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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.
| Attributes | |
| ACN | 1283861 |
| Time | |
| Date | 201508 |
| Local Time Of Day | 1201-1800 |
| Place | |
| Locale Reference | ZZZ.ARTCC |
| State Reference | US |
| Environment | |
| Flight Conditions | IMC |
| Light | Daylight |
| Aircraft 1 | |
| Make Model Name | B737 Next Generation Undifferentiated |
| Operating Under FAR Part | Part 121 |
| Flight Phase | Cruise |
| Flight Plan | IFR |
| Component | |
| Aircraft Component | Engine |
| Person 1 | |
| Function | Pilot Flying |
| Qualification | Flight Crew Air Transport Pilot (ATP) |
| Events | |
| Anomaly | Aircraft Equipment Problem Critical Inflight Event / Encounter Weather / Turbulence |
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.
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.