Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 87; Flight Crew Total 12868; Flight Crew Type 6600
Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Inflight Event / Encounter Weather / Turbulence
Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification
Weather
During stable cruise; FL350; M.79; PF-FO; PM-CA; A/P and A/T engaged; clear skies; approaching clouds not associated with weather radar returns; approaching Continental Divide; 58NM NNW DEN; ISA +7; FMS OPT FL360; aircraft experienced a slow increase of IAS toward MMO. PF immediately disconnected A/T to control IAS. IAS continued to increase toward MMO. PF immediately disconnected A/P and slowly raised the nose of the aircraft to control IAS; MAX ALT 35250 MSL. MMO was exceeded by approximately 5-7KTS for 10 seconds. As IAS started to decrease away from MMO; PF lowered nose and recovered to FL350 engaging A/P and A/T; however; IAS continued to decrease quickly (MMO -40KTS); aircraft was now performance limited and unable to maintain altitude and airspeed. Thrust positioned at MAX CON; FMS IAS -1; IAS continued to struggle. PM notified DEN Center that the aircraft had encountered an 'atmospheric anomaly' and was unable to maintain altitude and airspeed and needed an immediate descent to recover. An immediate descent to FL340 was granted. PF was able to complete a stable recovery at FL340.Post event investigation by the PM using various sources revealed that the aircraft was flying over a 'valley area' where surface altitude gradients are particularly intense and seem not only able to produce orographic wind effects; but also intensify these effects due to a possible funnel effect caused by the valley while approaching the Continental Divide. In this area the surface altitude gradient increases more than 6000 MSL; (8280 MSL to 14255 MSL); over a span of 10NM. Surface winds; over the last 2+30 hours prior to this event averaged 27021G33.
B737NG Captain reported altitude and airspeed deviations resulted from a mountain wave encounter near DEN.