Flight Crew Air Transport Pilot (ATP)
Distraction; Training / Qualification; Workload
Aircraft Equipment Problem Critical; Inflight Event / Encounter Weather / Turbulence
Flight Crew Became Reoriented; Flight Crew FLC complied w / Automation / Advisory
Aircraft; Weather
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.
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.