At FL380 B757 EICAS alerted airspeed unreliable; mach/speed trim; and rudder ratio in an area of mod...

Narrative:

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.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2013 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.

Attributes
ACN	1074393
Time
Date	201303
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZZ.ARTCC
State Reference	FO
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Make Model Name	B757 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Flight Plan	IFR
Component
Aircraft Component	Pitot-Static System
Person 1
Function	Pilot Not Flying First Officer
Events
Anomaly	Aircraft Equipment Problem Critical Inflight Event / Encounter Weather / Turbulence

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.