to see this during acceleration and high output. The event began with the selection of Flaps 15; which made our initial reaction to the event seem to be related to the airframe rather than the engine. We obviously had several checklists to choose from.

Data retrieved from NASA's ASRS site as of July 2013 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.