A B737-300 air data computer malfunctioned causing inoperative flags in the first officer's airspeed...

Narrative:

The first officer was the PF and the B autopilot disengaged. The first officer took control of the aircraft and was trying to maintain altitude and airspeed; but he actually was descending. I called; 'monitor your altitude;' because he was descending. At that moment; all the inoperative flags from his airspeed indicator; altimeter and ivsi were in view. I assumed control and started a smooth climb to FL360 after losing 450'. We notified ATC of the altitude deviation; he acknowledged; and informed us not to worry because there was no traffic around us. We requested to descend to FL280 to stay out rvsm airspace. The indication on the first officer's panel showed a continuous climb and a speed increase. The descent was IMC conditions and when we passed FL330 on VMC; the first officer's airspeed indicator showed 5 to 10 knots over the barber pole. The overspeed clacker sounded and then all his instruments returned to normal (the actual airspeed was M.74). We continued our flight and notified maintenance control and dispatch through the ACARS. After arriving; maintenance replaced the number two air data computer; and after that we continued our trip.

Original NASA ASRS Text

Title: A B737-300 Air Data Computer malfunctioned causing INOP FLAGS in the First Officer's airspeed indicator; altimeter and IVSI. The indications returned to normal eventually but the ADC was replace at the next station.

Narrative: The First Officer was the PF and the B autopilot disengaged. The First Officer took control of the aircraft and was trying to maintain altitude and airspeed; but he actually was descending. I called; 'Monitor your altitude;' because he was descending. At that moment; all the INOP FLAGS from his airspeed indicator; altimeter and IVSI were in view. I assumed control and started a smooth climb to FL360 after losing 450'. We notified ATC of the altitude deviation; he acknowledged; and informed us not to worry because there was no traffic around us. We requested to descend to FL280 to stay out RVSM airspace. The indication on the First Officer's panel showed a continuous climb and a speed increase. The descent was IMC conditions and when we passed FL330 on VMC; the First Officer's airspeed indicator showed 5 to 10 knots over the barber pole. The overspeed clacker sounded and then all his instruments returned to normal (the actual airspeed was M.74). We continued our flight and notified Maintenance Control and Dispatch through the ACARS. After arriving; Maintenance replaced the number two air data computer; and after that we continued our trip.

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.

Attributes
ACN	971746
Time
Date	201109
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Flight Plan	IFR
Component
Aircraft Component	Air Data Computer
Person 1
Function	Pilot Not Flying Captain
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 284
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance

A B737-300 air data computer malfunctioned causing inoperative flags in the first officer's airspeed indicator; altimeter and ivsi. The indications returned to normal eventually but the air data computer was replace at the next station.