B737 Next Generation Undifferentiated
Flight Crew Air Transport Pilot (ATP)
Distraction; Workload; Situational Awareness; Confusion
Deviation - Speed All Types; Inflight Event / Encounter Weather / Turbulence
Flight Crew FLC complied w / Automation / Advisory; Flight Crew Took Evasive Action
Weather
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!
While cruising at FL380 over Florida; we entered some high clouds (storm blow off). I noticed some trace ice building on the wiper blade. The Captain noticed it also and I saw him check the Static Air Temperature (SAT). As expected; the temperature was well below -40 C (it was -49 C) so no action was taken. A short time later; I noticed ice beginning to build on the wiper nut. That surprised me; so I turned on the engine anti-ice as a precaution. I then noticed the airspeed bleeding off. Before I could say anything; we began to get buffet alert and max airspeed .737M messages on the FMC. I looked at the N1 and they were pegged at the CLB limit; but the airspeed kept bleeding off. Just as I was saying something to the Captain; he was checking the SAT again and it read -27C at FL380. I stated that we needed to descend and he agreed. He requested and we were cleared to a lower altitude from center. The airspeed had slowed to about 220 KIAS by this point which was still above the hook and the Angle of Attack (AOA) was still below the red band. The descent solved the problem; the temp dropped back down the rest of the flight was uneventful.
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.