Narrative:

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.

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Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.