A B737NG crew at FL380 entered thunderstorm blow off clouds at a -47ºC OAT and a short time later th...

Attributes
ACN	1293781
Time
Date	201509
Local Time Of Day	1201-1800
Place
Locale Reference	ZMA.ARTCC
State Reference	FL
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
Person 1
Function	Pilot Not Flying Captain
Qualification	Flight Crew Air Transport Pilot (ATP)
Person 2
Function	Pilot Flying First Officer
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Deviation - Speed All Types Inflight Event / Encounter Weather / Turbulence

Narrative:

Entered an extensive area of cirrus blow off from dissipating thunderstorm. Altitude 38;000 feet; OAT was -49 (I checked it as we entered the clouds). We were cruising at .78. About 3 or 4 minutes after entering clouds; noticed our speed at .73! The first officer and I were caught off guard. Selected max N1 but were unable to accelerate. About this same time noticed our optimum altitude had decreased from 39;000 to 35;000. And the OAT had increased to -27! This all made sense now. Requested and received descent to 32;000. I have never experienced one like that. Checked charts and we were a full 30 degrees above standard. The area was very localized. We reported the temperature variance to ATC as there were numerous aircraft on the hiley arrival on the same transition. This was a real eye opener!

Original NASA ASRS Text

Title: A B737NG crew at FL380 entered thunderstorm blow off clouds at a -47ºC OAT and a short time later the FMC issued a BUFFET Alert with a -27ºC OAT; N1 at max CLB and a max altitude of FL350. The flight descended with light icing.

Narrative: Entered an extensive area of cirrus blow off from dissipating thunderstorm. Altitude 38;000 feet; OAT was -49 (I checked it as we entered the clouds). We were cruising at .78. About 3 or 4 minutes after entering clouds; noticed our speed at .73! The FO and I were caught off guard. Selected max N1 but were unable to accelerate. About this same time noticed our optimum altitude had decreased from 39;000 to 35;000. And the OAT had increased to -27! This all made sense now. Requested and received descent to 32;000. I have never experienced one like that. Checked charts and we were a full 30 degrees above standard. The area was very localized. We reported the temperature variance to ATC as there were numerous aircraft on the HILEY arrival on the same transition. This was a real eye opener!

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.

A B737NG crew at FL380 entered thunderstorm blow off clouds at a -47ºC OAT and a short time later the FMC issued a buffet alert with a -27ºC OAT; N1 at max climb and a max altitude of FL350. The flight descended with light icing.