Commercial Fixed Wing
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Loss Of Aircraft Control; Inflight Event / Encounter Weather / Turbulence
Flight Crew Regained Aircraft Control
Weather
Cruise FL 380 above a high undercast; seatbelt sign on; passengers and flight attendants seated. The ride had been smooth when we suddenly encountered light to moderate turbulence.  I checked the weather radar; which was tilted down 1 degree and there were no returns ahead. Concerned there may be embedded precipitation below; I adjusted the tilt further down. This showed green returns ahead embedded in the undercast. These returns turned yellow as we approached.  Concerned about overflying the embedded returns; the First Officer and I discussed deviation options.  During the discussion; we encountered rapidly intensifying airspeed and altitude fluctuations.  The aircraft began an un-commanded climb and rapid acceleration.  The First Officer immediately notified Air Traffic Control of severe turbulence and requested a higher altitude commensurate with the climb. ATC granted the higher altitude. I disconnected the autopilot and autothrottles but was unable to prevent an overspeed condition. In retrospect; the event may better be described as severe wave action. There was a feel of a slight and smooth aircraft loading; but not the significant jarring action typically associated with turbulence. The airspeed and altitude variations continued and we requested a block altitude which ATC immediately granted. With the rapid increase and loss of airspeed; aircraft control became my first priority.  We were visually above the undercast and any possible embedded weather; so I elected not to increase aircraft loading by turning or banking the aircraft. I continued straight and may have inadvertently flown over convective activity embedded below.  Discussions with the Flight Attendants after the encounter indicated they did not feel the ride was too bad and they had not received any complaints or comments from the passengers. There were no injuries; damage or passenger complaints. When the ride settled down we notified Company via ACARS of the event and continued to our destination. Appropriate logbook writeups were made and an aircraft inspection performed by Maintenance. While operating with manual radar in areas of possible convective activity embedded below; more aggressive use of the radar tilt and occasional look downs may allow more time for deviation planning.
While in cruise at FL 380; we were having a mostly smooth ride.  As we began to encounter some light chop to light turbulence we seated the flight attendants and passengers and asked ATC for more ride reports ahead.   The last report from ATC said they had nothing more than light chop ahead so far.  We had elected to stay at FL 380 due to the ride and being bit too heavy earlier in the flight for FL 400. We were above a continuous cloud deck far below us with multiple cloud layers ahead of us; but all below us.  Approaching SPI VORTAC we discussed the clouds ahead and their height relative to us and whether or not we should proceed along our route or take a turn to the north to avoid possible turbulence associated with them.  We could not see any specific build ups; but turned the radar on and began scanning ahead for possible convective activity or embedded thunderstorms in the clouds. Before we could make a turn and before reaching the clouds ahead; we experienced a rapid increase in airspeed.   The Captain immediately tried to reduce power before we started to overspeed. Once we began to overspeed it was apparent we had encountered a very heavy mountain wave or turbulence; and due to the loss of airspeed control I immediately told Center we had encountered severe turbulence. As the Captain worked to control our speed; our altitude began to fluctuate above our cruise altitude about 300 feet. I asked the Captain if he wanted to climb with the wave and asked Center for FL 390.  Center cleared us to FL 390.   As we climbed to FL 390 it became apparent we were not out of the wave; so I asked Center for a block altitude to give us room to work with speed and altitude.  Center gave us a block from FL 390-FL 400.   We then slowly climbed to FL 400 as the wave activity and airspeed fluctuations seemed less as we climbed.  We leveled at FL 400 and the turbulence was mostly gone. We held the block attitude for while until we were sure we were clear of the turbulence. The entire event took place in VMC conditions. The turbulence was not severe in itself; but the mountain wave and rapid uncontrolled airspeed increase was great enough that I felt we needed to call it severe. By the time the event was over; the wind and weather was behind us.  I also felt this would let ATC know we may not be able to hold RVSM altitude separation. ATC was very helpful and we were not in the vicinity of other aircraft during this event. After the event we checked on the flight attendants and verified the passengers were OK and not appearing alarmed.  The flight attendants assured us they were OK and the passengers were not commenting and did not seem alarmed. This event was much more noticeable to us on the flight deck; due to primarily being airspeed related; more than turbulence. After landing; the Captain and I fully debriefed and also debriefed with the flight attendants so they would have a better understanding of what had transpired. This is a glaring reminder for constant vigilance for turbulence and weather. Early warning is key for the safety of the passengers and aircraft.   When we began to discuss the look of clouds ahead and to the left of our course; we should have made a precautionary turn; even when we felt we had enough room above and felt they did not produce a hazard by the look of them. A small turn north may have made all the difference. Also; we did not have any tops reports; which would have helped us; but is also a reminder to not only keep asking for them; but to give them out whenever possible for the safety of other aircraft and crews as well.
Air Carrier Flight Crew reported mountain wave activity inflight.