Narrative:

After takeoff changed to departure frequency and proceeded to fly at 4000 MSL - just under the cloud deck (student under foggles) until approximately 40 miles to destination airport. All was going well - student almost ready for practical. Received weather info over VOR and both noted that the ovc was still above pattern altitude; therefore the charted minimum was very doable. After requesting IAF for a GPS approach into destination airport is when things happened rapidly: here is my recollection of all the events that took place and resulted in a safe landing in a non radar environment. Loss of airspeed indicator: noticed it was accelerating. Cross checked: altimeter-still holding 4;000; heading indicator/GPS-still holding heading. Turn coordinator/attitude indicator-still showing wings level. Pitot heat: noted we added pitot heat after we entered the clouds. Figured the pitot tube; drain hole; and static vent (all located on the pitot mast) had been compromised. Alternate air vent was opened immediately to keep the other static instruments reliable. RPM's. Noted student had full power and we were still only at 4;000 ft. Confirmed carb heat still in full hot position. Still not enough? Strange -- no icing noted on surface. Figured carb heat valve not opened fully. At this time I took over. [We] contacted center to amend approach to GPS 02 which would eliminate another 30 miles that GPS 20 would require. (It was a training flight and time was not an issue during preflight planning stage). Requested direct to FAF and noted 20 NM (I think). [I was] aware of our power situation not being sustainable in these conditions; noted closer airport from our present situation. Then noticed a runway underneath us through the broken layer (large enough to descend and see bottom of overcast) and decided to go for it. Informed center....then realized it was a highway not a runway. Power improved at the lower altitude and had enough to climb at 200 FPM - I climbed and was going to proceed to nearest airport until center suggest a different airport with VFR conditions. I accepted and asked for an immediate heading prior to setting GPS. At 3;500 MSL he had radar contact (and clear communications) to help me out. Center also contacted an airline crew to help relay comm's in case we were cut off again. It worked. I think it was 13 miles away. IFR student was asked to get an approach chart out for the new destination and figure the closest FAF to nearest runway. IFR student asked to check VFR sectionals for possible obstacles in our path in case we could not hold MEA altitude. Noticed 5 NM from airport and slowly descending from 4;000 ft looking through broken layer and saw the runway. Center cleared us for the visual. CTAF anticipated by IFR student - and announced to local traffic of full stop landing. The airport environment - what a sweet sight. Suggestion: why is there no radar in the lower altitudes of the northern half of our state? Why is the communications system intermittent at lower altitudes within the northern part of our state? Would have made the emergency workload more manageable. Lesson learned: keep pitot heat on starting at taxi. Carb heater is being checked. Wonder if it was an exhaust problem; too. A pilot is always learning.

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Original NASA ASRS Text

Title: PA28 instructor with student experiences loss of airspeed indication and possible carburetor ice during a training flight. Carburetor heat and pitot heat were both on at the time and thought to be insufficient. Flight diverts to a suitable airport with ATC assistance.

Narrative: After takeoff changed to Departure frequency and proceeded to fly at 4000 MSL - just under the cloud deck (student under foggles) until approximately 40 miles to destination airport. All was going well - student almost ready for practical. Received weather info over VOR and both noted that the OVC was still above pattern altitude; therefore the charted minimum was very doable. After requesting IAF for a GPS approach into destination airport is when things happened rapidly: here is my recollection of all the events that took place and resulted in a safe landing in a non radar environment. Loss of airspeed indicator: noticed it was accelerating. Cross checked: altimeter-still holding 4;000; heading indicator/GPS-still holding heading. Turn coordinator/attitude indicator-still showing wings level. Pitot heat: noted we added pitot heat after we entered the clouds. Figured the pitot tube; drain hole; and static vent (all located on the pitot mast) had been compromised. Alternate air vent was opened immediately to keep the other static instruments reliable. RPM's. Noted student had full power and we were still only at 4;000 FT. Confirmed carb heat still in full hot position. Still not enough? Strange -- no icing noted on surface. Figured carb heat valve not opened fully. At this time I took over. [We] contacted Center to amend approach to GPS 02 which would eliminate another 30 miles that GPS 20 would require. (It was a training flight and time was not an issue during preflight planning stage). Requested direct to FAF and noted 20 NM (I think). [I was] aware of our power situation not being sustainable in these conditions; noted closer airport from our present situation. Then noticed a runway underneath us through the broken layer (large enough to descend and see bottom of overcast) and decided to go for it. Informed Center....then realized it was a highway not a runway. Power improved at the lower altitude and had enough to climb at 200 FPM - I climbed and was going to proceed to nearest airport until center suggest a different airport with VFR conditions. I accepted and asked for an immediate heading prior to setting GPS. At 3;500 MSL he had radar contact (and clear communications) to help me out. Center also contacted an Airline crew to help relay Comm's in case we were cut off again. It worked. I think it was 13 miles away. IFR student was asked to get an approach chart out for the new destination and figure the closest FAF to nearest runway. IFR student asked to check VFR sectionals for possible obstacles in our path in case we could not hold MEA altitude. Noticed 5 NM from airport and slowly descending from 4;000 FT looking through broken layer and saw the runway. Center cleared us for the visual. CTAF anticipated by IFR student - and announced to local traffic of full stop landing. THE AIRPORT ENVIRONMENT - WHAT A SWEET SIGHT. Suggestion: why is there no RADAR in the lower altitudes of the northern half of our state? Why is the communications system intermittent at lower altitudes within the northern part of our state? Would have made the emergency workload more manageable. Lesson learned: keep pitot heat on starting at taxi. Carb heater is being checked. Wonder if it was an exhaust problem; too. A pilot is always learning.

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.