Commercial Fixed Wing
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Total 25000
Aircraft Equipment Problem Critical; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence
Aircraft Equipment Problem Dissipated
Aircraft; Weather
While enroute to [the destination airport] ATC issued us a descent clearance from FL310 to FL250. The First Officer was the flying pilot and initiated the descent in vertical speed mode at about 1;000 fpm. Descending through approximately 28;800 feet ATC asked us to expedite through FL270 for traffic. The First Officer then changed to IAS MODE with a selected speed of 290 kts and reduced the throttles.I turned to access some information from my IPAD. Moments later I heard the First Officer say 'what the heck' and I looked over to see what was going on. The autopilot had increased the vertical speed to near 6;000 fpm quite rapidly while attempting to maintain the selected 290 kts. As we descended I observed my airspeed indication decreasing to 230 kts; about 10 kts below min maneuvering speed. I looked at the First Officer's airspeed and it was back at 230 kts also. A buffet could also be felt along with this slow airspeed indication. At this time the autopilot began to capture altitude quite early due to the rapid descent rate. As part of the capture the auto throttles came out of clamp and went right to max thrust trying to attain the speed commanded of 290 kts. I looked at the standby airspeed and it showed 350 kts; 10 kts over VMO (Maximum Operating Speed). The auto throttles were disconnected and throttles reduced as we realized that we had some kind of unreliable airspeed event in progress. I checked pitot heat was on and had a good current draw for all pitot tubes. Approximately 10 seconds later the Captain's airspeed indication steadily increased to 350 kts and the overspeed warning sounded. About 5 seconds later the First Officer's airspeed increased to 350 kts. Now all three airspeed indications were the same. We felt that this was a correct indication and that the erroneous airspeed event was concluded. The entire event from 'what the heck' until back to normal airspeed indications at below VMO was probably less than a minute. While trying to sort out what we had with two airspeeds indicating the same 230 kts and a buffet being felt; the autothrottles unclamped and went straight to max thrust before we came to the conclusion that the standby airspeed was the lone correct indication based on the autopilot giving such a rapid descent. Initially it gave the appearance of a low speed event with two out of three airspeed indicators showing slow speed along with the buffet being experienced. Because of this it took a few seconds to convince our minds that a thrust decrease was in order. Against what we were seeing and feeling. Normally a single airspeed in disagreement with the other two would be the suspect indication.We flew in nearly continuous light rain on our way up from [the departure airport]. Airfoil and engine anti-ice were on. We observed no airframe icing. There were no flags; annunciations; aural warnings or any stall indications. The only abnormal warning was the overspeed warning once the airspeed indicators began indicating correctly.The flight was continued approximately 15 minutes to [our destination]. The rest of the flight was uneventful.The overspeed event was unfortunate but unavoidable given the speed at which the event unfolded. We suspect that somehow the pitot tubes iced up at cruise or water entered the system blocking the sensing of dynamic pressure. The increase of static pressure in the descent caused the steady decrease in airspeed.
Air carrier Captain reported an unreliable airspeed indication resulted in an over speed condition.