B757 Undifferentiated or Other Model
Troubleshooting; Situational Awareness; Human-Machine Interface
Aircraft Equipment Problem Critical; Inflight Event / Encounter Weather / Turbulence
Aircraft Equipment Problem Dissipated; Flight Crew Took Evasive Action; Flight Crew Became Reoriented; Flight Crew Exited Penetrated Airspace
Weather; Aircraft
We were at cruise altitude of FL380 at approximately Mach .78 in IMC conditions; when the airspeed abruptly began going erratic along with EICAS messages of AIRSPEED UNRELIABLE; MACH/SPEED TRIM; and RUDDER RATIO. We quickly realized that we had entered an area of moderate icing and that it had caused the pitot tubes to become blocked. The Captain requested that I coordinate a descent to FL320 and I was quickly given permission by Center Control to descend to FL320. The Captain turned on the engine and wing anti-ice and hand flew the aircraft down to FL320; while I set a slower airspeed of Mach .75 in the speed window and monitored airspeed indications. As we approached approximately FL350; the airspeed stopped fluctuating; and the EICAS messages disappeared. We continued our descent to FL320 and I reviewed the QRH to ensure that we hadn't missed any pertinent items. However; as we discussed; all indications were appropriate for icing of the pitot tubes; and not further action on our part was necessary once we had departed those conditions.
At FL380 B757 EICAS alerted AIRSPEED UNRELIABLE; MACH/SPEED TRIM; and RUDDER RATIO in an area of moderate icing. The crew; assumed pitot icing; descended to FL350 where the alerts ceased.