A B737-300 first officer reported the loss of his air data computer; resulting in erroneous airspeed...

Narrative:

I was the pilot flying. At cruise while I was looking at the overhead panel to change the cabin temperature; the B autopilot disengaged. This was followed by all flags on my airspeed indicator; the altitude off flag and the vertical speed indicator displaying. My airspeed was indicating over vmo; and my altimeter was climbing. In the short time that we recognized that it was probably an air data computer failure and transferred control to the captain; I had gently reduced power and allowed the aircraft to descend no more than 500 ft below cruise altitude. The unreliable airspeed memory items and check list were accomplished. ATC was informed of the altitude deviation; and we returned to our cruise altitude. I asked ATC if there was a loss of separation; and they said; 'no; and there is no one below you.' we asked for and then were cleared to 28;000 ft. As we descended; all of my instruments returned to normal. The aircraft did not over speed. It was only erroneous indications on my instruments. The air data computer was removed and replaced.

Original NASA ASRS Text

Title: A B737-300 First Officer reported the loss of his air data computer; resulting in erroneous airspeed indications; as well as warning flags for other associated components. The aircraft deviated from its assigned altitude by 500 FT; but the air data computer returned to normal in the descent.

Narrative: I was the pilot flying. At cruise while I was looking at the overhead panel to change the cabin temperature; the B autopilot disengaged. This was followed by all flags on my airspeed indicator; the altitude OFF flag and the vertical speed indicator displaying. My airspeed was indicating over Vmo; and my altimeter was climbing. In the short time that we recognized that it was probably an air data computer failure and transferred control to the Captain; I had gently reduced power and allowed the aircraft to descend no more than 500 FT below cruise altitude. The unreliable airspeed memory items and check list were accomplished. ATC was informed of the altitude deviation; and we returned to our cruise altitude. I asked ATC if there was a loss of separation; and they said; 'No; and there is no one below you.' We asked for and then were cleared to 28;000 FT. As we descended; all of my instruments returned to normal. The aircraft did not over speed. It was only erroneous indications on my instruments. The air data computer was removed and replaced.

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	971563
Time
Date	201109
Local Time Of Day	0601-1200
Place
Locale Reference	ZJX.ARTCC
State Reference	FL
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Component
Aircraft Component	Air Data Computer
Person 1
Function	First Officer Pilot Flying
Experience	Flight Crew Last 90 Days 208
Events
Anomaly	Aircraft Equipment Problem Critical Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance

A B737-300 first officer reported the loss of his air data computer; resulting in erroneous airspeed indications; as well as warning flags for other associated components. The aircraft deviated from its assigned altitude by 500 ft; but the air data computer returned to normal in the descent.