Narrative:

Departure was delayed due to ramp closure for passing thunderstorm. Climb and initial cruise was normal. At level cruise FL330 in clouds with engine & wing anti-ice on and functioning normally. No turbulence noted. Captain's airspeed indicated 270 knots (M.75) both engine EPR at approx 1.88 indicated and all other parameters appeared normal prior to mach trim inop light on eoap annunciating.first officer began mach trim inop checklist. Shortly thereafter autothrottle and autopilot both disconnected simultaneously. Captain took over manual control and a scan of instruments indicated captain's airspeed at approximately 270 kts mach .75.at this time first officer announced 'watch your speed; you're getting slow.' first officer's airspeed indicator showed approximately 230 kts and captain's airspeed indicated 270 kts M.75. Captain manually added power and requested descent. Informed ATC unable rvsm. We were given; and commenced a cruise descent to FL270.performed 'unreliable airspeed' memory items. Pitot heat checked at all positions and indicated normal amperage draw in each position. Captain continued to fly the aircraft using standby a/s indicator which appeared to agree with first officer's instrument. As we descended captain's airspeed indicator slowly reduced with altitude to approx 240 kts in parity with first officer's instrument as we accelerated in descent. Continued acceleration at FL270 to return to normal speed (approximately 270 kts IAS).flight attendants called. We told them we would call them back.mach trim inop checklist completed. At FL270 captain's airspeed indicator appeared to return to normal indication in parity with first officer's and stby indicators. Returned flight attendant call. They reported 'an electrical burning smell around row 7 that now seemed to be dissipating significantly'. As ca turned around to view circuit breakers on epc a very faint electrical smell seemed to be present. At this time all indications had returned to normal. No circuit breakers observed popped.requested direct routing and began descent. No emergency declared as no priority handling was required. Remainder of the flight was normal and autopilot and auto throttle functions returned to normal for remainder of the flight. Approach and landing were routine. Entered discrepancy in aircraft logbook with reference to previous similar occurrence by another crew 3 days previous. Contacted maintenance and waited to brief the amt's in person at the aircraft. Captain's airspeed indicator became unreliable in cruise flight.additional vigilance in cross-checking airspeed indicators for comparison at each fuel checkpoint on flight plan.

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Original NASA ASRS Text

Title: MD-83 Captain reported noting an unreliable Captain's airspeed indicator in cruise flight that corrected itself in the descent.

Narrative: Departure was delayed due to ramp closure for passing thunderstorm. Climb and initial cruise was normal. At level cruise FL330 in clouds with engine & wing anti-ice on and functioning normally. No turbulence noted. Captain's airspeed indicated 270 knots (M.75) both engine EPR at approx 1.88 indicated and all other parameters appeared normal prior to Mach Trim Inop light on EOAP annunciating.First officer began Mach Trim Inop checklist. Shortly thereafter Autothrottle and Autopilot both disconnected simultaneously. Captain took over manual control and a scan of instruments indicated Captain's airspeed at approximately 270 kts Mach .75.At this time FO announced 'watch your speed; you're getting slow.' FO's airspeed indicator showed approximately 230 kts and Captain's airspeed indicated 270 kts M.75. Captain manually added power and requested descent. Informed ATC unable RVSM. We were given; and commenced a cruise descent to FL270.Performed 'Unreliable Airspeed' memory items. Pitot heat checked at all positions and indicated normal amperage draw in each position. Captain continued to fly the aircraft using standby a/s indicator which appeared to agree with FO's instrument. As we descended Captain's airspeed indicator slowly reduced with altitude to approx 240 kts in parity with FO's instrument as we accelerated in descent. Continued acceleration at FL270 to return to normal speed (approximately 270 kts IAS).Flight attendants called. We told them we would call them back.Mach Trim Inop checklist completed. At FL270 Captain's airspeed indicator appeared to return to normal indication in parity with FO's and STBY indicators. Returned FA call. They reported 'an electrical burning smell around row 7 that now seemed to be dissipating significantly'. As CA turned around to view circuit breakers on EPC a very faint electrical smell seemed to be present. At this time all indications had returned to normal. No circuit breakers observed popped.Requested direct routing and began descent. No emergency declared as no priority handling was required. Remainder of the flight was normal and autopilot and auto throttle functions returned to normal for remainder of the flight. Approach and landing were routine. Entered discrepancy in aircraft logbook with reference to previous similar occurrence by another crew 3 days previous. Contacted Maintenance and waited to brief the AMT's in person at the aircraft. Captain's airspeed indicator became unreliable in cruise flight.Additional vigilance in cross-checking airspeed indicators for comparison at each fuel checkpoint on flight plan.

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.