A B737-300 in climb at FL290 to level off at FL330. Captain's and first officer's altimeters and air...

Attributes
ACN	605340
Time
Date	200401
Day	Thu
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : zzz.airport
State Reference	US
Altitude	msl bound lower : 29000 msl bound upper : 23000
Environment
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zzz.artcc
Operator	common carrier : air carrier
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Flight Phase	climbout : intermediate altitude cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 150 flight time total : 12000 flight time type : 2400
ASRS Report	605340
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Events
Anomaly	aircraft equipment problem : critical other anomaly other
Independent Detector	other flight crewa other flight crewb
Resolutory Action	flight crew : diverted to another airport other
Consequence	other other
Supplementary
Problem Areas	Flight Crew Human Performance Chart Or Publication Aircraft
Primary Problem	Aircraft

Narrative:

Departed ZZZ for ZZZ1. All preflight indications were normal. Captain and first officer airspeed checked in agreement at 80 KTS and V1. 29.92 was set at transition altitude FL180. At FL290, crew noticed disagreement in altimeters (first officer high approximately 600 ft). Crosschecked standby altimeter (which agreed with first officer altimeter) and confirmed 29.92 set both sides. Initial leveloff at FL330 (on first officer side), also noticed autothrottle not keeping speed. (.76 mach set, first officer airspeed indicating .78 mach, captain airspeed indicating .74 mach). Autothrottle obviously trying to catch captain's speed. No flags present. Autothrottle turned off and 'airspeed/mach unreliable' emergency checklist accomplished. Reference action states 'unreliable indications without flags generally are caused by a pitot static system problem which cannot be fixed in-flight.' called dispatch and phone patch to maintenance controller. Maintenance controller got engineering ok to attempt to cycle air data computer circuit breakers in-flight, which did not help. We had elected to divert to nearest suitable airport (ZZZ2) per flight manual guidance. 'If indications are unreliable, land at nearest suitable airport.' during descent captain's airspeed and altimeter errors gradually reduced and eventually matched those of first officer and standby instruments. I received ACARS message from dispatch saying flight operations dispatch recommended landing ZZZ, since indications were now normal. I consented and diverted back to ZZZ.

Original NASA ASRS Text

Title: A B737-300 IN CLB AT FL290 TO LEVEL OFF AT FL330. CAPT'S AND FO'S ALTIMETERS AND AIRSPD INDICATORS IN DISAGREEMENT, EXCEEDING ALLOWABLE LIMITS. RETURN LAND AT DEP ARPT.

Narrative: DEPARTED ZZZ FOR ZZZ1. ALL PREFLT INDICATIONS WERE NORMAL. CAPT AND FO AIRSPD CHKED IN AGREEMENT AT 80 KTS AND V1. 29.92 WAS SET AT TRANSITION ALT FL180. AT FL290, CREW NOTICED DISAGREEMENT IN ALTIMETERS (FO HIGH APPROX 600 FT). XCHKED STANDBY ALTIMETER (WHICH AGREED WITH FO ALTIMETER) AND CONFIRMED 29.92 SET BOTH SIDES. INITIAL LEVELOFF AT FL330 (ON FO SIDE), ALSO NOTICED AUTOTHROTTLE NOT KEEPING SPD. (.76 MACH SET, FO AIRSPD INDICATING .78 MACH, CAPT AIRSPD INDICATING .74 MACH). AUTOTHROTTLE OBVIOUSLY TRYING TO CATCH CAPT'S SPD. NO FLAGS PRESENT. AUTOTHROTTLE TURNED OFF AND 'AIRSPD/MACH UNRELIABLE' EMER CHKLIST ACCOMPLISHED. REF ACTION STATES 'UNRELIABLE INDICATIONS WITHOUT FLAGS GENERALLY ARE CAUSED BY A PITOT STATIC SYS PROB WHICH CANNOT BE FIXED INFLT.' CALLED DISPATCH AND PHONE PATCH TO MAINT CTLR. MAINT CTLR GOT ENGINEERING OK TO ATTEMPT TO CYCLE ADC CIRCUIT BREAKERS INFLT, WHICH DID NOT HELP. WE HAD ELECTED TO DIVERT TO NEAREST SUITABLE ARPT (ZZZ2) PER FLT MANUAL GUIDANCE. 'IF INDICATIONS ARE UNRELIABLE, LAND AT NEAREST SUITABLE ARPT.' DURING DSCNT CAPT'S AIRSPD AND ALTIMETER ERRORS GRADUALLY REDUCED AND EVENTUALLY MATCHED THOSE OF FO AND STANDBY INSTS. I RECEIVED ACARS MESSAGE FROM DISPATCH SAYING FLT OPS DISPATCH RECOMMENDED LNDG ZZZ, SINCE INDICATIONS WERE NOW NORMAL. I CONSENTED AND DIVERTED BACK TO ZZZ.

Data retrieved from NASA's ASRS site as of July 2007 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.

A B737-300 in climb at FL290 to level off at FL330. Captain's and first officer's altimeters and airspeed indicators in disagreement, exceeding allowable limits. Return land at departure airport.