Date: 201205
Local time of day: 0001-0600
Location: ZZZ.TRACON, US
Flight conditions: IMC
Light: Night
Aircraft: A320
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class E ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Air Data Computer
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Lighter-Than-Air; Flight Crew Air Transport Pilot (ATP)
Human factors: Distraction; Human-Machine Interface; Troubleshooting; Workload; Communication Breakdown; Confusion
Second reporter function: First Officer; Pilot Not Flying
Anomaly: Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew FLC complied w / Automation / Advisory; Flight Crew Overcame Equipment Problem; Flight Crew Regained Aircraft Control; Flight Crew Became Reoriented; Flight Crew Took Evasive Action
Contributing factors: Aircraft; Manuals; Chart Or Publication
Primary problem: Aircraft

Narrative: We were cleared to descend to cross an en route intersection at 10;000. During the descent and at 280 KTS I noticed my airspeed decreasing. I assumed the autothrottles were not working properly and switched off autothrottles. When speed continued Towers minimum selectable I selected autopilot off and pushed the nose over. The First Officer was busy as this was our third or fourth route change and two runway changes and he was busy. Then he noticed on the descent checklist that we had degraded to CAT 3 single and was investigating that anomaly. In the meantime I was struggling to control the aircraft because my airspeed was wrong and I was getting close to overspeeding the aircraft by pushing the nose over and advancing the throttles. The First Officer called airspeed. He was referring to the impending overspeed on his side - I thought he was referring to the impending stall that I was sensing on my side. In the heat of this struggle we crossed the intersection at 11;700 vice 10;000 assigned and told ATC that we could not make it as we had a problem. After we discovered whose instruments were correct via reference to the standby instruments I switched to Captain on Air Data 3 and switched to Autopilot 2 and autothrottles back on. It was very confusing and now we were behind. I was not confident that the speeds were correct and the First Officer was very busy; but doing a great job as pilot not flying. He was reading the EMERGENCY QRC MACH AIRSPEED UNRELIABLE; setting up the FMGC for the new runway and communication with ATC. After confirming the aircraft was under control we proceeded hurriedly; but uneventfully to touchdown on 27L.

Callback: The reporter stated that this event lasted about 60 seconds and what ran through his mind was the A330 accident. The short period of this event was very confusing because there were no other indications of an airspeed malfunction nor any clue as to what the crew should do other than begin the AIRSPEED UNRELIABLE procedure. He was so shaken by this event that he removed himself from the remainder of the trip.

Second narrative: We were given a descent to cross the intersection at 10;000. As we started our descent I began the approach/descent checklist as the Captain was the pilot flying. Shortly thereafter we received a runway change from 27R to 27L. I entered the new runway into the FMGC and continued the checklist. Upon reaching the 'Status check' part of the checklist I noticed that CAT three single was displayed on the ECAM status page. Neither I nor the Captain knew why. I had just entered the new runway and I was trying to figure out if there were any correlations to the new approach. While I was trying to figure out why the Captain disengaged the autopilot and soon after disconnected the autothrottles; we were passing through 13;000 FT. The Captain then advanced thrust and then went to climb thrust and engaged the autothrust stating the autothrust was not working. I didn't understand the statement because it was commanding idle thrust and we were in open descent mode. Again the autothrust began reducing trust toward idle. We were at around 280 KTS and 11;000 FT when the Captain disconnected autothrust and went to climb thrust. As the airspeed climbed I asked the Captain to watch the speed and he said 'I know.' We climbed to 11;300 FT and airspeed was still increasing. I told the Captain that we were going to overspeed. That is the time we figured out that his airspeed indicator was incorrect with cross checking all three indicators. Captain switched to Captain on AIRDATA 3; the airspeed overspeed by about 5 KTS. The throttles were brought to idle and speed brakes deployed. By this time we were too high to make the intersection crossing and we notified ATC. We were given a descent to 9;000 FT and we were given a heading. We were later slowed and given a heading to intercept 27L. With all the confusion we passed slightly through the LOC intercept and we were given a heading of 310 to join. Total 1 1/2 dot south of centerline; we then centered LOC and the remainder of approach was normal.


Synopsis: An A320 Captain's airspeed decreased toward V1s during descent; so he disconnected the autopilot and autothrottles to manually increase airspeed when the First Officer discovered the aircraft was overspeeding. They determined the Captain's airspeed was in error and the air data computer was at fault.