Flight Crew Last 90 Days 120; Flight Crew Total 11000; Flight Crew Type 1800
Human-Machine Interface
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence
Aircraft Automation Overrode Flight Crew; Flight Crew Regained Aircraft Control; Flight Crew Returned To Clearance
Aircraft; Human Factors; Weather
In smooth air cruising Mach .78 FL390 winds 280/90. Had checked weather en route which showed we would be going through southern tip of moderate turbulence box. Queried ATC; who reported only light chop. Started to pick up light mountain wave for ten seconds then hit a spot of moderate turbulence and moderate to severe mountain wave. I turned seatbelt sign on and made PA. Simultaneously the aircraft oversped 5 to 10 KTS. ECAM for autopilot disengage. Aircraft began rapid ascent which the Captain was able to stop at FL400. Advised ATC and returned to FL390. No injuries; no traffic separation issues. Advised company and ATC. Hard to tell how much of the altitude gain was due to the wave and how much was due to the airbus high speed protection mode. This mode; which kicks in at Mmo +.01; disengages autopilot and pitches up to slow aircraft. Between the ECAM noise; which takes a couple seconds to silence; and the altitude horn; it's a lot of distraction. The most dangerous aspect of this event is the potential for mid air collision. With the accuracy of the GPS we are often flying directly under or over other planes. With the Airbus programmed to enter steep climb; a nose to nose encounter is a real possibility. I would suggest a standard offset on domestic routes if even just .1 mile to avoid this possibility. Also the Airbus shouldn't be programmed to dump the autopilot while simultaneously taking control of the plane to deviate from the flight path. For reference; max speed for airbus is .82; maximum altitude is FL391.
The reporter did not notice the Mach number reached during the pitch up but believes that the Captain may have delayed the forward stick input between five to nine seconds and was gentle due to passenger considerations.  Autothrust remained engaged and active and the spoilers were not deployed.
A319 First Officer reports an altitude deviation from FL390 to FL400 due to a mountain wave induced overspeed and the aircraft overspeed protection system intervention.