A321
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Flight Crew Total 20275
Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Inflight Event / Encounter Weather / Turbulence
Flight Crew Returned To Clearance; Flight Crew Overcame Equipment Problem
Weather
Just south of ALS. In cruise at FL350; .78 Mach; we encountered mild mountain wave and requested a lower altitude. Cleared to FL290; we began the descent; but had descended only about 200 ft. when the aircraft started climbing rapidly. VVI reached 1;600 fpm up. Mach went from .78 to .815; airspeed went into the red by 5 kts.; over speed warning actuated. Max altitude was about FL355. Then it turned around; and resumed the descent to FL290. We notified Denver Center about the mountain wave and the altitude deviation. Notified Dispatch about the strong mountain wave. Notified Maintenance about the over speed. Interestingly; the entire event was in smooth air. Flight Attendants and passengers never knew anything had happened. No rough ride; just a very smooth roller coaster. My first clue was the VVI going from down to up. My comment; 'Why are we climbing?'
A321 Captain reported a mountain wave encounter that caused an altitude deviation and an overspeed condition.