Air carrier flight crew reported they were unable to maintain speed and altitude when they encounter...

Attributes
ACN	1253287
Time
Date	201504
Local Time Of Day	1201-1800
Place
Locale Reference	ZDV.ARTCC
State Reference	CO
Environment
Flight Conditions	IMC
Aircraft 1
Make Model Name	Widebody Transport
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Flight Plan	IFR
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Person 2
Function	First Officer Pilot Not Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	ATC Issue All Types Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance Deviation - Speed All Types Inflight Event / Encounter Weather / Turbulence

Narrative:

We were just south of ALS (alamosa) VOR at FL340 in level cruise flight; IMC; with moderate icing and moderate turbulence. We started to see the airspeed start to bleed off from cruise mach. After setting max cruise power and still unable to maintain speed (at this point we had bled off about 10-15 knots of speed) I told the first officer to notify center that we would like to descend to FL320. Because of the convective storm we were flying through; and radios were completely garbled; and we were unable to communicate. I elected to start to descend to FL320 without ATC clearance after losing another 15 knots and within 20 knots of the lower foot. At fl 330 we were able to talk to ATC and they gave us clearance to continue our descent to FL320.possible mountain wave activity

Original NASA ASRS Text

Title: Air Carrier flight crew reported they were unable to maintain speed and altitude when they encountered mountain wave activity at FL340 in the vicinity of Alamosa VOR.

Narrative: We were just south of ALS (Alamosa) VOR at FL340 in level cruise flight; IMC; with moderate icing and moderate turbulence. We started to see the airspeed start to bleed off from cruise mach. After setting max cruise power and still unable to maintain speed (at this point we had bled off about 10-15 knots of speed) I told the First Officer to notify Center that we would like to descend to FL320. Because of the convective storm we were flying through; and radios were completely garbled; and we were unable to communicate. I elected to start to descend to FL320 without ATC clearance after losing another 15 knots and within 20 knots of the lower foot. At FL 330 we were able to talk to ATC and they gave us clearance to continue our descent to FL320.Possible mountain wave activity

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.

Air carrier flight crew reported they were unable to maintain speed and altitude when they encountered mountain wave activity at FL340 in the vicinity of alamosa VOR.