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.
Synopsis
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.
Second reporter narrative
The Captain reported to ATC that we were unable to maintain FL380 and needed to get clearance for a lower altitude. We were therefore given a descent to FL360 which we complied with. Upon reaching that altitude; we started to investigate why the deviation had occurred. The aircraft flew fine at FL360. We talked about the Vertical Speed issue as a possibility and also worked on the performance numbers. Upon running all of the numbers we discovered that not only should we have been able to maintain altitude but that we should have been good to go at as slow an airspeed of .70 Mach at FL380. We therefore decided to attempt to climb back up to FL380 (at current weight our optimum was nearly FL390). In VNAV the aircraft climbed at approximately 1;700 FPM and had slowly decreased to 1;350 FPM by 37;400 FT. At that altitude we nearly instantaneously lost twenty knots of airspeed. The aircraft reduced its Vertical Speed to between 100 and 200 FT per minute and could not climb any faster nor increase airspeed until reaching FL380. Only upon leveling off completely did the aircraft slowly increase airspeed back to cruise speed. Based on that performance; the Captain and I decided it would be more prudent to return to FL360 which we received clearance for and did. We continued to encounter mountain wave turbulence from that point on (+/- 20 KTS) until well north. While the Vertical Speed climb undoubtedly put us in a position to be more vulnerable to mountain wave the first time we were climbing to FL380; we think that the ultimate cause of the problem was mountain wave turbulence. There were no reports of mountain wave turbulence prior to the incident. Following the incident there were several reports of it including reports up and down the mountain range we flew along. However; based on this the wave would not have been avoidable except perhaps with greater awareness crossing mountain ridges. The contributing factor of the Vertical Speed versus VNAV in the initial climb was preventable. Two things would have helped. First; greater and less casual communication between Crew Members regarding flight mode changes would be helpful. Second; a greater emphasis upon ensuring that the flight mode selected is what the aircraft is actually flying.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.