B737-700 first officer experiences an engine rollback at FL400 to 60% N1; then a slow recovery. High...

Narrative:

During enroute cruise at FL400 and flight planned mach; the number 2 engine rolled back to approximately 60% N1. The rollback was rough; like a cough; and was noticeable through both N1 gauge indication and aircraft feel. During the rollback; the autothrottles disengaged. As PF; I slightly reduce power on number 2 as the engine stabilized and started to recover to normal power. No malfunction indicators or messages were received. The engine operated normally for the remainder of the flight. The sat was approximately -58 degrees and the anti-ice was off. At the time we were deviating around radar returns for weather; but were not in any rain; turbulence; or dense clouds. We couldn't really tell the thickness of the weather because it was still dark. Shortly before the event; I had turned on the wing lights to check for icing and the wings were clear. Suggest investigation of potential high altitude; low temperature (below -40 sat) engine icing.

Original NASA ASRS Text

Title: B737-700 First Officer experiences an engine rollback at FL400 to 60% N1; then a slow recovery. High altitude low temperature internal engine icing is suspected.

Narrative: During enroute cruise at FL400 and flight planned Mach; the Number 2 engine rolled back to approximately 60% N1. The rollback was rough; like a cough; and was noticeable through both N1 gauge indication and aircraft feel. During the rollback; the autothrottles disengaged. As PF; I slightly reduce power on Number 2 as the engine stabilized and started to recover to normal power. No malfunction indicators or messages were received. The engine operated normally for the remainder of the flight. The SAT was approximately -58 degrees and the anti-ice was off. At the time we were deviating around radar returns for weather; but were not in any rain; turbulence; or dense clouds. We couldn't really tell the thickness of the weather because it was still dark. Shortly before the event; I had turned on the wing lights to check for icing and the wings were clear. Suggest investigation of potential high altitude; low temperature (below -40 SAT) engine icing.

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.

Attributes
ACN	1126006
Time
Date	201310
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Flight Conditions	IMC
Light	Night
Aircraft 1
Make Model Name	B737-700
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Component
Aircraft Component	Turbine Engine
Person 1
Function	First Officer Pilot Flying
Experience	Flight Crew Last 90 Days 129
Events
Anomaly	Aircraft Equipment Problem Critical Inflight Event / Encounter Weather / Turbulence

B737-700 first officer experiences an engine rollback at FL400 to 60% N1; then a slow recovery. High altitude low temperature internal engine icing is suspected.