Narrative:

After departing las on the shead 7 SID from runway 19L; we were cleared direct to shead. We proceeded direct to shead across the mountain range and continued the climb out. The first officer was flying the aircraft with the autopilot coupled with LNAV and VNAV selected. Sometime after passing through approximately FL290; the first officer selected vertical speed at 2;000 ft per minute because the aircraft was 'searching' for a stable climb rate. He failed to verbalize this and I failed to recognize it due to replacing the commercial chart approach pages. Passing through FL370; I heard the altitude alert bell and stated thirty seven for thirty eight. I then saw the yellow airspeed warning band starting to rise up; but we were still approximately thirty knots below our current speed. At about FL378 the autopilot clicked off and a.74 was flashing in the MCP speed display. We were still well above the yellow airspeed bar but the aircraft began to descend. We did not receive the stall warning 'stick shaker' but this was very uncomfortable. The aircraft was descending and the first officer and I both added full thrust. I informed ATC we were descending to FL370 and told the first officer to descend. ATC asked if everything was ok and I informed him that we had just had a large airspeed loss and we weren't able to maintain FL380. We were cleared to FL360 and descended to that altitude. After leveling at FL360 we were asked again by ATC if we were ok or need assistance. I told them we were ok and we thought we encountered mountain wave turbulence as we were leveling off at FL380. He said he hadn't had any reports of it yet and began to query other aircraft. We reviewed the performance data; the flight release; and the load sheet several times. According to the FMC our optimum altitude was FL388 and the performance data said we were good well above FL400 down to .70 mach. We weighed 131.1; the temperature was -34C; and the FMC cruise speed was .777. After reviewing this several times and being satisfied we were well within our weight and optimum altitude at FL380; we decided to try to return to our filed altitude. Again the autopilot was coupled in LNAV and VNAV with auto throttles selected and we began a climb at FMC climb speed of about .77 mach at 1;500 FPM. At FL375; we almost instantly lost 20-25 KTS and the climb rate reduced to 100-200 FPM. We very slowly climbed to FL380 and leveled off but it felt very uncomfortable and we were back in some turbulence and decided to return to FL360. At this time; we started receiving ATC mountain wave +/- 10-20 KT reports. We also encountered more mountain wave turbulence as our route was over the mountain ranges quite a bit. I sent a message to dispatch and asked if he would review/verify the release and load sheet weights as we were unable FL380 and that I would call at our destination. The remainder of the flight was uneventful and we agreed that we must have encountered mountain wave just as we were leveling off at FL380 the first time. When we arrived at our destination I verified with the ramp that the cargo weight was indeed only 4;600 pounds; and called to inform dispatch of what had happened. We discussed all the possible causes and again agreed that we had encountered mountain wave turbulence. In over twelve years here at the company and in over twenty years of flying I have never encountered turbulence like this before. I informed the oncoming crew of the events and; by this time; the entire area along our route of flight was boxed in multiple turbulence alerts on his weather packet.

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Original NASA ASRS Text

Title: A B737-700 climbing northbound at FL370 on the LAS SNEAD 7 RNAV encountered a mountain wave which caused a 20-25 KT airspeed decrease and an uncommanded descent.

Narrative: After departing LAS on the SHEAD 7 SID from Runway 19L; we were cleared direct to SHEAD. We proceeded direct to SHEAD across the mountain range and continued the climb out. The First Officer was flying the aircraft with the autopilot coupled with LNAV and VNAV selected. Sometime after passing through approximately FL290; the First Officer selected Vertical Speed at 2;000 FT per minute because the aircraft was 'searching' for a stable climb rate. He failed to verbalize this and I failed to recognize it due to replacing the Commercial Chart approach pages. Passing through FL370; I heard the altitude alert bell and stated thirty seven for thirty eight. I then saw the yellow airspeed warning band starting to rise up; but we were still approximately thirty knots below our current speed. At about FL378 the autopilot clicked off and A.74 was flashing in the MCP speed display. We were still well above the yellow airspeed bar but the aircraft began to descend. We did not receive the stall warning 'stick shaker' but this was very uncomfortable. The aircraft was descending and the First Officer and I both added full thrust. I informed ATC we were descending to FL370 and told the First Officer to descend. ATC asked if everything was OK and I informed him that we had just had a large airspeed loss and we weren't able to maintain FL380. We were cleared to FL360 and descended to that altitude. After leveling at FL360 we were asked again by ATC if we were OK or need assistance. I told them we were OK and we thought we encountered mountain wave turbulence as we were leveling off at FL380. He said he hadn't had any reports of it yet and began to query other aircraft. We reviewed the performance data; the Flight Release; and the load sheet several times. According to the FMC our optimum altitude was FL388 and the performance data said we were good well above FL400 down to .70 Mach. We weighed 131.1; the temperature was -34C; and the FMC cruise speed was .777. After reviewing this several times and being satisfied we were well within our weight and optimum altitude at FL380; we decided to try to return to our filed altitude. Again the autopilot was coupled in LNAV and VNAV with auto throttles selected and we began a climb at FMC climb speed of about .77 Mach at 1;500 FPM. At FL375; we almost instantly lost 20-25 KTS and the climb rate reduced to 100-200 FPM. We very slowly climbed to FL380 and leveled off but it felt very uncomfortable and we were back in some turbulence and decided to return to FL360. At this time; we started receiving ATC mountain wave +/- 10-20 KT reports. We also encountered more mountain wave turbulence as our route was over the mountain ranges quite a bit. I sent a message to Dispatch and asked if he would review/verify the release and load sheet weights as we were unable FL380 and that I would call at our destination. The remainder of the flight was uneventful and we agreed that we must have encountered mountain wave just as we were leveling off at FL380 the first time. When we arrived at our destination I verified with the Ramp that the cargo weight was indeed only 4;600 LBS; and called to inform Dispatch of what had happened. We discussed all the possible causes and again agreed that we had encountered mountain wave turbulence. In over twelve years here at the company and in over twenty years of flying I have never encountered turbulence like this before. I informed the oncoming Crew of the events and; by this time; the entire area along our route of flight was boxed in multiple turbulence alerts on his weather packet.

Data retrieved from NASA's ASRS site as of April 2012 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.