Narrative:

The flight was [for] cross country training and an IFR flight plan had been filed.we were frequently passing through regions of IMC and we were not encountering any difficulty flying the aircraft. Approximately 40 NM from our destination; we received a low fuel indication for the left wing tank. I quickly recalculated our fuel burn and cross checked the values with the flight-plan prepared by the student. I knew from the pre-flight preparations and experience that we should have plenty of fuel and doubted the low fuel warning. However; to be on the safer side; we informed the air traffic controller of the fuel situations and requested immediate landing clearance.just prior to starting a slow descent for landing; approximately 25-30NM out; we once again encountered instrument meteorological conditions (IMC). A few seconds into IMC; I noticed the air speed indicator (asi) almost instantaneously read zero knots. I immediately took flight controls from the student; turned on pitot heat and pulled the alternate static knob. The aircraft was trimmed for 100 KIAS and I retained the aircraft configuration. I asked the student to verify all actions taken by using the aircraft checklist. We passed out of IMC in a few minutes and the asi was still reading zero knots. The asi however; came alive within the next 2-3 minutes as we descended into visual meteorological conditions.ATC provided landing clearance. I made a descending circular turn near the approach end of the runway and landed the aircraft without incident (these actions were communicated to ATC prior to execution).note 1: night flying conditions; a low fuel indication and the loss of an asi prompted the PIC to [advise ATC] and land the aircraft immediately.note 2: though the outside air temperature (OAT) varied between 10-15 degrees celsius; it is likely a stray ice partial may have blocked the pitot tube. No visible icing or ice particles was observed on the wind or windshield throughout the flight. Also; about 5 min after the pitot heat was switched on; the asi became alive and appeared to function properly.note 3: the pitot tube and fuel was inspected imminently upon landing and later by an aircraft mechanic. The aircraft had 10.3 gal (usable) in the right fuel tank and 8.1 gal (usable) in the left fuel tank. There were no defects noted in the pitot tube and; the aircraft was declared airworthy for the [return] flight.

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Original NASA ASRS Text

Title: The instructor and student pilots aboard a C172 IFR training flight in IMC encountered apparent icing induced loss of airspeed indication along with inexplicable low fuel quantity warnings while enroute. Airspeed indications returned to normal after leaving IMC and post flight maintenance determined both tanks contained significant amounts of fuel; totaling nearly 20 gallons.

Narrative: The flight was [for] cross country training and an IFR flight plan had been filed.We were frequently passing through regions of IMC and we were not encountering any difficulty flying the aircraft. Approximately 40 NM from our destination; we received a Low Fuel Indication for the left wing tank. I quickly recalculated our fuel burn and cross checked the values with the flight-plan prepared by the student. I knew from the pre-flight preparations and experience that we should have plenty of fuel and doubted the Low Fuel Warning. However; to be on the safer side; we informed the air traffic controller of the fuel situations and requested immediate landing clearance.Just prior to starting a slow descent for landing; approximately 25-30NM out; we once again encountered instrument meteorological conditions (IMC). A few seconds into IMC; I noticed the air speed indicator (ASI) almost instantaneously read zero knots. I immediately took flight controls from the student; turned on pitot heat and pulled the alternate static knob. The aircraft was trimmed for 100 KIAS and I retained the aircraft configuration. I asked the student to verify all actions taken by using the aircraft checklist. We passed out of IMC in a few minutes and the ASI was still reading zero knots. The ASI however; came alive within the next 2-3 minutes as we descended into visual meteorological conditions.ATC provided landing clearance. I made a descending circular turn near the approach end of the runway and landed the aircraft without incident (These actions were communicated to ATC prior to execution).NOTE 1: Night flying conditions; a low fuel indication and the loss of an ASI prompted the PIC to [advise ATC] and land the aircraft immediately.NOTE 2: Though the outside air temperature (OAT) varied between 10-15 degrees Celsius; it is likely a stray ice partial may have blocked the pitot tube. No visible icing or ice particles was observed on the wind or windshield throughout the flight. Also; about 5 min after the pitot heat was switched on; the ASI became alive and appeared to function properly.NOTE 3: The pitot tube and fuel was inspected imminently upon landing and later by an aircraft mechanic. The aircraft had 10.3 Gal (usable) in the Right fuel tank and 8.1 Gal (usable) in the left fuel tank. There were no defects noted in the pitot tube and; the aircraft was declared airworthy for the [return] flight.

Data retrieved from NASA's ASRS site 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.