Date: 201703
Local time of day: 1201-1800
Location: CLT.Airport, NC
Flight conditions: VMC
Light: Daylight
Aircraft: Airbus 318/319/320/321 Undifferentiated
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class A ZDC
ATC / advisory: Center ZDC
Aircraft component: Altitude Hold/Capture
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Workload; Situational Awareness; Distraction; Communication Breakdown; Troubleshooting
Second reporter function: Enroute
Anomaly: ATC Issue All Types; Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Aircraft Automation Overrode Flight Crew; Flight Crew FLC Overrode Automation; Flight Crew Became Reoriented; Flight Crew Returned To Clearance; General Maintenance Action
Contributing factors: Human Factors; Aircraft; Procedure
Primary problem: Aircraft

Narrative: At FL260 and with a speed restriction of 290 knots just before the fix BLUEJ we were instructed to descend via the CHSLY2 south transition at BURRZ. Being that we were under the altitude for BLUEJ we inserted a hard altitude for BURRZ of FL240 as per company policy on the special info message section of the release. We had 9000 ft selected on the MCP as the bottom altitude for the arrival. At some point before BURRZ the airplane began descending below FL240. We were briefing a possible runway change and did not stop the descent until FL236. At the same time ATC called and asked about our altitude. I replied that we were trying to control altitude and would call him back. The airplane was not responsive through the MCP panel at all. The controller cleared us to descend to FL230. At that time he instructed us to call Washington Center and gave us a phone number. I replied that we were busy trying to control the altitude of the aircraft and would call him back. We then received the phone number and switched to Atlanta Center and had an uneventful approach and landing. We wrote up the MCP and altitude hold in the logbook and contacted maintenance. I do not know the outcome as we had to swap airplanes for our next leg.The CHSLY arrival is all but unusable in the A320 series. There needs to be a software change and the controllers need to stick with their procedures and stop issuing so many speed and altitude restrictions in conjunction with the arrival.

Callback: The reporter stated all Airbus FMGC's contain a known anomaly affecting the CHSLY2 RNAV descent computation algorithm. ATC; the air carrier; and Airbus are aware of the anomaly. When below the BLUEJ waypoint FL270 constraint; crews must fly at FL260 so as to cross BURRZ at FL240. Their aircraft started descending earlier than expected because of the FMGC algorithm anomaly while the pilots were briefing a possible runway change. The reporter stated what happened to their aircraft is a daily occurrences now covered in Company communications about crew actions to mitigate the deviation. In this particular case; the aircraft's descent could not be controlled. The option would have been to disconnect the autopilot because ALT HOLD did not stop the uncommanded descent.

Second narrative: I gave Aircraft X descend via the CHSLY3 arrival and he busted his window before BLUES.


Synopsis: Airbus First Officer and Center Controller reported descent below cleared altitude. The aircraft began an uncommanded descent below FL240 and the crew could not stop the descent until FL236. Reportedly known Airbus FMGC anomaly on this STAR contributed to the early descent.