Narrative:

Apparently encountered the 'ice crystal icing' phenomena while in cruise; in IMC; at FL380; storms in our area but not close. FMC indicated 'buffet limit'; as the autothrottles retarded from .78 to a commanded .74 mach. Max altitude/optimum altitude in FMC went from 38.4/39.2 to 34.5/34.5; as we slowed at FL380. Total air temperature (tat) was 0 degrees; and we did have moisture that appeared as light rain on the windshield. There was no noise or odors associated with it; as mentioned in the QRH. We did not overfly any storms; which the QRH warns against; nor were there any radar returns indicating possible engine ice crystal icing. The checklist was accomplished; and as the engine anti ice (a/I) was turned on; the FMC slowly returned to normal; and the mach slowly climbed back to the previous .78. We did not end up disconnecting the autothrottles; as they did respond with power after we recognized the problem; but if it happens again I will most likely disconnect them.recommend watching for the 0 degrees tat; and especially an uncommanded speed drop while in cruise in IMC.

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Original NASA ASRS Text

Title: A B737-NG FMC indicated 'BUFFET LIMIT' as the autothrottle reduced thrust to maintain M.74 from M.78 at FL380. The Total Air Temperature was near Zero; with no apparent icing and no radar returns but the crew suspected engine core icing and followed the QRH to counter the suspected Ice Crystal Icing phenomenon.

Narrative: Apparently encountered the 'Ice Crystal Icing' phenomena while in cruise; in IMC; at FL380; storms in our area but not close. FMC indicated 'buffet limit'; as the autothrottles retarded from .78 to a commanded .74 mach. Max altitude/optimum altitude in FMC went from 38.4/39.2 to 34.5/34.5; as we slowed at FL380. Total Air Temperature (TAT) was 0 degrees; and we did have moisture that appeared as light rain on the windshield. There was no noise or odors associated with it; as mentioned in the QRH. We did not overfly any storms; which the QRH warns against; nor were there any radar returns indicating possible engine ice crystal icing. The checklist was accomplished; and as the engine Anti Ice (A/I) was turned on; the FMC slowly returned to normal; and the mach slowly climbed back to the previous .78. We did not end up disconnecting the autothrottles; as they did respond with power after we recognized the problem; but if it happens again I will most likely disconnect them.Recommend watching for the 0 degrees TAT; and especially an uncommanded speed drop while in cruise in IMC.

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.