Narrative:

After leveling at cruise altitude FL180; the captain called to identify the problem; in response to the master caution light. As pilot monitoring; I identified a pitot heat 1 caution light. We followed the abnormal checklist instructions to first check that the pitot heat was selected on and second to monitor the #2 flight instruments. Approximately one minute after complying with the checklist; the autopilot disengaged. The 802 panel displayed a FD navigation data invld caution message. The captain flew the aircraft by hand while I checked a message on the uns. The uns had lost its TAS 1 information. We agreed that this was normal for the situation. We continued using auxiliary navigation information from the number 2 FGC. Number one airspeed was now fluctuating near 0 KTS. The master caution and pitot heat 2 caution lights illuminated. Again; I referred to the abnormal checklist. Since there is no guidance for failure of both pitot heaters; the captain directed me to get an ATC clearance to descend and return to ZZZ. Both pilots checked the circuit breaker panels for faults. All circuit breakers remained in. Center cleared us to ZZZ descending to 16000 ft. ADI pitch/roll mismatch messages appeared on the 802 panel. Judging the number 2 flight instruments to match the standby instruments and sometimes-visible horizon; the captain used rev on his EFIS controller to copy the number 2 nd and FD panels. The captain also switched his static source to alternate. I retained my source to normal. There was a 120-140 foot difference between altimeters. The captain sensed a yawing motion and advanced the condition levers to 1050 RPM believing that ice was accumulating on the propellers. Center queried our icing conditions. I observed and reported only light rime ice on the windshield wiper posts and wing leading edge. Center transferred our flight to approach for a continued descent down to 8000 ft. During the descent; the stall warning horn chirped intermittently. Considering our pitch; power; and lack of IAS/TAS the captain determined that the warning was spurious. Descending out of 11000 ft; we exited icing conditions. Approach cleared us to level at 9000 ft for the remainder of its control. Passing through 10000 ft; both airspeed indicators revived. The yawing motion ceased. I checked the alternate static source on my side. Both pilots returned to normal static source. The captain reduced prpm to 900. We ceased using navigation and att rev. Reaching the intersection; we navigated by our vors. I called the flight attendant informing her that we would be returning to ZZZ for a normal landing in approximately 20 minutes. I explained that we had trouble with the instruments due to icing in the clouds and assured her that all was well now. I then made a PA announcement of the same information. By now; approach was vectoring us for ILS runway xx. We were able to remain clear of clouds. Sat was approximately +7. We briefed the approach; completed the descent approach checklist; and landed on runway xx. There was an electrical or mechanical failure of both pitot heaters. Because the flight was in icing conditions; the pitot tubes froze interrupting ram air pressure to the airspeed indicators and adcs. Loss of TAS information caused the further failures of ahrs pitch and navigation output. Confused by the TAS; the rudder pressure regulators began alternating from 900 to 1500 psi. This caused the twitchy yawing motion. The pitot heat systems should be modified to full independence.callback conversation with reporter revealed the following information: the reporter could provide no additional information as to the cause of the pitot heater failures. He reiterated his belief that there is some common circuit for these two pitot heaters to allow them to fail simultaneously without tripping their circuit breakers.

Related reports:

Google
 

Original NASA ASRS Text

Title: DHC 8 First Officer reports dual pitot heat failures at FL180 in icing conditions. Secondary failures include Captain's ADI; airspeed; and NAV data. First Officer's instruments along with standby instruments remain functional. After descending out of icing conditions all systems returned to normal except for pitot heat; followed by an uneventful landing at departure airport.

Narrative: After leveling at cruise altitude FL180; the Captain called to identify the problem; in response to the master caution light. As pilot monitoring; I identified a PITOT HEAT 1 caution light. We followed the abnormal checklist instructions to first check that the pitot heat was selected on and second to monitor the #2 flight instruments. Approximately one minute after complying with the checklist; the autopilot disengaged. The 802 panel displayed a FD NAV DATA INVLD caution message. The Captain flew the aircraft by hand while I checked a message on the UNS. The UNS had lost its TAS 1 information. We agreed that this was normal for the situation. We continued using AUX navigation information from the number 2 FGC. Number one airspeed was now fluctuating near 0 KTS. The master caution and PITOT HEAT 2 caution lights illuminated. Again; I referred to the abnormal checklist. Since there is no guidance for failure of both pitot heaters; the Captain directed me to get an ATC clearance to descend and return to ZZZ. Both pilots checked the circuit breaker panels for faults. All circuit breakers remained in. Center cleared us to ZZZ descending to 16000 FT. ADI PITCH/ROLL MISMATCH messages appeared on the 802 panel. Judging the number 2 flight instruments to match the standby instruments and sometimes-visible horizon; the Captain used REV on his EFIS controller to copy the number 2 ND and FD panels. The Captain also switched his static source to alternate. I retained my source to normal. There was a 120-140 foot difference between altimeters. The Captain sensed a yawing motion and advanced the condition levers to 1050 RPM believing that ice was accumulating on the propellers. Center queried our icing conditions. I observed and reported only light rime ice on the windshield wiper posts and wing leading edge. Center transferred our flight to Approach for a continued descent down to 8000 FT. During the descent; the stall warning horn chirped intermittently. Considering our pitch; power; and lack of IAS/TAS the Captain determined that the warning was spurious. Descending out of 11000 FT; we exited icing conditions. Approach cleared us to level at 9000 FT for the remainder of its control. Passing through 10000 FT; both airspeed indicators revived. The yawing motion ceased. I checked the alternate static source on my side. Both pilots returned to normal static source. The Captain reduced PRPM to 900. We ceased using NAV and ATT REV. Reaching the intersection; we navigated by our VORs. I called the Flight Attendant informing her that we would be returning to ZZZ for a normal landing in approximately 20 minutes. I explained that we had trouble with the instruments due to icing in the clouds and assured her that all was well now. I then made a PA announcement of the same information. By now; Approach was vectoring us for ILS Runway XX. We were able to remain clear of clouds. SAT was approximately +7. We briefed the approach; completed the descent approach checklist; and landed on Runway XX. There was an electrical or mechanical failure of both pitot heaters. Because the flight was in icing conditions; the pitot tubes froze interrupting RAM air pressure to the airspeed indicators and ADCs. Loss of TAS information caused the further failures of AHRS pitch and NAV output. Confused by the TAS; the rudder pressure regulators began alternating from 900 to 1500 psi. This caused the twitchy yawing motion. The pitot heat systems should be modified to full independence.Callback conversation with reporter revealed the following information: The reporter could provide no additional information as to the cause of the pitot heater failures. He reiterated his belief that there is some common circuit for these two pitot heaters to allow them to fail simultaneously without tripping their circuit breakers.

Data retrieved from NASA's ASRS site as of April 2012 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.